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).

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 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

- Disconnect theEnergy Management Control Module (J644) 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 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. Refer to => [ Engine Control Module, Removing and Installing ] Service and Repair.

- Using a Multimeter, check the Energy Management Control Module (J644) electrical harness connector to the Motronic Engine Control Module (ECM) (J623) electrical harness connector T94 or the electrical harness connector T94 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). Refer to => [ Engine Control Module, Removing and Installing ] Service and Repair.

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.