Month: September 2016


Antifreeze / Coolant Description:
Antifreeze/coolant serves four purposes in a vehicle cooling system.

1.      To lower the freezing point below 32º F / 0º C. (To help prevent freeze up)
2.      To raise the boiling point above 212º F / 100º C. (To help prevent boil over)
3.      To protect the cooling system against corrosion.
4.      To lubricate water pump seals.



Types of Coolant / Antifreeze:

In today’s day and age, there are many variations of coolant/antifreeze and can be quite confusing. But they are all based on ethylene glycol (EG) or propylene glycol (PG). From this, six groups come out. Colors vary from types, to the different manufacturers.

1. Inorganic Acid Technology (IAT) – This is the old typical green antifreeze/coolant that contained Silicates, Phosphates and/or Borates. The average life of IAT was from: 2 years/30,000 miles to about 3 years/36,000miles (dependent on manufacturer’s interval specs).

2. Organic Acid Technology (OAT) – This variation does not contain silicates or phosphates. A version of this is called Dex-Cool that has been used in GM cars since 1996, and Volkswagen/Audi uses a similar version called G12, G12+ and G12++ of this antifreeze/coolant. They contain 2-EHA and /or Sebacate for corrosion protection. This is a long life coolant with an interval of around 5 years/50,000 miles (dependent on manufacturer’s interval specs).
3. Hybrid Organic Acid Technology (HOAT) – Is a combination of organic acid technology (OAT) with a low dose of silicates or phosphates. Most antifreeze/coolant is of this variation. This is a long life coolant with an interval of around 5 years/50,000 miles (dependent on manufacturer’s interval specs).
4. Nitrited Organic Acid Technology (NOAT) – Is an organic acid technology (OAT) that has no silicates or phosphates. It contains nitrates and/or molybdenum as part of their inhibitor package. This coolant is an extended life coolant and does not require supplements. This extended life coolant has a service interval of around 8 years / 750,000 miles (dependent on manufacturer’s interval specs).
5. Nitrited Molybdate Organic Technology (NMOAT) – This coolant is similar to NOAT that has nitrates, but also has Molybdate for diesel wet sleeve liner protection.
6. Poly Organic Acid Technology (POAT) – This a long life coolant that is propylene glycol based, organic acid technology. Currently Amsoil is the only manufacturer for this coolant.




Antifreeze vs. Coolant

Antifreeze – Is undiluted and must mixed to a 50/50 antifreeze to water ratio. Distilled water should be used to dilute antifreeze. Use the correct ratio recommended by vehicle manufacturer. As a note, running straight antifreeze in an engine, will result in a hotter running engine. Antifreeze does not have the heat transferring abilities as water.


Coolant – Is pre-diluted and ready to use. Coolant does not require water to be added


SCA (Supplemental Coolant Additive) / DCA (Diesel Coolant Additive) – Because additives in conventional (IAT) and hybrid organic acid (HOAT) coolants are consumed during the vehicle’s operation, they must be refortified with supplemental coolant additives (SCA/DCA) in order to maintain effective corrosion protection. On the other hand, OAT and NOAT coolants do not require SCAs. Also note that the inhibitor package does not determine the level of freeze or boil over protection; the inhibitor is responsible for corrosion protection only.





⇒Most antifreeze / coolants are dangerous to humans and animals if ingested. They can cause severe injury or death. Use caution with children and animals since most coolants are sweet tasting.
⇒Dispose of used antifreeze / coolant to proper recycling center.
⇒Wear personal protective equipment.
⇒Use caution when working with hot coolant.





