AutoFocus: Scan tool 'passes' on Volkswagen Passat

April 21, 2010

The problem: Scan Tool will not communicate with any vehicle system.

The vehicles in question: Audi A4 and Volkswagen Passat ABS/EDL equipped vehicles.

The system description: The ABS/EDL system uses an Electronic Differential Lock (EDL), which is a low speed, traction assist system that acts automatically without the driver’s intervention. EDL uses many of the ABS components to adjust drive wheel speed for improved traction, steering control and stability. EDL operates during acceleration up to speeds of 25 mph (40 km/h) on front-wheel drive cars. On vehicles equipped with 4Motion, EDL operates up to speeds of 50 mph (80 km/h).

(EDL supplements the standard ABS functions on: Bosch 5.0 ABS, Bosch 5.3 ABS, Bosch 5.7 ABS, Teves MK 04, ITT MK 20 IE ABS, ITT MK 60 IE ABS.)

To prevent the drive wheels from spinning during low speed acceleration, EDL monitors the drive wheels using the wheel speed sensors. If excessive drive wheel spin is detected, the self-priming pump in the hydraulic unit extracts brake fluid from the master cylinder reservoir, and then pumps the fluid through a solenoid valve and into the brake caliper of the wheel with the highest rotation rate. As a result, the drive torque is transmitted through the differential to the drive wheel with the most traction. In the event of calculated high brake system temperatures (caused by repeated severe EDL operation), the safety circuits within the ABS control module may de-activate EDL control to prevent the brakes from overheating. EDL will become operational again, once the ABS control module recognizes a calculated brake temperature within a specified range.

If a fault occurs in the ABS, both the ABS and EDL will be disabled and the ABS warning light will illuminate. The ABS/EDL system receives a "Standing Time" signal (also known as a Time Interval Management signal) generated in the instrument cluster each time the ignition switch is turned to the “on” position. This time signal is used to calculate brake temperature based on the number and interval of applied EDL events. ABS code 01203 indicates a problem with this time signal.

What symptoms might I expect to encounter?

1. ABS light and brake light are both illuminated; brake light may flash.

2. Trouble code 01203 stored in ABS system.

3. No scan tool communication possible with ABS.

4. No scan tool communication possible with other vehicle systems at all.

5. Abnormal instrument cluster operation/display.

Approach to diagnosis: The ABS controller can fail in such a way internally that it will back feed voltage into the instrument cluster via the ‘standing time’ signal wire. When both the ABS and brake lights are illuminated and/or flashing, always start with a diagnosis of the ABS controller. If the voltage back feed from the ABS controller is large enough, it can adversely affect CAN communication between all modules on the CAN bus and can cause the instrument cluster displays to operate incorrectly.

How should I proceed with diagnosis?

1. Using a full function factory compatible scan tool, check and verify that communication is possible with the ABS system. Use Address Word 03 to access the ABS.

2. If scan tool communication is possible with the ABS system, check for the presence of code 01203 in system memory.

3. If scan tool communication is not possible with the ABS system, check and verify that scan tool communication is possible with any other vehicle system (engine, transmission, SRS, etc.).

4. If communication is possible with other systems, check for correct switched ignition power to the ABS controller pin #15 (black/blue wire). There must be 12 volts with the ignition on.

5. Check ABS controller pins #17 and #18 (both red/black wires) for 12 volts at all times.

6. Check ABS controller pins #16 and #19 (both brown wires) for good ground at all times.

7. If tests 1 through 6 all pass, proceed as follows: Using a lab scope, check for the presence of a Standing Time signal from the instrument cluster to the ABS controller pin #10 (green wire). This signal will appear as a series of quick 11-volt to zero volt square-wave pulses a moment or so after ignition is turned to the ‘on’ position. The signal typically lasts only a few seconds. The signal must be a clean square-wave form and cycle all the way to zero volts. If the signal does not reach zero volts on the down slope, it is not correct.

8. If no signal or an incorrect signal is present, turn the ignition switch off. Disconnect the ABS ECU. Turn the ignition on and recheck for a signal at pin #10 on the wiring harness side.

9. If a correct 11-volt square-wave signal is now present, likely there is an internal driver problem with the ABS ECU.

10. If there is still no signal, check the wiring from the ABS ECU to the instrument panel. Pay special attention to the orange connector at the driver’s side kick panel connector station for corrosion or damage. If the wiring is okay, there is a likely failure of the instrument cluster itself and additional diagnosis of the cluster will be necessary.

Technical Tips:

1. The Standing Time signal originates in the instrument cluster and is sent out to those controllers that need it (examples: ABS controller, climate control controller).

2. If a scan tool can not communicate with any controller on the CAN bus, unplug the ABS controller and recheck for communication on all other systems.

3. The ABS controller is available as a separate unit. Replacement of the entire ABS hydraulic assembly is not required.

4. The ABS controller can be accessed and replaced through the driver’s side wheel well in many cases without removing the hydraulic assembly from the vehicle. The inner fender wall will need to be removed to gain access.

What tools or accessories will I need to make the diagnosis?

1. Volkswagen Full Function Factory Compatible Scan tool;

2. Lab Scope; and

3. Digital Volt Ohm Meter (DVOM).