Can-Bus Terminal Resistance, Checking

Special tools and workshop equipment required 

• Multimeter.
• Wiring diagram.

Test requirement 

• A CAN-Bus malfunction was recognized.
• The 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 Engine Control Module (ECM) -- 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 Engine Control Module (ECM) -- 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 Engine Control Module (ECM) --.

If the CAN-Bus communication is interrupted. Communication from the scan tool to the Engine Control Module (ECM) -- may not be possible.

Test procedure 

-- Perform a preliminary check to verify the customers complaint. Refer to PRELIMINARY CHECK .

Start diagnosis 

-- Disconnect the Instrument Cluster Control Module -J285- electrical harness connector.

-- Using a Multimeter, check the Instrument Cluster Control Module -J285- electrical harness connector terminals T32a/12 to T32a/13 for resistance.

Specified value: 60 to 72 Ω (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.

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, REPLACING .

-- Using a Multimeter, check the Instrument Cluster Control Module -J285- electrical harness connector T32a to the Engine Control Module (ECM) -J623- electrical harness connector T94 for resistance.

Instrument Cluster Control Module -J285- electrical harness connector terminals	Engine Control Module (ECM) -J623- electrical harness connector T94 terminals or test box sockets
T32a/12 (Can_Bus Low)	67
T32a/13 (Can_Bus High)	68

Specified value: 1.5 Ω 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 .

-- Perform a road test to verify repair.

If the DTC does not return:

Repair complete, Generate readiness code. Refer to READINESS CODE .

-- 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) -J623-. Refer to ENGINE CONTROL MODULE, REPLACING .

-- 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. Refer to DIAGNOSTIC MODE 03 - READ DTC MEMORY .
2. If necessary, erase the DTC memory. Refer to DIAGNOSTIC MODE 04 - ERASE DTC MEMORY .
3. If the DTC memory was erased, generate readiness code. Refer to READINESS CODE .