Month: August 2016

Relay Function and Operations:



A relay is a simple electromagnetic switch used in many circuits in transit bus electrical systems. The main relay function is as a remote-control switch, where a component control by a relay can be energized by switch far away. In operation, the relay uses a small current control circuit to control a larger current circuit. Transit bus relays are know as “change over relays”. In the near future relays will become obsolete on bus electrical systems; for the time being there are millions still in operation on transit buses. For the purpose of this blog, we will focus on 12V relays; there are however many types of relays with many current carrying capacities.




Most relays in transit bus circuits are Bosch 5 terminal; 85, 86, 30, 87, 87a.

12v relay, 24v relay, relay specs
Relay Specifications

The best place to find the specifications for the relay you are dealing with is on the relay itself. Most good quality relays will have a diagram and specifications included in the housing.

Relay Specification Case
Relay Specification Case




During operation, current flows through two separate paths in the relay; the control and switched circuit. The control side is the low amperage side of the relay, the switched side is the high amperage side of the relay.


The control circuit   85 & 86 contains the relay’s electromagnetic coil.
It is typically controlled by a switch in the current path between the power source (source control) and the coil or between the coil and ground (ground control). When the control circuit is complete the relay coil creates a magnetic field that attracts the metallic switch and changes over the contacts.

The switched circuit contains two sets of contacts.
⇒NO (Normally Open) contacts 30 & 87
⇒NC (Normally Closed) contacts 30 & 87a
When the relay coil is energized, it moves the NO contacts closed and moves the NC contacts open.

–NO (Normally open contacts) – close when relay coil energizes.
NC (Normally closed contacts) – open when relay coil energizes.

relya drawing, relay schematic, control side, switched side
Relay Circuits



Schematic Drawings:

The relay is always shown in the OFF state on schematic drawings. The standard schematic drawing symbol for a relay varies between manufacturers. The representation below is a generic sample.

OFF State

Relay Terminals:

Terminal 30 – is the switch common terminal, this terminal will be used to supply power or ground to terminal 87a when relay is OFF and 87 when relay is ON. Most applications provide power through this terminal.

Terminal 87a – is connected to terminal 30 when the relay is OFF (not energized).

Terminal 87 – is connected to terminal 30 when the relay is ON (energized).

Terminal 86 – is the power supply for the coil. It is very important to observe polarity when using relays that have a diode. Connecting 86 to ground will create a short circuit that can blow fuses, burn the relay diode or worst yet, damage an ECU or module. Resister type relays are not polarity sensitive and 86 can be ground.

Terminal 85 – is the ground for the relay coil.

relay terminals, bottom view relay



Resistor Suppressed and Diode Suppressed Relays:

Relay coils like any other coil create a magnetic field that collapses when the coil current is interrupted. This collapsing magnetic field creates a voltage spike in the 100’s of volts out of the negative side of the coil. To suppress or quench this spike a resistor or diode is place in parallel to the coil. The resister or diode gives the voltage spike an alternate path to dissipate the voltage spike away from electrical components. If the spikes are not suppressed, damage to electronic components may occur.

diode relay, resistor relay
Suppressed Relay Diagrams



Relay Testing:

There are two ways to check the condition of a relay. The best method is to check under operation by measuring voltage, the other method is to check resistance.

Measuring Resistance: use a DMM or ohmmeter to measure resistance.

Coil Resistance –
12 volt = 85 ± 5Ω
24 volt = 255 ± 15Ω
Note: Due to variances from different relay manufacturers, coil resistance values vary. Values shown reflect Bosch relays, refer to manufacturer for correct specs.

30 to 87a– should read continuity, less that 1 ohm of resistance. High resistance, indicates the contacts are corroded and pitted.

30 to 87 – should read open (OL), any resistance here indicates the contacts are touching due to corrosion, pitting, or a weak spring.

Testing Resistance
Testing Resistance


Measuring Voltage: use a DMM or voltmeter to measure voltage.

Terminal 30 – there should be source voltage available at this terminal.

Terminal 86 – there should be source voltage available at this terminal when the circuit is energized.

Terminal 85 – there should be ground (no voltage) present at this terminal when the circuit is energized.

Terminal 87 – there should be source voltage at this terminal when the circuit is energized. This voltage is the supply voltage to the load.

Terminal 87a – there should be no voltage at this terminal when the circuit is energized.

Relay Testing Voltage
Relay Testing Voltage




Spark Plugs 101

***Update on spark plug installation section, regarding using anti-seize.


Spark plugs are the ignition source for the air/fuel mixture in engines. The spark that’s delivered by the spark plug must overcome the intense combustion chamber pressures and temperatures.  They require between 12,000 – 25,000 volts to fire correctly, but can be as high as 45,000 volts. The actual electricity (spark) does not ignite the air/fuel mixture, but rather the heat generated by the electricity (spark) ignites the mixture. Spark plug gap (air gap between the center electrode and side electrode) ranges from .035 – .070 inch.  A spark plug can fire up to 35 times per second in flame temperatures of over 3000°F (1649°C). The spark created at the electrode tip ionizes the air/fuel mixture causing it to slit and form a conductive path. The gas ionization initiates a flame kernel that grows into a flame front and will travel thru the combustion chamber. Voltage at the spark plug gap is determined by several factors:

