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 Motronic Engine Control Module (ECM) J220 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.

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

Function
The Motronic Engine Control Module (ECM) J220 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) J220 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) J220.

Start diagnosis
- Disconnect the Data Bus On Board Diagnostic Interface J533 electrical harness connector.

NOTE: The Motronic Engine Control Module (ECM) J220 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) J220.

If the specified value was obtained:

Checking wiring
If the manufacturers test box is being used. Perform the following step.
- Install the Adapter Cable (121-pin) VAG1598/31.

If the manufacturers test box is not being used. Perform the following step.
- Remove the Motronic Engine Control Module (ECM) J220.
- Using a Multimeter, check the Data Bus On Board Diagnostic Interface J533 electrical harness connector to the Motronic Engine Control Module (ECM) J220 electrical harness connector T121 for resistance.

Data Bus On Board Diagnostic Motronic Engine Control Module
Interface J533 electrical (ECM) J220 electrical harness
harness T20 connector connector T121 terminals or
terminals test box sockets
6 (Can_Bus Low) 58
16 (Can_Bus High) 60

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.
- Erase the DTC memory. Clearing Diagnostic Trouble Codes
- 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 diagnosis.

If the DTC does return and no malfunction is detected in the wiring and the voltage supply was OK:
- Replace the Motronic Engine Control Module (ECM) J220.
- Assembly is performed in the reverse of the removal.

Final procedures
After the repair work, the following work steps must be performed in the following sequence:
1. Check the DTC memory. Reading Diagnostic Trouble Codes
2. If necessary, erase the DTC memory. Clearing Diagnostic Trouble Codes
3. If the DTC memory was erased, generate readiness code. Monitors, Trips, Drive Cycles and Readiness Codes

End diagnosis.