CAN Bus Instrument Cluster to Data Bus OBD Interface, Checking
CAN Bus Instrument Cluster to Data Bus OBD Interface, Checking
Observe all safety precautions: => [ Safety Precautions ] Generic Scan Tool
View clean working conditions: => [ Clean Working Conditions ] Clean Working Conditions
Use only gold-plated terminals when servicing any component with gold-plated electrical harness connector terminals.
For wiring diagrams, component locations, and connector views, Refer to the applicable wiring diagram.
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 Instrument Cluster Control Module (J285) electrical harness connector.
• 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 T32/29 to T32/30 for resistance.
Specified value: 60 to 72 ohms (at approx. 20° C)
If the specified value was not obtained:
- Check the wiring for a short circuit to Battery (+), or an open circuit.
- 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). Refer to the appropriate service manual.
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). Refer to the appropriate service manual.
- Using a multimeter, check the Data Bus On Board Diagnostic Interface (J533) electrical harness connector to the Instrument Cluster Control Module (J285) electrical harness connector T32 for resistance.
Specified value: 1.5 ohms Max.
If the specified value was not obtained:
- Check the wiring for a short circuit to Battery (+), or an open circuit.
- 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 Instrument Cluster Control Module (J285). Refer to the appropriate service manual.
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 Modes 01 - 09.
2. If necessary, erase the DTC memory. Refer to => [ Diagnostic Mode 04 - Erase DTC Memory ] Diagnostic Modes 01 - 09.
3. If the DTC memory was erased, generate readiness code. Refer to => [ Readiness Code ] Readiness Code.