J1507 / J1708 Operation and Layout
J1587 / J1708
SAE J1708 is a standard used for serial communication between ECUs / ECMs/ and controllers on a heavy-duty vehicle. J1708 defines the physical layer, which are the hardware (wiring) and basic protocol that is needed for J1587 and J1922 to transmit messages.
J1708 Physical aspects:
⇒ 2- wire 18 gauge
⇒ Can be run up to 130 ft. (40m)
⇒ Wires twisted to cancel out unwanted frequencies
⇒ Does not require EOL (End of the Line Resisters)
⇒ Almost always used in conjunction with protocol J1587
⇒ Hardware based on the electronic properties from the RS-485*
⇒ Serial byte-oriented communication
⇒ 9600 bps transmission rate
⇒ The 6-pin grey Deutsch connector is used when running only J1708
⇒ The 9-pin black Deutsch connector is used for running J1939, but it has provisions to run J1708 on pins G and H.
* RS-485 – Also known as TIA-485, is a standard defining the electrical characteristics of drivers and receivers (ECU/EMC/controllers) for use in serial communication systems.
Is the specification which defines messages that are transmitted on the J1708. It specifies the transport, network and application protocol layers.
⇒ J1587 is being replaced by J1939, but is still found in newer vehicles to help operate lower priority communications between controllers. This helps keep the J1939 from being tasked with too many controllers.
⇒ J1939 and J1587 will communicate with each other and exchange messages through a gateway module to bridge the two different protocols together.
⇒ The disadvantage that J1587 has over J1939 is the communication speed. J1587 transmit data at 9600 bps, while J1939 transmits data at 250,000 bps which is real time.
J1587 messages start with:
MID (Message Identifier) this identifies and indicates the source address of the transmitting node (which ECU/ECM/controller the message is coming from, e.g. engine, transmission, axle, et.)
⇒ the next message that follows is:
PID (Parameter Identifier) this indicates the parameter data under the MID the data corresponds to (e.g. request parameter, invalid data parameter, pressures, temperatures, et.)
⇒ other messages are:
SID (Subsystem Identification Numbers) these are used to identify a section of a control system without a related PID. These are usually assigned to a replaceable subsystem (e.g. fuel injector, fuel control valve, speed sensor, fuel shut of valve, et.) and fall under an MID.
⇒ the next one relates to trouble codes:
FMI (Failure Mode Identifier) describes the type of failure detected in the subsystem identified by the PID or SID. The FMI, and either the PID or SID combined to form a diagnostic trouble code (e.g. data erratic, intermittent or incorrect, voltage above normal, voltage below normal, e.t.)
The J1708 runs the length of the vehicle and the ECM / ECU/ controllers connect to it. It consists of two wires.
⇒ J1587 +
⇒ J1587 —
⇒ No EOL (End of Line) resisters are used.
Note: The wires are twisted to cancel out frequencies.
Diagnostic 9-pin Deutsch Connector:
Heavy duty J1708 / J1587 applications use a 6-pin grey Deutsch connector to interface with test equipment and software to J1708 / J1587. Communicating to nodes and testing the J1708 / J1587 can be done through the Deutsch connector.
Operation and Testing
J1587 / J1708 Chassis voltage ground check Pin-E:
J1587 / J1708 Voltage check Pin-C + (Vehicle battery) and Pin-E —(Chassis ground):
J1587 / J1708 Voltage check Pin-A J1587 + and Pin-E — (Chassis ground):
J1587 / J1708 Voltage check Pin-B J1587 — and Pin-E — (Chassis ground):
J1587 / J1708 – Shorted wire J1587 + to wire J1587 —:
J1587 / J1708 – ECU Shorted Internally, Stopping Communication:
J1587 / J1708 – Wire J1587 + or Wire J1587 — Shorted to Ground: