CAN-Bus Terminal Resistance, Checking

CAN-Bus Terminal Resistance, Checking

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

Test requirement
- CAN-Bus malfunction was recognized.
- The Motronic 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 Motronic Engine Control Module (ECM) J623 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 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.

Procedure
- Disconnect the Data Bus On Board Diagnostic Interface T32b electrical harness connector.

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

- Using a Multimeter, check the Data Bus On Board Diagnostic Interface J533 electrical harness connector terminals 6 to 16 for resistance.

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

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:
- Replace the Motronic Engine Control Module (ECM) J623.

If the specified value was 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 Motronic Engine Control Module (ECM) J623.
- Using a Multimeter, check the Data Bus On Board Diagnostic Interface J533 electrical harness connector to the Motronic Engine Control Module (ECM) J623 electrical harness connector T94 for resistance.

Data Bus On Board Diagnostic Motronic Engine Control Module
Interface J533 electrical harness (ECM) J623 electrical connector
connector terminals T94 terminals or test box sockets
6 (Can_Bus Low) 67
16 (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:
- Replace the Data Bus On Board Diagnostic Interface J533.
- Assembly is performed in the reverse of the removal.

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