CAN-Bus Terminal Resistance, Checking

CAN-Bus Terminal Resistance, Checking

Special tools, testers and auxiliary items required
- Multimeter.
- Wiring diagram.

Test requirement
- A CAN-Bus malfunction was recognized.
- The Engine Control Module (ECM) J623 fuses OK.
- Battery voltage at least 12.5 volts.
- All electrical consumers such as, lights and rear window defroster, switched off.
- Vehicles with automatic transmission, shift selector lever into position "P" or "N".
- A/C switched off.
- Ground (GND) connections between engine/transmission/chassis OK.
- Ignition switched off.

Function
The Engine Control Module (ECM) communicates with other CAN-Bus capable control modules.

The control modules are connected by two data bus wires which are twisted together (CAN_High and CAN_Low), and exchange information (messages). Missing information on the CAN-bus is recognized as a malfunction by the Engine Control Module (ECM) and the other control modules connected to the CAN-bus.

Trouble-free operation of the CAN-Bus requires that it have a terminal resistance. This central terminal resistance is located in the Engine Control Module (ECM).

If the CAN-Bus communication is interrupted. Communication from the scan tool to the Engine Control Module (ECM) may not be possible.

Test procedure
- Perform a preliminary check to verify the customers complaint. General Diagnosis

Start diagnosis
- Disconnect the Instrument Cluster Control Module J285 electrical harness connector.

NOTE: The Engine Control Module (ECM) J623 must remain connected for the following step.

- Using a Multimeter , check the Instrument Cluster Control Module J285 electrical harness connector terminals T32a/12 to T32a/13 for resistance.

Specified value: 60 to 72 Ohms (at approx. 20 degrees C)

If the specified value was not obtained:

Checking wiring

If the manufacturers test box is being used. Perform the following step.
- Install the test box.

If the manufacturers test box is not being used. Perform the following step.
- Remove the Engine Control Module (ECM) J623.
- Using a Multimeter , check the Instrument Cluster Control Module J285 electrical harness connector T32a to the Engine Control Module (ECM) J623 electrical harness connector T94 for resistance.

Instrument Cluster Control Engine Control Module (ECM) J623
Module J285 electrical harness electrical harness connector T94
connector terminals terminals or test box sockets
T32a/12 (Can_Bus Low) 67
T32a/13 (Can_Bus High) 68

Specified value: 1.5 Ohms Max.

If the specified value was not obtained:
- Check the wiring connection for an open circuit, short circuit to Battery (+) or Ground (GND).
- Check the wiring connection for damage, corrosion, loose or broken terminals.
- If necessary, repair the faulty wiring connection.

If no malfunction is found in the wiring:
- Erase the DTC memory. Diagnostic Mode 04 - Erase DTC Memory
- Perform a road test to verify repair.

If the DTC does not return:

Repair complete, Generate readiness code. Monitors, Trips, Drive Cycles and Readiness Codes
- End of diagnosis.

If the DTC does return and no malfunction is detected in the wiring and the voltage supply was OK:
- Replace the Engine Control Module (ECM) J623.

Final procedures
After repair work, the following work steps must be performed in the mentioned sequence:
1. Check the DTC memory. Diagnostic Mode 03 - Read DTC Memory
2. If necessary, erase the DTC memory. Diagnostic Mode 04 - Erase DTC Memory
3. If the DTC memory was erased, generate readiness code. Monitors, Trips, Drive Cycles and Readiness Codes

End of diagnosis.