⇒Air/ fuel mixture
⇒Spark plug gap
⇒Spark plug electrode material and design
⇒Combustion chamber pressure
⇒Combustion chamber temperature
⇒Spark wear
⇒Improper installation (not torqued)





Spark Plug Parts
Spark Plug Breakdown                                                                                                                                                                                                                                                                                                                                                                            



Available Voltage – The voltage the ignition system provides the spark plugs.
Bridging – When deposits accumulate on the spark plug tip and connect the center electrode and side electrode creating an improper spark path.
Carbon Tracking – Unwanted ionization (spark) outside the spark plug wire or coil that can lead to misfires.
Center Wire – Carries secondary ignition voltage, high voltage spark.
Central Electrode – Carries secondary voltage across the gap to the side terminal (cathode).
Compression Pressure – The required voltage increases in direct proportion to cylinder pressure or as density increases.
Core Nose Tracking – Happens when the spark does not jump the gap from the center electrode, but runs from the center electrode along the ceramic insulator up inside the plug where it meets the metal casing and grounds out.
Corona Discharge (Corona Effect) – It is a rare phenomenon that causes a bluish white glow on spark plug wires or around spark plug. The cause for corona discharge is the ionization of the surrounding air. The ionization releases electrons from the oxygen molecule that is resultant energy release creates photons (light) to be produced. Certain contaminants enhance the appearance
Corona Stain – A discoloration around the ceramic insulator surface between wire boot and metal case of plug. It is caused by charged oil particles in the air around the ceramic insulator surface that are attracted by the corona discharge. These particles will stick to the insulator surface. The staining should not be confused with combustion gases leaking thru the spark plug. The staining will not affect function or performance of the spark plug.
Crush Washer – This the sealing washer between the spark plug and the cylinder head. When torqued to the proper specifications, the washer will collapse to form a leak proof seal.
Deposits – Are accumulation of deposits on the spark plugs on the center electrode, side electrode, shell, and center electrode insulator. The deposits can bridge for an improper spark path. Can be caused by:
⇒Excessive oil leaking into combustion chamber
⇒Fuel quality
⇒Engine’s operating temp or duration
Detonation – Caused by extensive pre-ignition (pinging), incorrect air/fuel mixture, or too advanced ignition timing. The center electrode ceramic insulator may appear cracked or broken away.
Dielectric – An insulator prevents current flow.
Dielectric Strength – The maximum non-conductivity of a material before it breaks down and becomes conductive.
Erosion (Spark Erosion) – Each time the spark plug fires, minute metal particles from the electrodes are worn away. Some of the affects of too much erosion (wear) are:
⇒Higher fuel consumption
⇒Added strain on the ignition system, the expanded gap requires higher voltage to fire.
Exotic Metals – See Noble Metals.
Flame Front – After the flame kernel, the first part of the air/fuel mixture that starts to burn is called the flame front as it travels igniting the rest of the mixture.
Flame Kernel – The initial shape of a freshly ignited air/fuel mixture during the first few milli seconds of combustion that happen at the spark plug electrode.
Flashover – This is when voltage sent to the spark plug does not fire between the center and side electrode. Instead it shorts between the exterior terminal nut and the metal casing of the spark plug causing a misfire.
Fouling – Occurs when the spark plugs firing tip becomes coated with excessive oil, fuel, or combustion deposits so that is unable to produce a spark. Can be caused:
⇒Too rich air/fuel mixture
⇒Excessive oil leaking into combustion chamber
⇒Too “cold” heat range spark plug
⇒Low speed engine operation
⇒Retarded engine timing
⇒Cold running engine

Gap – The voltage that is needed to ionize the electrode gap. The larger the spark gap the higher the required voltage is needed to ionize the electrode gap.
Heat Range – Refers to the operating temperature of the spark plug. It is determined by the length of the electrode insulator (ceramic) nose to the metal casing.
Heat Transfer – The function of a spark plug to remove heat from the combustion chamber.
Insulator – The ceramic portion of the spark plug that is made of sintered alumina. It is very hard and has a high dielectric strength.
Ionizing – Becoming electrically conductive, as in the air/fuel mixture becomes ionized, it creates as path for the spark to travel from the center electrode to the side electrode.
Kv – Kilo-volt.
Noble Metals – Metals used in the electrode tips that can be yttrium, iridium, tungsten, palladium, platinum, silver and gold.
Overheated – An overheated spark plug will have deposits that have melted on the insulator tip and give the insulator tip a glazed or glossy appearance.
Precious Metals – See Noble Metals.
Pre-Ignition (pinging or knocking) – It is the ignition of the air/fuel mixture before the spark from the spark plug ignites it. Can be caused:
⇒Low octane
⇒Ignition timing
⇒Lean air/fuel mixture
⇒Hot spots in combustion chamber (carbon deposits)
⇒Too “high” heat range spark plug
⇒Engine overheating

Quenching – Is the heat generated by the spark and is absorbed by the center or side electrode instead of the air/fuel mixture. This is unwanted, because the desired effect is for the air/fuel mixture to absorb all the heat. Manufacturers are creating different designs to reduce the quenching effects.
Required Voltage – The voltage required to produce a spark.
Resistor Type Spark Plugs – A ceramic resister of 4.5K-5K Ohms built into the center wire of a spark plug to suppress spark generated electromagnetic noise that can interfere with a vehicle’s on board electronics and radio signals.
Ribs – Protrusions on the ceramic insulator to help seal the spark plug boot.
Shadowing – The design effect of the ground electrode blocking the spark from the air/fuel mixture. Spark plug designs made to minimize this effect for better combustion.
Spark Corona – See Corona Discharge (Corona Effect)
Terminal – Top of plug that connects to ignition system coil or spark plug wire.



