Can-Bus Terminal Resistance, Checking

Special tools and workshop equipment required 

• Multimeter.
• Wiring diagram.

Test Requirements 

• A CAN-Bus malfunction was recognized.
• The Engine Control Module (ECM) - J623- fuses OK.
• Battery voltage at least 12.5 V.
• 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 inside 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 customer's complaint. Refer to PRELIMINARY CHECK .

If the manufacturers test box is being used, perform the following step.

-- Connect the Test Box 105 Pin VAG1598/42 with Adapter Cable1598/39-1 to the Engine Control Module (ECM) -J623-. Refer to Component Locations Overview .

If the manufacturers test box is not being used, perform the following step.

-- Remove the Engine Control Module (ECM) -J623-. Refer to ENGINE CONTROL MODULE, REPLACING .

-- Using a Multimeter, check the central termination resistor -- by checking for resistance between terminals 58 and 60 of the Engine Control Module (ECM)J623.

Specified value: 60 to 72 Ω (at approx. 20° C)

If the specified value was not obtained:

-- Replace the Engine Control Module (ECM) -J623-. Refer to ENGINE CONTROL MODULE, REPLACING .

If the specified value was obtained:

Checking wiring

If the manufacturers test box is being used, perform the following step.

-- Connect the Test Box 105 Pin VAG1598/42 with Adapter Cable1598/39-1 to the Engine Control Module (ECM) -J623- wiring harness connector. Refer to Component Locations Overview .

-- Using a Multimeter, check the central termination resistor -- by checking for resistance between terminals 58 and 60 of the Engine Control Module (ECM)J623 electrical harness connector "A" (T81).

Engine Control Module (ECM) -J623- electrical harness connector T81 terminals or test box sockets
58 (Can_Bus Low)
60 (Can_Bus High)

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 Engine Control Module (ECM) -J623-. Refer to ENGINE CONTROL MODULE, REPLACING .

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 .