Acidic (pH) – Liquid having a pH rating of 1 – 6 on the pH scale (sour). Acids fall in this scale.
Alkaline (pH) – Liquid having a pH rating of 8 – 14 on the pH scale (sweet). Caustics fall in this scale.
Antifreeze – This type is 100% antifreeze and has to be diluted with water to achieve desired freeze point. Running 100% antifreeze is not advisable.
Basic (pH) – see Alkaline.
Benzoate – A corrosion inhibitor that is the main inhibitor in HOAT antifreeze/coolant and G-05 antifreeze/coolant.
Benzotriazole (BZT) – An inhibitor that protects brass and copper against corrosion.
Boiling Point – The temperature that a liquid changes from liquid to gas.
Borate – A chemical used in antifreeze/coolant SCAs to maintain the pH as it ages. It is a pH buffer.
Carboxylates – Organic acid anti rust inhibitors.
Caustic – A solution that has a pH of 12 to 14 pH.
Cavitation – The formation of bubbles and the immediate implosion of them. This produces a shock wave that causes pitting and erosion where it is occurring inside the cooling system.
Coolant – This type comes pre-mixed with water in a 50/50 mix. It is ready to be used and should not be diluted with water.
Coolant System Flush – A chemical or water cleaning of a cooling system to remove old antifreeze/coolant and particles from a cooling system.
Colligative Agent – Antifreeze/coolant is a colligative agent that raises and lowers the boiling point and freezing point in a cooling system.
Convective Heat Transfer – Heat transfer thru fluid movement in liquid or gas. Cooling systems work in this manner.
Corrosion – A destructive reaction of a metal with another material, e.g. oxygen, water, chemical (either acidic or alkaline).
Corrosive – A chemical (liquid, solid, or gas) that can cause destructive damage to body tissues, metals and other materials.
Deionized water (DI) – see Distilled water.
Denatonium – A bitter tasting agent added to antifreeze/coolant to make it bitter.
Distilled water – Water that has had minerals ions removed. In a cooling system, hard water contains calcium and magnesium that will form deposits in a cooling system.
Electrolysis – A chemical process that induces the transfer of metal by way of electrical current flow.
Ethylene Glycol – A colorless, odorless, syrupy, sweet alcohol that is toxic
Hybrid Inorganic Acid Technology – Antifreeze/coolant that is either ethylene glycol or propylene glycol that has a low dose of silicates or phosphates added.
Freezing Point – The temperature that a liquid changes from liquid to solid.
Galvanic Corrosion – This happens when dissimilar metals are in electrical contact. The Anode (+) metal will corrode faster than the Cathode (-) metal.
Glycerol – It was used as antifreeze/coolant before it was replaced by ethylene glycol.
Hybrid Organic Acid Technology (HOAT) – Antifreeze/coolant that is either ethylene glycol or propylene glycol that has a low dose of either silicates or phosphates.
Hydrometer – Measures the density of the coolant to the density of water. Reads freezing point and boiling point of antifreeze/coolant.
Inorganic Acid Technology (IAT) – Antifreeze/coolant that is either ethylene glycol or propylene glycol that has silicates and phosphates added (traditional green).
Ion – Is an atom or a molecule in which the total number of electrons is not equal to the number of protons, giving it a net positive or negative charge.
Molybdate – An additive to prevent cylinder liner cavitation.
Neutral – Having a pH of 7. The middle of the pH scale.
Nitrate – An anti-corrosion additive in preventing rust.
Nitrite – An additive to prevent cylinder liner cavitation.
pH – Potential for hydrogen.
pH Scale – A scale of 1-14, 1-6 being acidic, 8-14 being alkaline, and 7 being neutral.
Phosphates – An inorganic chemical that insulates the metal in a cooling system against corrosion.
Plasticizer – A substance that softens certain plastics, silicones, and nylons.
Propylene Glycol – Similar to Ethylene glycol but not as sweet tasting, but is not as toxic to environment.
Organic Acid Technology – Antifreeze/coolant that is either ethylene glycol or propylene glycol that has no silicates and phosphates added (Dex-cool and VW/Audi coolants).
Refractometer – An instrument that measures the refractive index of a liquid.
Reserve Alkalinity – A coolants ability to neutralize acid.
Sebacate – An organic acid added to coolant for corrosion protection.
Silicates/Silicone – An additive to antifreeze/coolant that reacts with the metals in a cooling system to protect against corrosion, particularly with aluminum. It is based from sand and has deemed abrasive to coolant systems and certain water pump seals.
Sodium Hydroxide – An additive to help maintain pH balance.
Sodium Silicate – Used in block sealers, head gasket sealers, and radiator sealers. Called liquid glass that forms a powerful sealant.
Surface Tension – Is the property of the surface of a liquid that allows it to resist an external force.
Surfactant – Is a compound that lowers the surface tension of antifreeze/coolant and allows better heat transfer. Used in additives designed to help lower cooling system temps.
Triazole / Thiazoles – An additive for corrosion protection, particularly with brass and copper.
Tolyltriazole (TTZ) – An inhibitor that protects brass and copper against corrosion.
2-EHA (2-ethylhexanoic acid) – An organic acid added to coolant for corrosion protection. It is a plasticizer that can damage silicone rubber gaskets and certain nylon gaskets.