Thermal Characteristics:

Combustion chamber temperatures can range from 3600-4500 degrees F (2000-2500 degrees C) and the heat transfer into the spark plug firing end can range from 750-930 degrees F (400-500 degrees C) at light load situations. At heavy loads the spark plug firing end temperature can range from 1560-1830 degrees F (850-1000 degrees C).



Spark Plug Heat Range:

Spark plugs insulator tip and center electrode are designed to work within the ideal heat range of about 930-1560 degrees F (500-850 degrees C). If the spark plug firing tip exceeds over 1900 degrees F (1040 degrees C) the air/fuel mixture in the combustion chamber tends to pre-ignite. If the temperature is less than 750 degrees F (400 degrees C) then the insulator tip and center electrode tend to foul with carbon and oil deposits. This can lead to misfires, poor drivability and higher emissions. The spark plug heat range is measured how fast heat is transferred away from the insulator tip and electrode through the case threads into the cylinder head and cooling system. The length, projection of the insulator nose, surface area, the center core electrode material, and thermal conductivity of the insulator material determine the control of heat range.

In a hotter range spark plug, the longer insulator nose creates a longer path for heat to dissipate to the threads and into the head and cooling system.  This allows a higher heat build up in the tip of the spark plug.
In a colder range spark plug, the shorter insulator nose creates a shorter path for heat to dissipate to threads and into the head and cooling system. This allows heat to be transferred easier, and not so much heat is builds up at the spark plug tip.

Factors affecting spark plug temperature:

Ignition timing – Ignition timing too advanced.
Compression pressure – The higher the compression ratio, the higher the spark plug temperature.
Engine speed and load – The higher the engine load and speed the higher the temperature.
Air/fuel ratio – A lean air/fuel mixture will cause higher spark plug temperatures as opposed to a rich air/fuel that will cause a cooler spark plug temperature, but a very rich air/fuel mixture can cause fouling.
Tightening Torque – Improperly tightened (loose) spark plugs cannot transfer heat properly and spark plug temperature will increase. Over tightened spark plugs will decrease temperature very slightly.
Low octane gasoline – Has a faster burn rate and is more susceptible to knock, increasing temperature.
Cooling system problems – A cooling system that is not cooling properly will cause all the components on the engine to run at higher temperatures.
Excessive engine deposits – Too many deposits in the combustion chamber will increase temperatures.
Hot range spark plugs – Spark plugs that are in the hot range will retain more heat.


Spark Plug Hot Medium Cold
Spark Plug Hot Medium Cold


Spark Plug Operating Tip Temps
Spark Plug Operating Tip Temps


Spark Plug Temp Dissipation
Spark Plug Temp Dissipation



Spark Plug Material Types:

Exotic Material (Noble Metal) Spark Plugs:
Spark plug design is the same basic one for all. The main difference is in the electrode design and materials. Manufacturers are using noble metals to increase efficiency and longer service intervals. The use of these metals offers high resistance to spark erosion and the corrosive environment of the combustion chamber. Spark plugs can go 100,000 miles on vehicles these days with these types of spark plugs. Some of the noble metals used on the electrode tips are:

The use of these  noble metals allow the use of a smaller center wire, which has sharper edges but will not melt or corrode away. Some of the advantages of fine wire electrodes are:
⇒Less voltage required to fire
⇒More consistent spark
⇒Better air/fuel mixture around electrode for more efficient burn
⇒Less quench effect
⇒Longer durability with the use of noble metals
⇒Long life

The smaller electrode absorbs less heat from the spark and initial flame energy. The only drawbacks to noble metal plugs are price because of the metals used, but is outweighed by their long life and may not be available for older vehicle applications.


Conventional Spark Plugs:

Conventional spark plugs differ from noble metal plugs in the spark plug electrode tip. The electrode center tip can be:
⇒Nickel alloy

⇒Combination of Nickel-iron, copper, or chromium

The side electrode is:
⇒High nickel steel

 The drawbacks of conventional spark plugs to noble spark plugs are that they are not as efficient as noble metal spark plugs and have a much shorter life. The real plus of conventional spark plugs is price.



Electrode Designs:

Manufacturers are consistently making refinements to the electrode design to provide a more efficient spark to ignite the air/fuel mixture and provide longer life. Some designs incorporate channels in the center electrode and also on the side electrode. Other designs use notches and bridges across and on the electrodes. The use of noble metals has made the electrodes even more efficient. One design is to incorporate multiple side electrodes of 2 to 4 electrodes. Only one electrode will work at a time, the reason for this is to give the spark more than one route to travel. Electricity is lazy and will travel to the path with least resistance, and with this style of plug it will travel to the electrode with least resistance. As electrodes wear down the spark will go to the next of lower resistance.

