CAN-Bus Terminal Resistance, Checking

Can-Bus Terminal Resistance, Checking

This 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 S102 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 Instrument Cluster Control Module J285 electrical harness connector.
- Disconnect the Engine Control Module (ECM) 2 J624 electrical harness connector.

NOTE: The Motronic 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 12 to 13 for resistance.

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

If the specified value was not obtained:
- Check the wiring.

Checking internal resistance, Engine Control Module (ECM) 2 J624
- Disconnect the Instrument Cluster Control Module J285 electrical harness connector.
- Disconnect the Motronic Engine Control Module (ECM) J623 electrical harness connector.

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

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

Specified value: 60 to 72 Ohms (at approx. 20 degrees 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.
- Using a Multimeter , check the Instrument Cluster Control Module J285 electrical harness connector to the Motronic Engine Control Module (ECM) J623 electrical harness connector T94 or the Engine Control Module (ECM) 2 J624 electrical harness connector T94a for resistance.

Instrument Cluster Motronic Engine Control Module (ECM) J623
Control Module J285 electrical harness connector T94 terminals
electrical harness or Engine Control Module (ECM) 2 J624
connector terminals electrical harness connector T94a test box sockets
12 (Can_Bus Low) 67
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, 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.

Final procedures
After repair work, the following work steps must be performed in the following 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.