ASTM – American Society of Testing and Materials
ATC – Automatic Temperature Compensated
DCA – Diesel Coolant Additive
DI – deionization
DMM – Digital Multi Meter
DVOM – Digital Volt Ohm Meter
EG – Ethylene Glycol
ELC – Extended Life Coolant
ESI – Extended Service Interval
HOAT – Hybrid Inorganic Acid Technology
IAT – Inorganic Acid technology
LLC – Long Life Coolant
NORA – National Oil Recyclers Association 

NPG – Non-Aqueous Propylene Glycol
OAT – Organic Acid Technology
PG – Propylene Glycol
pH – potential for hydrogen
SCA – supplemental coolant additive





Antifreeze/Coolant Pros and Cons:


⇒Lowers freeze point
⇒Raises boiling point
⇒Lubricates water pump
⇒Prevents corrosion


⇒Does not have a good heat transfer compared to water
⇒Harmful to environment if improperly disposed
⇒Mixing of two different antifreeze/coolants can damage a cooling system
⇒Antifreeze/coolant not replaced at service intervals can damage a cooling system
⇒Improper antifreeze to water ratio mix can affect desired protection and will affect corrosion protection, freeze point and boiling point.





Antifreeze/Coolant Checks:

If you follow four simple rules, you can prevent antifreeze/coolant problems.

⇒Use the manufacturers recommended type for the vehicle.
⇒Replace at manufactures recommended intervals or add SCA / DCA. *
⇒Never mix different antifreeze/coolant types.
⇒Perform antifreeze/coolant checks at service intervals (at oil changes).


* Note: When using antifreeze, it has to be mixed with water to achieve the desired freeze point protection. Never use tap water; it contains minerals, silica, chlorine and fluoride, all of which can leave deposits in the cooling system.  This is why only distilled water should be used. Coolant should not be diluted with water, it comes pre-mixed 50/50 and is ready to be used.



Antifreeze/Coolant Tests: The following tests will check to see if the freeze point (antifreeze to water ratio) is correct, pH and reserve alkalinity are in specs for corrosion protection, and check for unwanted current in the cooling system that could lead to electrolysis damage. The tests are fairly simple, but they require test tools to perform checks. In some cases more than one test must be performed on a cooling system, more so if the system has been having problems.


1. Coolant Test Strips – By far the easiest test to perform. They check for pH level (corrosion protection), reserve alkalinity, sulfates, nitrates, chlorine and freeze point/ boiling point. When purchasing test strips, make sure they are compatible with antifreeze/coolant in cooling system. Also, they are variances in testing from manufacturer to manufacturer of the strips. If using SCA/DCA, test strips will determine how much to add or if antifreeze/ coolant needs replacing. Follow the instructions to achieve accurate test results. The strip is dipped into antifreeze/coolant and colors on strip are compared to the chart on the kit.

Coolant antifreeze test strips
Coolant Strips and Color Examples

⇒Easy to use

⇒Low price
⇒Fast results



⇒Must be used at specific temperatures or readings must be temperature compensated.
⇒Have a shelf life (expiration date)
⇒Won’t work for color blind people
⇒Correct strips must be used for particular cooling system being checked.


2. Hydrometer – Measures the gravity density of antifreeze/coolant. They convert a specific gravity measurement into a freeze point level.  Hydrometers are made of a container (glass or plastic) that are designed to draw in the sample fluid. Inside the sample chamber there is a float that contains a ballast weight made of either pellets, disks, floating balls or an arrow. The chamber has a scale and where the ballast floats to the corresponding scale the reading is taken. Hydrometers are accurate only with ethylene glycol antifreeze/coolant and will not work with propylene glycol antifreeze/coolant.

