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Break pressure switch failure -ABS problem

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Old 01-17-2011, 02:53 AM   #1
A 8
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Default Break pressure switch failure -ABS problem

Hi all, hoping for some help.

I randomly get the ABS warning light when I start the car. I took it to a non authorized Audi mechanic who read the fault code and it said that the break pressure switch or its curcuit failed.
It took some time for them to decide exactly what the break pressure switch was but they concluded it was the switch that senses if you hit the break pedal, it's the same that turns on the break lights. They changed it but it did not help. I dismantled the old switch, 2 poles and concluded it cant be the right switch.

Anyone else have this problem or know where the "break pressure switch" is located?

Thanks/
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Old 01-17-2011, 08:09 AM   #2
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There is no brake pressure switch in your car. That is an older type brake light switch, not used in Audi's anymore. A true brake pressure switch is usually screwed into the master cylinder, a sealed hydraulic switch used on earlier VW's and Audi's.

You DO have a 2 function mechanical switch at the brake pedal..brake lights and brake position sensor, which sort of detects pressure by how far brake is pushed in (position).

You obviously need to get to a "qualified" independent mechanic (or dealer if necessary), to get qualified diagnosis...read codes, inspect obvious components (including the proper installation of brake pedal switch and communication with that switch by ABS computer), etc. ABS light could be caused by a dirty or failing wheel speed sensor at any of the 4 wheels or even an electrical problem at the ABS hydraulic unit under the hood. I would start with unplugging and reseating the the big plug on that unit...then checking for dirt/grease build up at the wheel sensors. If you go to a dealer, DO NOT be talked into replacing the hydraulic unit..they do NOT go bad and cost huge $$....but the electric module on the side of that unit can fail and can be repaired at a reasonable price by places like modulemaster online...a good indy mechanic knows this and can take care of it.
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Old 01-17-2011, 09:17 AM   #3
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I bet your mechanic pulled this code. http://www.audipages.com/Tech_Articl...itcherror.html
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Old 01-17-2011, 10:32 AM   #4
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Thanks, good info!

The mechanic pulled the code when it was active and I have used an authorized mechanic on this issue before and they pulled the same fault. I have also checked the break switch install myself including soldering the cables to the connectors just to make sure...no difference. Typically I get the problem when the car has been sitting in a damp environment for a while. It goes away after a night in my warm(ish)garage or after an hour of driving. I always need to restart to clear the fault so its not a condition that gets checked continously.

If it was the abs sensors I assume the fault codes would indicate just that so I am thinking the problem must be at the other end of this curcuit, do you know if that is at the ABS hydraulic unit or is there another control unit I need to look for?

I know the hydraulic units electrical connectors was a problem on older models but mine has the "newer" version with the connector mounted vertically.
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Old 01-17-2011, 03:24 PM   #5
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Diagrams seem to show that brake light switch portion of pedal switch goes dierctly to light control module (relay)(1.5mm wire red w/black, terminal 2 on switch) and the pedal position sensor portion goes directly to the ECM (.35mm wire white w/red term 4).
Terminal 1: 1.5mm red w/white from fuse to brake light.
Terminal 3: .35mm black w/yellow from fuse to pedal position.
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Old 01-18-2011, 02:16 AM   #6
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Good info again, I will open the ecu box and maybe reseat the connector.
Thanks/
Michael
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