Spark Plug Tip Design
Spark Plug Tip Designs



Reading Spark Plug:

A spark plug that has been running on an engine can tell what combustion characteristics are happening by the color of the electrode and insulator tip. This is called reading a spark plug and can determined many variables that can be happening during combustion. When spark plugs are removed, keep track of the corresponding cylinder they came out of. This way if a potential problem with a particular cylinder is occurring, it can be identified much easier. The following pictures show some of the characteristics in the combustion chamber.

Spark Plug Condition Chart
Spark Plug Condition Chart





Spark Plug High Performance Tuning:

There are high performance engine tuning tricks with spark plugs. Some of these are:

⇒Switching over to noble metal material spark plugs that have a thin electrode design or the multiple electrode design. They require less voltage to fire and have better air/fuel flow around electrode.
⇒Finding the correct heat range spark plug. Engines that have been modified tend to have different combustion characteristics than stock, so finding the correct heat range is vital for proper performance. Reading the spark plugs is a way to determine correct heat range. Caution should exercised when trying different heat ranges of spark plugs. Running an engine with a very high heat range spark plug can cause damage to the engine.
⇒Indexing spark plugs is another tuning trick. This requires indexing washers that come in different thicknesses. The idea behind this is to have the open end of the electrodes facing the incoming air/fuel mixture for better combustion. The different size washers allow for this adjustment.





There are some special tools for spark plug servicing and repair:

Spark plug gap tool – This tool sets the air gap between the center electrode and the side electrode (follow manufacturer’s specifications and procedures).

Spark Plug Gap Tools
Spark Plug Gap Tools


 Spark Plug Gap Pliers – These pliers sets the air gap between the center electrode and the side electrode (follow manufacturer’s specifications and procedures).

Spark Plug Gap Pliers
Spark Plug Gap Pliers


Spark Plug Boot Pliers – These pliers help remove the ignition wire boot without damaging the boot.

Spark Plug Boot Remover Pliers
Spark Plug Boot Remover Pliers


Spark Plug Socket – Spark plug sockets comes in magnetic, clip style or rubber boot to prevent the spark plug from falling out.

Spark Plug Socket
Spark Plug Socket


Spark Plug Hole Repair Tools – Damaged spark plug holes whether stripped or cross threaded can be repaired by cleaning the threads, or inserting a thread adapter. Many kit variations are available for these types of repairs.

Spark Plug Thread Repair Kits
Spark Plug Thread Repair Kits


Broken Spark Plug Remover – Removes broken spark plugs without dropping porcelain into cylinder.

Broken Spark Plug Remover
Broken Spark Plug Remover


Spark Plug Anti-Fouler Insert – These inserts pull the spark plug away from the combustion chamber. They are used in engines that are passing too much oil into the combustion chamber and oil fouling the plugs.

Spark Plug Anti-Fouler
Spark Plug Anti-Fouler


Spark Thread Start Tool – Helps in starting to thread spark plugs into cylinder head without the risk of cross threading. A piece of hose can also be used to install.

Spark Plug Installer
Spark Plug Installer



Installation Tips:

Spark plug replacement is an important service maintenance procedure to maintain good vehicle performance, fuel economy and good emissions. Some simple procedures will make this a trouble free procedure. Keep in mind that some vehicles will require special extension and adapters to reach them. Here are some tips on spark plug replacement, but follow manufacturers specifications and procedures.
⇒When removing spark plug wires, use spark plug boot pliers not to damage wires.
⇒Use compressed air to blow out spark hole before removing plug. This will prevent any dirt or debris from falling into cylinder when removing spark plug.
⇒It is a good idea to have two spark plug sockets. One socket to remove and one to install. The removal socket can get dirty and oily, so the install socket will always be clean and not contaminate the new spark plugs. Contaminating the new spark plugs can cause carbon tracking and misfires.
⇒If recommended by manufacturer check spark gap. Some plugs can’t be gaped or the manufacturer ships them pre-gaped. Again refer to manufacturer’s specifications on installation.
⇒Uses a piece of hose inserted into porcelain insulator or use an install tool to help start the threads of the spark plug into the cylinder head. This will help prevent cross threading into cylinder head.
⇒Torque spark plugs to manufacturers specified torque. Improperly torqued spark plugs can cause misfires, make spark plugs run hotter, and can damage spark plug coils.
⇒Use dielectric grease on spark plug boot or coils if recommended by manufacturer.
****UPDATE: In the original article it was stated to use anti-seize on the spark plugs threads prior to installation. Which was the statement below:
⇒Use a small amount of anti-seize on the spark plug thread to prevent galling of the threads of the cylinder head.
Anti-seize should not be used, unless the manufacturer specifies the use of it. Most spark plugs manufacturers and vehicle manufacturers do not recommend the use of anti-seize. Reasons for not using anti-seize:
⇒Anti-seize will reduce conductivity between spark plug and cylinder head, which can result in cylinder misfire and spark plug failure.
⇒If anti-seize gets on the tip of the spark plug the metals in the anti-seize will stick and bond to the electrode tip causing a misfire.