Hydrometer tester
Hydrometer Tester


⇒Low price
⇒Fast results
⇒Easy to use

⇒Must be used at specific temperatures or readings must be temperature compensated.
⇒Will not work with propylene glycol antifreeze/coolant (not accurate).
⇒Air bubbles in sample fluid will affect reading.
⇒Friction of ballast in test chamber will affect reading.
⇒Not very accurate, can be off plus or minus 8 degrees.
⇒Quality of tester can cause skewed readings.


3. Refractometers – Is a precision optical device capable of very accurate specific gravity readings for antifreeze/coolant, battery electrolyte, and windshield washer fluid. It is an optical tool, in which a sample of the fluid is placed on the test window in the refractometer body.  The sample is viewed thru the eyepiece, and where the reading intersects the scale the reading is measurement. The measurements are based on the “Refractive Index” in the way light passes thru the fluid. There are more modern units which are digital refractometers and are fully electronic. The units are also automatically temperature compensated (ATC) and temperature is factored into the reading. Again, as in any tool, follow the manufacturer’s instructions to achieve an accurate reading.

refractometer antifreeze coolant
Refractometer Showing Viewing Scale


⇒Very accurate
⇒Models can measure antifreeze/coolant, battery electrolyte, and windshield washer.
⇒Automatically temperature compensated (ATC).


⇒More expensive than hydrometer and test strips.
⇒Can be hard to read.


4. DMM (Digital Multi Meter) – Can be used to check current in a coolant system. Too much current in a cooling system can cause electrolysis and damage cooling system components. Current is generated under two conditions:
The antifreeze/coolant has been in the cooling system way past its service interval. As coolant ages, the additives in the antifreeze/coolant that keep it neutralized become depleted, over time the coolant starts to develop an electrical charge from passing over dissimilar metals. It starts acting like a battery and the electric current produced starts removing metal from the engine surfaces. This is called electrolysis and it affects aluminum components in particular.

⇒The other type of current produced in a cooling system is caused by bad electrical grounds. This causes current to flow back to the battery thru the cooling system. This will also damage metal components in a cooling system. Assuring all ground points are clean and secure will eliminate this problem.

The following is the procedure for testing a cooling system with a DMM:

Note: Use personal protective equipment, and use caution with running engines.

A)   Remove radiator or reservoir cap.
B)    Insert red DMM lead into radiator (do not let the lead touch the radiator or reservoir) and black lead to negative post of battery.
C)    If reading of DMM is .3 Volts (300mV) without engine running, then the anti-freeze is holding a charge and should be replaced.

Start the engine and read DMM readings. Interpret readings below:

Caution: Use caution when working on a hot and/or pressurized cooling system. Make sure all pressure is relieved from cooling system before removing coolant reservoir or radiator cap. 


DVOM DMM Coolant Antifreeze Testing
DMM Coolant Antifreeze Testing

Interpreting Results:

⇒ Below .1 Volts (100mV) = Cooling system is OK.

⇒ .1 Volts (100mV) to .2 Volts (200mV) = Cooling system is starting to have too much current flow, Antifreeze/coolant should be replaced or grounds should be checked.

⇒ .2 Volts (200mV) to .3 Volts (300mV) = Too much current flow thru cooling system, potential for electrolysis. Antifreeze/coolant should be replaced or grounds should be checked.


If readings are above .3 Volts (300mV) with the engine running, this could be caused by bad grounds and the cooling system is being used to transfer ground. To check to see if bad grounds are the cause of the high voltage using the DMM. Connect the DMM, as previously shown.

A)   With the DMM connected, crank the engine and observe DMM reading. If readings show big voltage spike, then bad grounds are probably the problem.

B)    With the engine running at high RPM of 1,500-2,000RPM and all loads of off, observe the voltage reading. Then turn on all vehicle loads and observe DMM with engine still at high RPM. If voltage readings increase, then bad grounds are probably the problem.


