Can Bus Terminal Resistance, Checking

Observe all safety precautions: SAFETY PRECAUTIONS 

View clean working conditions: CLEAN WORKING CONDITIONS 

Use only gold-plated terminals when servicing any component with gold-plated electrical harness connector terminals.

For wiring diagrams, component locations, and connector views, Refer to the applicable wiring diagram.

Special tools and workshop equipment required 

• Multimeter
• Wiring diagram

Test requirement 

• A CAN-Bus malfunction was recognized.
• The Motronic Engine Control Module (ECM) - J623- fuses OK.
  Fig 1: Identifying 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 (GND) connections between engine/transmission/chassis OK.
• Ignition switched off.

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-Bus High and CAN-Bus 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 -.

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 connector terminals 6 to 16 for resistance.

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

If the specified value was Not obtained:

-- Check the wiring connection for damage, corrosion, loose or broken terminals.

-- If necessary, repair the faulty wiring connection.

-- If No malfunctions were found replace the Replace the Motronic Engine Control Module (ECM) -J623 -. Refer to appropriate service information.

-- Go to Final procedures

If the specified value was 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 the appropriate service information.

-- Using a Multimeter, check the Data Bus On Board Diagnostic Interface -J533- electrical harness connector to the Motronic Engine Control Module (ECM) -J623- electrical harness connector T94 for resistance.

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

-- Go to Final procedures

If the specified value was obtained:

-- Replace the Data Bus On Board Diagnostic Interface -J533-. Refer to appropriate service information.

Final procedures 

-- Reinstall the engine cover with air filter, if necessary. Refer to the appropriate service information.

-- 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 .