Can-Bus Terminal Resistance, Checking
The following procedure is used to diagnose the CAN-Bus terminal resistance of the Motronic Engine Control Module (ECM) - J623- and the Engine Control Module (ECM) 2 -J624-.
Special tools and workshop equipment required
- Multimeter.
- Wiring diagram.
Test requirements
- A CAN-Bus malfunction was recognized.
- Fuse -SF6- 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 the Engine Control Module (ECM) 2 -J624- and 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-.
Checking internal resistance, Motronic Engine Control Module (ECM) -J623-
-- Disconnect theEnergy Management Control Module -J644- electrical harness connector.
-- Disconnect the Engine Control Module (ECM) 2 -J624- electrical harness connector.
The Motronic Engine Control Module (ECM) -J623- must remain connected for the following step.
-- Using a Multimeter, check the Energy Management Control Module -J644- electrical harness connector terminals 9 to 10 for resistance.
Specified value: 60 to 72 Ω (at approx. 20° C)
If the specified value was not obtained:
-- Check the wiring. Refer to CAN-BUS TERMINAL RESISTANCE, CHECKING .
Checking internal resistance, Engine Control Module (ECM) 2 -J624-
-- Disconnect theEnergy Management Control Module -J644- electrical harness connector.
-- Disconnect theMotronic Engine Control Module (ECM) -J623- electrical harness connector.
The Engine Control Module (ECM) 2 -J624- must remain connected for the following step.
-- Using a Multimeter, check the Energy Management Control Module -J644- electrical harness connector terminals 9 to 10 for resistance.
Specified value: 60 to 72 Ω (at approx. 20° C)
If the specified value was not obtained:
-- Check the wiring. Refer to CAN-BUS TERMINAL RESISTANCE, CHECKING .
Refer to CAN-BUS TERMINAL RESISTANCE, CHECKING .
If the manufacturers test box is being used. Perform the following step.
-- Install the test box. Refer to Wiring and Components, Checking with Test Box VAG 1598/42 .
If the manufacturers test box is not being used. Perform the following step.
-- Remove the Motronic Engine Control Module (ECM) -J623- or the Engine Control Module (ECM) 2 -J624-. Refer to ENGINE CONTROL MODULE, REMOVING AND INSTALLING .
-- Using a Multimeter, check the Energy Management Control Module -J644- electrical harness connector to the Motronic Engine Control Module (ECM) -J623- electrical harness connector T94a or the Engine Control Module (ECM) 2 -J624- electrical harness connector T94b for resistance.
| Energy Management Control Module -J644- electrical harness connector terminals | Motronic Engine Control Module (ECM) -J623- electrical harness connector T94a terminals or Engine Control Module (ECM) 2 -J624- electrical harness connector T94b test box sockets |
|---|---|
| 10 (Can_Bus Low) | 67 |
| 9 (Can_Bus High) | 68 |
| Motronic Engine Control Module (ECM) -J623- electrical harness connector T94a terminals or test box sockets | Engine Control Module (ECM) 2 -J624- electrical harness connector T94b terminals or test box sockets |
|---|---|
| 89 (Can_Bus Low) | 89 (Can_Bus Low) |
| 90 (Can_Bus High) | 90 (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, lose 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- or the Engine Control Module (ECM) 2 -J624-. Refer to ENGINE CONTROL MODULE, REMOVING AND INSTALLING .
Final procedures
After repair work, the following work steps must be performed in the following sequence:
- Check the DTC memory. Refer to DIAGNOSTIC MODE 03 - READ DTC MEMORY .
- If necessary, erase the DTC memory. Refer to DIAGNOSTIC MODE 04 - ERASE DTC MEMORY .
- If the DTC memory was erased, generate readiness code. Refer to READINESS CODE .
-- End of diagnosis.