Note: If voltage readings do not fluctuate, then coolant is problem.


If voltage is detected in cooling system, then you have to determine if current is coming from the antifreeze/coolant itself or from a bad ground. Test as Follows:

a)    With DMM connected in the above figure, remove the +positive cable from the battery and observe DMM voltage reading.

b)    If voltage reading did not change, then voltage is coming from the antifreeze/coolant itself. Replace antifreeze/coolant and re-check test.

c)    If voltage reading disappears after removing +positive battery cable, then the problem is bad grounds. Check and clean all ground connections. Re-check system to ensure current has dissipated to a safe level. In extreme cases, supplemental grounds may have to be installed.



⇒Test strips, hydrometers, and refractometers don’t check for current in systems.


⇒If low end (cheap) DMM is used, readings could be skewed.
⇒If cooling system is pressurized, test cannot be performed.



Preventative Products:

There are many products available to supplement a cooling system from preventative and repair. But beware of the use of these products; research the product before pouring it into your cooling system. Like the saying goes, the remedy maybe worse than the problem. Some of the products available are:

Stop leak – Stops radiator, head gasket, and block leaks.
Electrolysis Eliminator – additives that eliminate and prevent electrolysis.
Anode Inserts – It is a drop in element or is part of the radiator cap. It is a piece of metal that is sacrificial; the electrolysis will attack the anode piece instead of the cooling system.
SCA – Supplemental Coolant Additive, additives that maintain and fortify the antifreeze/coolants anticorrosive and freeze properties.
Coolant Filters – Used to filter and remove particles from cooling system. Some filters release SCA’s into the cooling system to maintain system.
DCA – Diesel Coolant Additive, see SCA.
Extra Grounds – In a vehicle with various supplemental electrical add-ons, extra grounds help get the current back to battery, rather than thru the cooling system.
Temperature Reducing Products – These products (Surfactants) promote better heat transfer by reducing the surface tension between the antifreeze/coolant and the cooling passages of the engine.



Coolant Types and Colors:

Antifreeze/coolant in natural form is colorless; manufacturers add color to help identify it. The problem is that different manufacturers are using similar colors for totally different antifreeze/coolant configurations. Be careful not mix antifreeze/coolant; this can cause serious cooling system problems. If in doubt of what is in the cooling system, completely flush out cooling system (this may require three or more flushes with water). This will help in not mixing the old antifreeze/coolant with the new and establish a good baseline for maintenance.


Why different antifreeze/coolant types? In Europe, they use phosphate free antifreeze/coolant. The hard water in Europe contains calcium and magnesium. When mixed with the phosphate, they form calcium or magnesium phosphate, which leads to scale build up in the cooling system. They use a relative low dose of silicates for corrosion protection. On the other hand Asians don’t use silicates because they deem it abrasive to water pump seals. They use a low dose of phosphates for corrosion protection. GM uses Dex-Cool since 1996 which doesn’t use either phosphate or silicates. It contains Sebacate and 2-EHA for corrosion inhibitors. Dex-Cool is also involved in a big controversy about damaging cooling systems. And the list of different antifreeze/coolant grows as manufacturers expand the types. The goal of all new antifreeze/coolant is for long life and intervals are slowly increasing more and more. Of all the colors of antifreeze/coolant, the color you don’t want to see is brown. Brown indicates contaminated, mixed antifreeze/coolant or rust in the system. A system with this problem requires immediate attention to avoid serious damage. For best results follow manufacturers recommended antifreeze/coolant for the vehicle.


Universal Antifreeze/Coolants: Aftermarket antifreeze/coolant makers are marketing Universal coolant that is compatible with all makes and types of antifreeze/coolant. They are OAT (Organic Acid Technology) that doesn’t have silicates or phosphate, they contain proprietary corrosion inhibitors. Some claim that they are lifetime, but again checking coolant at service intervals to make sure it is in specs. Again, before pouring anything into your cooling system, do some research on the product.  Make sure it is compatible with your cooling system.


