Richard Goulding

Thought I give something back to the forum after using them for years without posting. Maybe help someone out with a similar problem.

2000 A4 B5 Quattro 2.8 Sport (owned since 2003)

The testing procedure should apply to any car with passive sensors.

For about a year I have had problems with my ABS that was a nightmare to solve. It went to several garages including a 'specialist' with no joy. In the end I had to learn how to diagnose it myself.

Here are the symptoms/observations:

1.ABS pump noises (buzzing) when driving along the road without pressing the brake
2.Relay noises from under dash area at the same time as pump noises
3.ABS pump noises (buzzing) when releasing the brake after the car is fully stopped.
4.Lights dimming during buzzing.

Other symptoms that are often present but not in my case, are low speed activation of abs when the car is not skidding.

My car has the Bosch 5.3 ESP system which has a separate 'charger' pump which is the pump that is heard running. It has 'passive' wheel speed sensors which is important to the testing procedure.

Testing procedure for these kind of symptoms are (what you are looking for the wheel which is causing the problem):

Check the system for ABS fault codes using VCDS/VAG-COM to see if any present. In my case there were never any.

Monitor the individual wheel speeds using VCDS/VAG-COM (measuring block 1). Drive in a straight line below 19mph (connection is aborted over this speed for 'safety reasons'). You are looking for differences between the wheels greater than 2mph. But greater differences will confirm the particular wheel is at fault. In my case the speeds were normal.

At this point most 'specialists' I visited decided there was no problem and it was 'normal'!!!

Next, you need to start to checking the wheel signals in more depth. What you are trying to find is differences between the wheels.

Start with a wheel and jack up the car at that point. Find the connector to the wheel sensor. On my car they are behind a grommet in the wheel arch for the front wheels (black connector - red is wear sensor) and under the rear seat for the rear sensors.

Disconnect the sensor and attach a multimeter to the pins (that is, one probe to one pin other probe to other pin) on the sensor side connector (runs to the wheel).

Measure the resistance. It should be between 400 and 2300 ohms (mine varied between 1600 and 1700 on all wheels). You are looking for major differences in the wheels. If they are all similar then that is fine.

Next, leave the connectors as above and switch the multimeter to A/C volts. I used a range of 200 volts but 20 is better.

Spin the wheel. The voltage should increase uniformly the faster the wheel spins. For example 0.1v - 0.2v - 0.3v -0.4v. as the wheel gets faster and decrease uniformly as the wheel slows. If you stop the wheel abruptly it should drop immediately to 0. I couldn't get it above 0.5v by hand. If there is a problem, the voltages will be all over the place. For example 0.1v - 1.1v - 0.2v - 0.5v etc.

Test all wheels. It will be obvious which one is different to the others if it has a problem.

At this point you should have narrowed down the wheel with the problem and you need to remove the driveshaft to examine it but you will probably find that the reluctor ring is corroded and/or split. Mine had corrosion under the ring which had forced it into sensor which had worn it down causing weak signals. It also had a small crack in it.

The problem can also be caused by dirt on the sensor/ring or a failing wheel bearing causing uneven rotation. I would recommend removing the driveshaft to examine the reluctor ring as removing the sensor will destroy it. The sensor is usually ok if it passes the resistance checks and produces voltage.

To replace the ring just buy one from Ebay or elsewhere. Knock the old one off, clean the corrosion where it seats and heat the new one and put it on. Mine was a little loose so some epoxy glue was used.

Reassemble and redo the spin test to confirm repair. If this does not solve the problem, at least you're wheel speeds are eliminated as the problem.

You could use an oscilloscope to get more accurate patterns but the cheapest multimeter was enough to diagnose mine.

For the first time in a year my ABS is working as intended. The car was almost undriveable before and I ended up buying a new car Hyundai i30 1.6 diesel) as a daily driver and making the Audi a 'weekend' car.

Hope this helps someone out.