⇒Use of anti-seize will reduce the tightening torque friction, this can lead to over tightening of spark plugs. This can strip out cylinder heads or damage the spark plug.
⇒Most good quality spark plugs are coated or plated with metals that act like anti-seize. This is why manufacturers don’t recommend the use of it.
⇒The bottom line is to follow the vehicle manufacturer’s recommended installation procedures for spark plug replacement.


*Special thanks to Professor John Frala, Alternate Fuels Technology at Rio Hondo College in Whittier CA for your training and input.


















Allison Transmission Filter Change B400R & B500R

Type: Allison Transmission
⇒Models: B400R & B500R
⇒Versions: WTEC III, Gen 4 and Gen 5
⇒Service Intervals: Service intervals vary depending on vehicle application. Refer to preventative maintenance (PM) intervals or service manual.
⇒Filter cover bolts/nuts torque: 38-45 lb. ft. (51-61 N-m)
Drain plug torque 18-24 lb. ft. (25-32 N-m)
Oil service fill approximately 16 quarts (15 Liters)
Oil service fill approximately 30 quarts (28 Liters)


Oil filters can be replaced without having to drain transmission sump oil.





⇒Vehicle should be on level ground and wheels chocked.
⇒Wear personal protective equipment.
⇒Use caution when working with hot transmission fluid.





Allison High Capacity Filter Kit, Twin Filter Kit, Part #29548987
This kit contains all the O-rings, gaskets and filters for a complete service. There are extra O-rings included for different style applications.

Allison Filter Kit
Allison Filter Kit
Allison Filter Kit
Allison Filter Kit Contents



Allison B400R & B500R with versions of  WTEC III, Gen 4, and Gen 5 transmissions with 2″ sumps, use the same service filters. These filters are for “Main” and “Lube” and can be removed and replaced without removing the valve body.

Note: Allison filters installed at the factory are marked “Factory Installed” for warranty purposes.

Allison Filters
Factory Filters and Replacement Filters




Procedure: Note – B400R shown, same procedure for B500R. 


1. Remove oil drain plug (Oil filters can be replaced without having to drain transmission sump oil).

Allison, Valve Body, Fliters
Allison Valve Body

 2. Remove hex nuts to remove “Main” and “Lube” filter covers.

Allison, Filters, Covers
Allison Filters Covers

 3. Remove filter covers.

Allison Filter Covers
Removing Filters

 4. Remove old gaskets.

Allison Filter Change
Remove Old Gaskets

 4. Clean any old gasket materials from valve body and filter covers. Remove old O-rings from filter covers.

Allison filter Gaskets and O-rings
Clean Gasket and Remove O-rings.




1. Install new O-rings on filter covers. (Use the correct O-rings by matching them to the old O-rings.)

Allison Filter Seals
Filter Cover Seals

Note: Using the incorrect O-rings may cause damage to filter covers.

2. Install filters to filter covers.

Allison Filters
Filter O-rings and Filter Installed

3. Install gaskets to valve body.

Allison Filter Gaskets
Filter Gaskets Installed

4. Install filter cover assemblies into valve body.

Allison Filter Install
Filter Install

5. Install the twelve nuts securing filter covers to valve body.

Allison Filter Cover
Install Filter Cover Nuts

Note: Do not use nuts to draw in filter covers, damage can result to filter covers.


6. Torque nuts evenly to:
Filter covers 38-45 lb. ft. (51-61 N-m)
Drain plug to 18-24 lb. ft. (25-32 N-m)


7. Add fluid to transmission, start vehicle and check for leaks. Once vehicle is at operating temperature use shifter or Allison DOC to check for proper level. For information on transmission shifter functions. Go to;
Allison Transmission Shifter Functions and Prognostics











⇒Storage Pressure 3600psi

⇒Engine Management Pressure 45-100psi.

⇒Cylinder Life Span – 15/20/25 years

⇒Ignition Temperature 1200°F

⇒5.660 pounds = 1 Gasoline Gallon Equivalent (GGE) (≈125 scf)

⇒6.360 pounds = 1 Diesel Gallon Equivalent (DGE) (≈ 140 scf)

⇒Octane Rating 117




Natural gas is odorless and colorless, the local gas company adds chemicals (odorants) to give it a pungent smell, similar to rotten eggs. Natural gas is lighter than air, leaking natural gas will generally rise and disperse safely; in an enclosed shop area it can pose a danger. The nominal pressure in a CNG system is 3600psi at 70°F, pressure can rise to 4400psi on warmer days.

To be safe always:

⇒Follow manufacturer recommended safety practices

⇒Keep any ignition source away from CNG system

⇒Eliminate any source of static electricity

⇒Conduct work on ventilated area

⇒Never tightened or loosen any component while under pressure

⇒Be familiar with the system you are working on!




Transit bus engines powered by natural gas work like gasoline powered spark-ignited engines. The CNG fuel system on the bus transfers high-pressure natural gas from the storage cylinders to the engine. The high pressure gas is reduced to a lower pressure compatible with the engine fuel management system. Once the fuel is in the engine the air-fuel mixture is ignited by the spark plugs just like a gasoline engine.

About 12000 transit buses in the united states are fueled by CNG; thousands of refuse trucks and delivery fleets are also powered by CNG.