Waterless Coolant: This type of coolant uses no water in its composition. Since no water is used, it has a higher boiling point, a lower freezing point and there is no corrosion since there is no water. Also since no water is used, there is no pressure build up with heat. It contains no silicates or phosphates. While this technology is not new, it is worth doing research for the vehicle or application it will be used on. Response on how the product works is mixed, from great to bad.



⇒Better heat transfer since no water to create hot spots or cavitation.
⇒Waterless coolant boils at 375º F (191º C), while coolant mix of 50/50 boils at 263º F (128º C).
⇒Waterless coolant freezes at -40º F (-40º C), while coolant mix of 50/50 at freezes at -34º F
(-37º C).
⇒Conventional antifreeze requires a radiator / reservoir cap of about 15psi. to pressurize system to raise boiling point. Waterless coolant requires a 1 to 2 psi. cap, since there is no water, little pressure is produced.
⇒Since little pressure is generated in the cooling system, less stress is exerted on cooling system hoses, gaskets, O-rings, radiator, heater core, and water pump.
⇒With little pressure on the cooling system, leaks are less susceptible.
⇒It inhibits corrosion, since no water is used.
⇒With better heat transfer there is less load on electric fans or mechanical fans.
⇒Environmentally safe, since waterless coolant is propylene glycol (PG) which non-toxic to humans, animals and plants. While most conventional antifreezes are ethelyne glycol (EG) are toxic and have a sweet taste.




⇒If the vehicle has a coolant leak, it must replenished with waterless coolant. This can be a problem if it is a huge leak and waterless coolant is not available. If water or regular coolant is used, the waterless coolant will lose all of its benefits and have to be flushed.
⇒Before adding waterless coolant to a vehicle cooling system, it must be flushed with a special flush agent to remove any water and old coolant.
⇒Waterless coolant is more expensive that conventional coolant. It is two to three times more expensive.
⇒Waterless coolant is not readily available at most parts store. Purchasing coolant will most likely have be done from an online vendor.
⇒Currently one manufacturer produces waterless coolant.
⇒It would be recommended to always carry extra waterless coolant in case of leaks.
⇒In some applications, hotter engine operating temperatures have been reported.
⇒Has a higher viscosity than conventional antifreeze/coolants, putting more load on water pump and engine.



Resource Links:

Helpful resource links –




Open circuit measurements are never a good idea! This type of measurement is done with the circuit under no load and unwanted resistance of several million ohms may not show leading to a possible wrong diagnosis. In most cases opening the circuit will not affect your measurement. But, there will be the times that it will and you may end up chasing your tail for a whole shift without solving the problem.




A = Amperage or current

E = Voltage

I = Current or amperage

V = Voltage

R = Resistance

Rt = Resistance Total

It = Current Total

Vd = Voltage Drop



Ohm’s Law:

Before being able to understand why open circuit measurements are not reliable; let’s review the concept of ohm’s law. It explains the relationship between voltage, current, and resistance. Simply stated “it takes 1 volt to push 1 amp through 1 ohm of resistance”.

In a electrical circuit, the amount of current flow is what performs work. The higher the current flow is the more work that is performed, this may not necessarily be a problem unless the circuit fuse is blowing. On the other hand lower current flow will perform less work, this is a problem because the load/s in the circuit will not perform the require work. Any deviation in the voltage or resistance requirements will affect the current flow; thus, affecting the amount of work performed.


To perform circuit calculations the following formulas are used:

(Volts) V = R x A

(Resistance) R = V/A

(Amps) A or I = V/R

Note: Current has different names and symbols. It can be referred to as amperes or amps. The symbols in Ohm’s law can be I or A.


The Ohm’s Law pie or triangle chart is often used to simply the understanding of the formulas.

ohms law, V=Axr



Series Circuits:

A series circuit has only one path for current to flow, the following rules apply to series circuits:


1. Total resistance is the sum of each individual resistance.
2. Current flow is the same in the entire circuit.
3. The sum of voltage drops equal source voltage.
4. Voltage drop is proportional to the resistance of each component.


Let’s  use the circuit below to perform series circuit calculations. Refer to abbreviations section for formula nomenclature. 

