Can Bus Terminal Resistance, Checking
Special tools and workshop equipment required
- multimeter
- Wiring diagram.
Test requirements
- 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 connections between engine/transmission/chassis OK.
- Ignition switched off.
Test procedure
-- Observe safety precautions. Refer to SAFETY PRECAUTIONS
-- Observe Clean Working Conditions. Refer to CLEAN WORKING CONDITIONS
-- Perform a preliminary check to verify the customers complaint. Refer to PRELIMINARY CHECK .
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-.
Procedure
-- Disconnect the Data Bus On Board Diagnostic Interface -T32b- 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 T20 connector, terminals 6 to 16 for resistance.
Specified value: 60 to 72 Ω (at approx. 20° C)
If the specified value was not obtained:
-- 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 Repair information.
-- Using a multimeter, check the Data Bus On Board Diagnostic Interface -J533- electrical harness connector T20 to the Motronic Engine Control Module (ECM) -J623- electrical harness connector T94 for resistance.
| Data Bus On Board Diagnostic Interface -J533- electrical harness connector T20 terminals | Motronic Engine Control Module (ECM) -J623- electrical connector T94 terminals or test box sockets |
|---|---|
| 6 (Can Bus Low) | 67 |
| 16 (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, high resistance or short circuit to voltage or ground.
-- 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-.
-- 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:
Generate readiness code. Refer to READINESS CODE .
If the DTC does return, no malfunction is detected in the wiring and the voltage supply is OK:
-- Replace the Data Bus On Board Diagnostic Interface -J533-. Refer to the Repair information.
-- 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:
- 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 .