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.
  Fig 1: Fuses On E-Box In Engine Compartment, Left Side

  Courtesy of AUDI OF AMERICA, LLC
• 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.

Function 

The 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 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 Engine Control Module (ECM) -J623-.

Test procedure 

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

Start diagnosis 

-- Disconnect the Data Bus On Board Diagnostic Interface -J533- electrical harness connector.

-- 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:

Checking wiring 

-- Remove the 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 to the Engine Control Module (ECM) -J623- electrical harness connector T94 for resistance.

Data Bus On Board Diagnostic Interface -J533- electrical connector T20 terminals	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, short circuit to Battery positive 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 and the Terminal Resistance value was obtained:

-- Replace the Data Bus On Board Diagnostic Interface -J533-.

If no malfunction is found in the wiring and the Terminal Resistance value was Not obtained

-- Replace the Engine Control Module (ECM) -J623-. Refer to the Repair information.

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 .
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 .