jseklund

Alright, I've been going crazy for a few months trying to fix this issue on my mom's car. The ESP light and ABS brake light come on the car at extremely random times. I have tried warming up the brake lights, slamming on the brakes from various speeds, hard acceleration, swerving motions, you name it. NOTHING sets these lights off. Then you'll just be driving at a normal speed on a straight road and bang - they pop on.

Searching the forums, I found that the brake light switch is the typical culprit for these two lights coming on together. I went to Audi and bought a new switch, installed it, and everything appeared to be fine for about 2 days..then the lights came back on (again, this is random, so the switch appeared to have done nothing). It should also be noted that this is a 2001 A6 2.7T - and the switch is slighlty older than what many of us have on here. There is no recall replacement it appears - just the original version. The recall version will not fit the car's connectors.

So, I've been using the vag-com to diagnose. I've found that the wheel speed sensors all seem to work perfectly. Then I noticed something a little funny:

When I step on the brake, the vag-com registers the "Brake Light Switch" as "Activated". When I release the brake, it goes to "0". This is reliable and works fine. A second field, "Brake Test Switch", however, reads "Activated" all the time.

I tested my car (same setup, but 2004) and I noticed that the Brake Light Switch and Brake Test Switch both register the same values at all times. i.e. - stepping on the brake activates both, and both read "0" when the brake is released.

My question is - what is the "Brake Test Switch"? Is it the same switch as the brake light switch? Or is it different? I can't find anything on this. If this is a second switch - then it is obviously the problem. If it is the same switch, then I must have received a bad switch from Audi.

Thanks in advance for the help.

jseklund

I think I figured it out guys. The "Test" switch is triggered when the brake pedal move the first small amount through its motion. The plunger comes out 1-2 mm and the "test" switch activates. This turns off the light on the shifter with the foot on the pedal - and lets you move the shifter out of park I guess.

After about 3-4 mm, the brake switch activates and the lights go on.

In the above case, the "test" switch was ALWAYS active. The switch had not been tightened down enough and the plunger was never fully compressed when the pedal was not being pressed. This meant that the computer was always getting a signal that the pedal was being pressed enough to activate the "test" switch, but not enough for the brake lights. The car was driving REALLY sluggish because of it (makes sense).

Anyway, I tightened the switch and it appears to be working great now. Car drives like new. I hope this may help someone else who has a similar issue.

Thanks so much to all those who responded.

radhaz

I was actually looking up your question last night, but got distracted by a PT invite in FFXIV.
Great find~

jseklund

HAHA, I understand - we're all busy and often times we just don't know the answers. Why would anyone response if they didn't have an answer? I get that. I just wanted to poke some fun

2.7T Blues

I have the exact same car as your mom. I plugged it in to an expensive OBD II reader and said switch F is bad. Replacing it today
The hard part for me was removing the lower knee panel. Didn't realize you could remove the trim around the steering wheel to get to the screws then one from the fuse panel area. Mine screws in not sure how that didn't get adjusted right the first time???