Natural gas as a vehicle fuel is safer than gasoline or diesel, CNG is lighter than air, if a CNG leak should occur, the gas will disperse rapidly upwards into the atmosphere and dissipate. Diesel, gasoline are heavier than air, and will pool on the ground when a leak occurs, creating a fire hazard.

The risk of fire with CNG is very low when handled properly, the ignition temperature is about 1,200°F, compared with about 600° F for gasoline. It also has very narrow flammability limits, it will only burn in concentrations of 5-15%, below 5% is too lean and above 15% is too rich to burn. The high ignition temperature and limited flammability range make accidental combustion of CNG unlikely.


System Components:

cng, regulators, gauges, defuel switch, fuel lines,
Generic System Components

 Generic Layout:

cng layout
Generic CNG Fuel System Layout

System Operation:

When fueling – The CNG enters the bus through the fill valve/fill manifold; flows through the one-way check valve and into the storage cylinders.

During engine operation – the gas leaves the cylinders through the fuel lines and flows through the Manual Shut-Off Valve and high pressure fuel filter. The gas then travels through the high pressure regulator where the storage pressure (3600psi) drops to a lower operational pressure of about 125psi. The fuel now flows to an electronic fuel shut off valve and into the low pressure filter and then into the low pressure regulator. At the low pressure regulator the fuel drops to engine management pressure which is between 45-100psi, depending on type of engine. After the low pressure regulator fuel flows into the engine through a modulated solenoid (single point injection) to premix with the air as in enters the intake. On other engines the fuel is injected into the intake ports through set of injectors (multi-point injection).


Fill Manifold:

At the fill manifold several components make come together depending on the vehicle. in most cases, the fill manifold will house the fill valve, the pressure gauges, the defuel valve, and the manual shut-off valve.

defuel valve, proxinity swtich, pressure gauge
Generic Fill Manifold


Check Valve:

The check valve is designed as a safety measure to prevent fuel flow in reverse. It allows free fuel flow to the CNG cylinder and blocks flow in reverse.


CNG Fuel Cylinders:

The natural gas is stored in high pressure storage cylinders made of steel, aluminum or composites. The nominal storage pressure is 3600psi.

fuel cylinders, fuel tanks, cng tanks,
CNG Cylinders


The cylinders are categorized into 4 categories:

1.  Type 1 is an all steel cylinder made of metal formed in the shape of a cylinder. these types of tanks are the heaviest tanks on the market, but also the most affordable or cost-effective. Type I tanks are typically painted with a protective coating on the outside of the tank.

2. Type 2 cylinders are made with a thinner, steel liner.  The metal liner is wrapped with a composite wrapping around the body of the cylinder; this is called a “hooped wrapped” cylinder. The wrapping is made of glass fiber or carbon fibers or a combination of both.  The metal cylinder takes about 50% of the pressure and the wrapping the rest. Type 2 cylinders are lighter than Type 1 cylinders due to the reduction in metal and the use of lighter-weight composite materials. 

3. Type 3 is made with an all aluminum cylinder liner wrapped entirely in a composite or fiberglass wrap. This is what is called a “fully wrapped” cylinder. The composite material wrapped around the metal liner is typically made of glass fiber or carbon fibers.

4. Type 4 is an all composite cylinder, it does not use any metal it is their structural design.  These cylinders are made with a “gas-tightthick rubber membrane.  The plastic liner is then reinforced with a composite material using the “full wrapped” method, where the entire cylinder is wrapped.   The composite material wrapped around the metal liner is typically made of glass fiber or carbon fibers.

All cylinders are manufactured to meet an industry standard. The main difference between cylinders is cost and weight, type 1 cylinders are the least expensive and also the heaviest.


Cylinder Shut-Off Valves:

All cylinders are equipped with an electronic or manual shutoff valve. Some cylinders have an electronic solenoid inside the tank; others a manual valve on the manifold attached to the tank, yet others have both a manual valve and solenoid valve on the tank manifold.

electric shut-off, manual shut-off, cng shut-off, cylinder valve
Shut-Off Valves


Pressure Relief Device:

Each cylinder must have at least one pressure relief device. The PRD is designed to release cylinder pressure in case of a fire. The PRD is made of a fusible material seal that melts when the temperature on the device has reached 219°F.

The release of the pressure will keep the cylinder from bursting from the increase in pressure due to heat.

Keep in mind that the fuel line between the PRD and the fuel cylinder is always under pressure even if the cylinder valve is closed. To relieve pressure from the PRD or PRD line the cylinder must be defueled.

The PRD is a single use component and must be replaced after activation or when leaking. There are no serviceable parts on a PRD.


Fuel Lines:

Fuel lines are made of stainless steel for longevity and rust prevention. The lines are manufactured to be pressurized to 1.5 times service pressure and not to burst below 2.5 times service pressure.

sample figures:

3600psi – service pressure/5400psi – line max pressure/9000psi – burst pressure


Manual Shut-Off Valve:

The manual shut-off valve is designed to shut off fuel flow to the components past the one way check valve. There are some vehicles that will hold pressure on the high pressure side of the system even when the valve is in the OFF position. Ensure you know the layout of the system you are working on and always follow safety procedures!

shutoff valve, 1/4 turn valve
Manual Shut-Off Valve 1/4 turn


High Pressure Filter:

Positioned on the high-pressure side of the vehicle system between the storage cylinder and the high pressure regulator. The high pressure filter purifies the fuel by removing moisture and contaminants. The filter is a coalescing type and has an efficiency rating of 1 micron.

cng filter, coaslescing filter
High Pressure Filter


High Pressure Regulator:

The high pressure (HP) regulator drops the fuel pressure from cylinder pressure of 3600psi to approximately 99 -150psi.