Series Circuit Calculations

The formula for series circuits is Rt = R1 + R2 + R3 + Rn……

Rt = R1 + R2 = 5 + 5 = 10Ω

It = 12/10 = 1.2 A

Voltage Drop of the 1st light bulb is:

Vd1 = R x A = 5 x 1.2 = 6 volts ( this light bulb uses 6 of the 12 volts supplied)

Voltage Drop of the 2nd light bulb is:

Vd2 = R x A = 5 x 1.2 = 6 volts ( this light bulb uses the remaining voltage)

Total Circuit Voltage Drop is:
6 + 6 = 12v (same as source voltage)

Now let’s insert some unwanted resistance into the circuit.

series circuit, unwanted resistance, ohm's law

Rt = R1 + R2 + R3 + R4 = 1 + 5 + 5 + 2 = 13Ω

It = V/A = 12/13 = .923A (923 mA)

Voltage Drop through the bad connection:

Vd1 = R x A = 1 x .923 = .923V ( the bad connection uses almost 1 volt of the supplied voltage; this will affect the performance of the light bulbs (loads) in the circuit).

Voltage Drop of the 1st light bulb is:

Vd2 = R x A = 5 x .923 = 4.62 volts 

Voltage Drop of the 2nd light bulb is:

Vd3 = R x A = 5 x .923 = 4.62 volts

Voltage Drop of the corroded ground:

Vd4 = R x A = 2 x .923 = 1.85 volts ( this connection is using almost 2 volts; this will affect the performance of the light bulbs (loads) in the circuit).

Total Voltage Drop of this circuit is:

Vd1 + Vd2 + Vd3 + Vd4 = .923 + 4.62 + 4.62 + 1.85 = 12 volts



Parallel Circuits:

A parallel circuit has more than one path for current flow, the following rules apply to parallel circuits:


1. Amperage flow through each path depends on its resistance.
2. Total current is the sum of the current flow in each path.
3. If one path of circuit is broken, current will continue to flow in the other path.
4. Total circuit resistance always less than resistance of smallest resistor.


The formulas for parallel circuits are:

It = I1 + I2 + I3 + 1n……….


So, let’s use the circuit below to practice these formulas. Refer to abbreviations section for formula nomenclature.

parallel circuit, ohm's law

The total resistance of this circuit using both formulas is 2.5Ω:


The total current flow for this circuit is:

It = V/R = 12/2.5 = 4.8 A


Sample Circuit Calculations:

So, why are open circuit measurements a bad idea? When the circuit is open to measure voltage; the meter becomes a load in the circuit effectively creating a series circuit. Like in any other series circuit the voltage is divided between the loads proportional to the resistance. 

The total resistance of this circuit is:
Rt = 6 + 6 =12
The current flow through this circuit is:
A= V/R
A= 12/12
A = 1amp
So, the voltage drop of the bad connection will be:
V= R x A = 1 x 6 = 6 volts
The voltage available to the light bulb will be 6 volts which will make the bulb dim or inoperative.

ohms law, calculations, bad connection



DMM Connection:

The proper method of connecting the DMM to measure voltage is to connect the meter in parallel to the circuit; such as shown in the drawing below. The meter will take whatever voltage is available at the positive lead and drop that voltage thru its internal resistance. Using the numbers from the previous drawing; the available voltage after the first resistance (bad connection) is 6 volts. Since only 6 volts are available at that point that is what the DMM will drop and display as the measured value.

measuring voltage, dmm voltage

DMM Connection: Circuit Open = WRONG!

So, with the circuit disconnected and the DMM connected, you are now working with a series circuit. in the example given the meter is the second load. 


Going back to using Ohm’s Law, resistance in a series circuit is the sum of all. So in the circuit we are working with, Rt=  6 + 10,000,000 = 10,000,006
Our current flow for this circuit will be: A= V/R, A= 12/10,000,006 = .0000012 amps
That means our voltage drop across our loads are: V= A x R
Bad Connection:
V= 6 x .0000012 = .0000072 V
V = 10,000,000 x .0000012 = 12 V
The DMM will display 12V, which may lead you to conclude that the component (light bulb) is defective. When in reality the connection is defective, but, you cannot see it since you are measuring the wrong way! Remember….NEVER open the circuit when measuring voltage!


