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) (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).
Test procedure
- Perform a preliminary check to verify the customers complaint. Refer to => [ Preliminary Check ] Preliminary Check
Start diagnosis
- Disconnect the Data Bus On Board Diagnostic Interface (J533) electrical harness connector.
• 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° C)
If the specified value was not obtained:
Checking wiring
If the manufacturers test box is being used. Perform the following step.
- Install the Test Box 105 Pin (VAG1598/42).
If the manufacturers test box is not being used. Perform the following step.
- Remove the Motronic Engine Control Module (ECM) (J623). Refer to => [ Engine Control Module J623, Removing and Installing ] Engine Control Module J623, Removing and Installing.
- 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.
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:
- Erase the DTC memory. Refer to => [ Diagnostic Mode 04 - Erase DTC Memory ] Diagnostic Mode 04 - Erase DTC Memory.
- Perform a road test to verify repair.
If the DTC does not return:
Repair complete, Generate readiness code. Refer to => [ Readiness Code ] Monitors, Trips, Drive Cycles and Readiness Codes.
- End of diagnosis.
If the DTC does return and no malfunction is detected in the wiring and the voltage supply was OK:
- Replace the Engine Control Module (ECM) (J623). Refer to => [ Engine Control Module J623, Removing and Installing ] Engine Control Module J623, Removing and Installing.
Final procedures
After the 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.