The regulator is warmed through engine coolant recirculation to prevent it from freezing up during the pressure reduction process. The regulator has a pressure relief valve set to open at 270psi + 60psi, any pressure relieved due to a regulator malfunction will be vented via an externally mounted vent line.

cng regulator, hp regulator
High Pressure Regulator


Low Pressure Regulator:

The low pressure regulator drops the fuel pressure to below 45-100psi. The regulator may be externally mounted or integrated into the engine intake manifold. Depending on the fuel system this regulator may also be warmed with engine coolant to prevent it from freezing up during the pressure reduction process.

Low Pressure regualtor, cng regulator, impco regulator
Low Pressure Regulator – External Mounted


Low Pressure Filter:

The low pressure coalescing filter(s) is usually located near the engine. The low pressure filter removes residual contaminants and oil from the fuel before it enters the engine. Consult the engine manufacturer for maintenance and replacement guidelines.


Shut-Off Solenoid:

The shut-off solenoid shuts fuel flow to the engine under key-off condition.


Fuel Metering Single-Point:

Fuel flow to the engine is metered through a control valve with a PWM signal. The valve is normally closed NC.

cummins FVC, fuel control
Fuel Control Valve


Fuel Metering Multi-Point:

Another method of delivering CNG fuel to the engine is through a multi-point injection system; the fuel is delivered closer to the intake port of each cylinder.

Direct Injection:

Some later model engines are experimenting with direct injection into the combustion chamber similar to a diesel engine.


System Maintenance:

Filter Change:

See article on “CNG Fuel Filter R&R”.

CNG Fuel Filter R&R

Cylinder Maintenance:

END-OF-LIFE – CNG fuel cylinders have a useful life of 15, 20, or 25 years, as designated by the manufacturer. All cylinders must be replaced when they reached their expiration date, cylinders will have a label that states “DO NOT USE AFTER (EXPIRATION DATE).” On some buses the cylinder expiration date may also be found in the fueling connector area.

CYLINDER INSPECTION – CNG cylinders are exposed to environmental, chemical, and road hazards that can damage and threaten the integrity of the cylinder. Cylinders should be inspected by a qualified inspector every three years (36 months) or every 36,000 miles, whichever comes first in accordance with government regulations. Cylinders must also be inspected after any fire, accident or other incident that could cause damage to the cylinder.

CYLINDER REPLACEMENT – CNG cylinders should be replaced by a qualified personnel and a repair facility with the right equipment to safely vent the CNG from the tank and purge the tank with nitrogen to eliminate any pressure or fire potential associated with residual CNG in the cylinder. Once the cylinder is safely purged of any natural gas, the expired CNG cylinder must be rendered unusable usually by drilling two ½ or larger holes in the body of the cylinder; then discard following local regulations.


Allison Transmission Shifter Functions and Prognostics



Type: Allison Transmission
⇒Models: B400R & B500R
⇒Versions: WTEC III, Gen 4 and Gen 5




⇒Vehicle should be on level ground and wheels chocked.
⇒Wear personal protective equipment.




Allison Gen 4/5 transmissions have an optional feature called Prognostics. Prognostics is a diagnostic feature that alerts the operator when the transmission service is needed thru the shifter. Prognostics information can be also accessed with Allison DOC software.
It alerts service in these fields:
1. Transmission Oil life
2. Transmission Filter Life
3. Transmission Clutch life
When service is needed, the shifter will illuminate a wrench icon in the display. Along with these features, the shifter will display the standard features of oil level and DTC (Diagnostic Trouble Codes).

Allison Prognostics Service Arrow
Allison Prognostics Service Arrow   


Shifter functions, can be accessed by pushing the up and down arrows simultaneously, each time the up and down arrows are pressed the next function is accessed:

Allison Shifter Accessing Functions
Allison Shifter Accessing Functions                        

WTEC III – Has two functions of:
1) Oil level
2) Codes
Gen 4/5 with no Prognostics -Has two functions of:
1) Oil level
2) DTC (codes)
Gen 4/5 with Prognostics – Has 5 functions of:
1) Oil Level
2) Oil Life Monitor
3) Filter Life Monitor
4) Transmission Health Monitor (clutch life)
5) DTC (codes)
WTEC III, Gen 4 and Gen 5 can clear DTC’s (codes) by holding the “Mode” button down for 10 seconds while in DTC function.
To exit out of any function, press the neutral button.




1.    Oil Level Check – Preparing for oil level check:

⇒Make sure vehicle is on level ground
⇒Before starting vehicle, use dipstick to verify transmission oil condition (not burnt) and that there is sufficient oil in transmission ( that dipstick shows oil )
⇒Engine is at idle (no fast idle)
⇒Transmission is in neutral
⇒Transmission fluid temperature must be between 140º F and 220º F to get an accurate reading.
⇒Vehicle has been stationary and idling for approximately for two minutes for oil to settle to sump.