Before we talk about voltage drop (Vd), lets refresh a bit on the three electrical values used when discussing electrical circuits; voltage, current, and resistance.

Voltage is electrical force or the pressure that pushes electrons (electricity) through a circuit. The higher the voltage in a circuit the more potential flow there is. Voltage is measured in volts.

Current is the amount of electrical flow in a circuit. Current is the actual amount of electricity flowing past a given point in the circuit. In electrical circuits; the higher the voltage in the circuit the higher potential flow will be. Current is measured in amperes.

Resistance is the opposition to electrical flow. In electrical circuits there is always resistance; most resistance encountered will be at the load. Resistance other than at the load is unwanted and affects the operation of the circuit. Resistance is measured in units of Ohm (Ω).

So, all things in a circuit working as designed will make for a proper working circuit. When any of the three values above become higher or lower than intended the circuit operation is affected.

Lower Voltage = Low Current = circuit performs less work

Higher Resistance = Low Current = circuit performs less work

Higher Voltage= Higher Current = circuit performs more work; which in most cases is unwanted as components will heat up and the fuse will blow.

Lower Resistance= Higher Current = circuit performs more work; which in most cases is unwanted as components will heat up and the fuse will blow.

As you can see current is what performs the work in a circuit. When current is changed the circuit will not operate as designed.

Circuit Faults:

Most circuit faults can be traced back to unwanted resistance. This resistance creates a voltage drop that reduces the voltage available to the load and the overall current flow in the circuit.

So let’s look at a basic circuit…….

Basic Circuit Operation:

circuit readings
Basic Circuit Operation

What Does a Voltmeter Measure?

All meters whether single or combination, measure the difference between the two meter leads. When measuring voltage the meter displays the difference in voltage between the two meter leads. The same applies when measuring resistance and current.

voltmeter, voltage drop, difference in potential
Voltmeter Readings

Voltage Drop:

So what is voltage drop? Voltage drop is voltage used. Voltage is used in a circuit anywhere resistance is encountered.

How is voltage drop measured? It is measure with your voltmeter, multimeter, or DMM.

Can I measure voltage drop in the positive or negative side of the circuit? Yes, absolutely. A voltage drop measurement is done with circuit energized and is ideal for finding loose connections or corroded wiring that increases in resistance when heated by current flow.


Max Drop Chart


Load Voltage Drop:

The circuit load is the component that uses the voltage in the circuit since it should be the only resistance in the circuit.

Load Voltage Drop
Load Voltage Drop

Measuring Voltage Drop Through a Connector:

Connectors often develop unwanted resistance due to corrosion, loss of terminal tension, or physical damage. This resistance creates a voltage drop that affects the operation of the load. in essence; a series circuit is created when unwanted resistance is part of a circuit.

Voltage drop through connectors, switches, fuses, and wiring should be minimal and should not exceed 0.2V on 12V circuits and 0.4 on a 24V circuit.

Connector Voltage Drop
Connector Voltage Drop

Powertrain and computers circuits are very susceptible to failing when unwanted resistance is present. These are low voltage circuits for the most part; follow specifications when troubleshooting these types of circuits.

Vd, connector, voltage drop
Connector Voltage Drop

Measuring Voltage Drop in the Whole Circuit:

Remember! voltage drop is voltage used. Most of the circuit voltage should be used (dropped) at the load. In the drawing below you can see that the load is only using about half of the voltage because unwanted resistance at a connection is using the rest. The 12V load is very dim since 5.5 volts is not enough to make it light normally.

bad connection, ground voltage drop
Voltage Drop
battery cable, Vd, unwanted resistance
Battery Cable Voltage Drop

In conclusion:

Measuring voltage drop is the most effective way of finding unwanted resistance. Why? with the circuit under load voltage will be divided between all resistance points in the circuit. If you were to just perform a resistance measurement; the reading will probably read within specifications, potentially leading you to a wrong diagnosis.