CAN-Bus Terminal Resistance, Checking

CAN-Bus Terminal Resistance, Checking

The following procedure is used to diagnose the CAN-Bus terminal resistance of the Motronic Engine Control Module (ECM) (J623) and the Engine Control Module (ECM) 2 (J624).

Special tools, testers and auxiliary items required

• Multimeter.

• Wiring diagram.

Test requirement

• A CAN-Bus malfunction was recognized.

• Fuse (SF6) 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 Motronic Engine Control Module (ECM) (J623) communicates with the Engine Control Module (ECM) 2 (J624) and 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 Motronic Engine Control Module (ECM) (J623) 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 Motronic Engine Control Module (ECM) (J623).

Checking internal resistance,Motronic Engine Control Module (ECM) (J623)

- Disconnect the Energy Management Control Module (J644) electrical harness connector.

- Disconnect the Engine Control Module (ECM) 2 (J624) electrical harness connector.

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

- Using a Multimeter, check the Energy Management Control Module (J644) electrical harness connector terminals 9 to 10 for resistance.

Specified value: 60 to 72 ohms (at approx. 20° C)

If the specified value was not obtained:

- Check the wiring.

Checking internal resistance,Engine Control Module (ECM) 2 (J624)

- Disconnect the Energy Management Control Module (J644) electrical harness connector.

- Disconnect the Motronic Engine Control Module (ECM) (J623) electrical harness connector.

• The Engine Control Module (ECM) 2 (J624) must remain connected for the following step.

- Using a Multimeter, check the Energy Management Control Module (J644) electrical harness connector terminals 9 to 10 for resistance.

Specified value: 60 to 72 ohms (at approx. 20° C)

If the specified value was not obtained:

- Check the wiring.

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 Motronic Engine Control Module (ECM) (J623) or the Engine Control Module (ECM) 2 (J624). Refer to => [ Engine Control Module, Removing and Installing ] Engine Control Module, Removing and Installing.

- Using a Multimeter, check the Energy Management Control Module (J644) electrical harness connector to the Motronic Engine Control Module (ECM) (J623) electrical harness connector T94a or the Engine Control Module (ECM) 2 (J624) electrical harness connector T94b for resistance.









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, lose or broken terminals.

- If necessary, repair the faulty wiring connection.

If no malfunction is found in the wiring:

- Replace the Motronic Engine Control Module (ECM) (J623) or the Engine Control Module (ECM) 2 (J624). Refer to => [ Engine Control Module, Removing and Installing ] Engine Control Module, Removing and Installing.

Final procedures

After repair work, the following work steps must be performed in the following sequence:

1. Check the DTC memory. Refer to => [ Diagnostic Mode 03 - Read DTC Memory ] Diagnostic Mode 03 - Read DTC Memory.

2. If necessary, erase the DTC memory. Refer to => [ Diagnostic Mode 04 - Erase DTC Memory ] Diagnostic Mode 04 - Erase DTC Memory.

3. If the DTC memory was erased, generate readiness code. Refer to => [ Readiness Code ] Monitors, Trips, Drive Cycles and Readiness Codes.

- End of diagnosis.