The transmission oil level sensor has a max reading of 3 quarts high and 4 quarts low. Any higher than these limits the limits will not be shown.

Oil level sensor will compensate for fluid expansion when oil is at operating temperature, this may cause a deviation between dipstick and oil level sensor readings. The oil level sensor readings should be used to achieve the correct oil level and not the dipstick.


A.  Oil Level Check – 
 I. Push up and down arrows once

Allison Shifter Accessing Functions
Allison Shifter Accessing Functions
 If vehicle has just been was just driven, and oil level is selected, the shifter will go into a countdown mode to allow the oil to settle to the sump (approximately 2 minutes).    

Allison Oil Countdown


Shifter will display transmission oil level status.

Allison Oil Level Display
Shifter Displaying Oil Level Status
Depending on the version of Allison, the shifters will display oil level in different ways –
Oil level reading low:

Allison Shifter Oil Low

Oil level reading high:
Allison Shifter Oil Display High
Oil level reading Ok:
Allison shifter Oil Level OK


If oil level check does not meet ECU / TCM requirements, the shifter will display an alert designator or alert of condition not allowing an oil level check. The following are the alert designators:

Allison WTEC Gen 4 Shifter Codes



Allison Gen 5 Display Designators

Allison Gen 5 Shifter Messages


Oil level can be also viewed with Allison DOC software.



2.  Oil Life Monitor – This feature gives a count down 99% to 0% of the oil life. At 2% it will activate the service wrench icon on the shifter display. It is based on the vehicle’s duty cycle of operation. Which are:
⇒Operating hours
⇒Output revolutions
⇒Shift density (shifts per mile)
⇒Retarder accumulator applies

If the oil life monitor is not reset at 0%, the shifter will read a negative percentage of up to -100%. The negative percentage indicates how far past due the oil change is. And DTC P0897 will be logged with a check transmission light.


A. Oil Life Monitor Checking and Resetting – 
I. Push up and down arrows twice.

Allison Shifter Oil Life
Shifter Displaying Oil Life Remaining
II. To reset oil life monitor after performing service, hold mode button down for 10 seconds. This will reset oil life back to 99%.
Allison Oil Life Mode Reset
Using Mode Button to Reset Oil Life

III. Shifter will display 99%

Allison Oil Life Reset


Oil life can be also viewed and reset with Allison DOC software.



3.   Filter Life Monitor – Filter life is sensed by the PS2 (Pressure Switch 2) which is a Normally Closed (N.C.) switch located in the valve body. When the PS2 switch receives pressure, it signals the TCM that the filters need servicing. The switch takes a pressure difference between unfiltered main pressure and filtered lock up (TCC). When the pressure difference is great enough the Normally Closed switch goes open and signals TCM that filter service is needed.

A. Filter Life Monitor Checking and Resetting –
I.  Push up and down arrows three times

Allison Filter Life Shifter
Display Showing Filter Life Status

II. To reset filter life monitor after servicing main and lube filters, hold mode button down for 10 seconds.

Allison Filter Life Reset Shifter
Filter Life Reset with Mode Button


Filter life monitor on B400R and B500R will reset automatically if TCM receives no signal from PS2 filter switch.

Filter life monitor can be also viewed and reset with Allison DOC software under the Prognostics tab.



4.  Transmission Health MonitorTransmission health monitors the running clearance of clutches C1, C2, C3, C4, and C5 to determine clutch plate wear. Clutch running clearance is based upon clutch fill volume, which indicates remaining clutch life. Lock-up clutch (TCC) is not monitored. When 10% of clutch life is reached, the wrench icon on the shifter will illuminate.


If the wrench icon is ignored for over a 100 hours indicating transmission health problem, DTC P2789 Transmission Clutch Life Expired will be logged active.

Transmission Health Monitor cannot be reset with the “Mode” button. It can only be reset with Allison DOC software under Prognostics tab.


A. Transmission Health Monitor Check –
I. Push up and down arrows four times.

Allison Transmission Health shifter
Transmission Health Displayed

II. If transmission health shows low, Allison DOC software must be used to troubleshoot and reset Transmission Health Monitor.

Allison Transmission Health Low
Transmission Health Low Status



5. DTC (Diagnostic Trouble Codes) Checking and Clearing – The shifter will show up to five DTC (Diagnostic Trouble Codes) active and inactive. The shifter will also clear active and inactive DTC’s. 

A. DTC Checking and Clearing –
I. With Prognostics feature: Push up and down arrows five times.
Without Prognostics Feature: Push up and down arrows two times.

Allison Shifter Accessing Functions
Allison Shifter Accessing Function

II. Display for WTEC III and Gen 4 will display DTC’s with flash codes.

Allison DTC Shifter


Allison Shifter Mode Button

Allison Shifter Mode Button

Allison Shifter Diagnostic Levels

III. Clearing DTC’s for WTEC III, Gen 4 and Gen 5, hold the mode button down for 10 seconds while in DTC mode. This will clear all “ACTIVE” and “INACTIVE” DTC’s

Allison ShifterMode Button Clearing


DTC’s can be also be viewed and cleared with Allison DOC software under the DTC page.