05 Audi A4 Brake Light Switch DTC 526

Alex Technician Denver, Colorado Posted   Latest   Edited  
2005 Audi A4 2.0L (F BPG) 6-spd (01X) — WAUDF78E55A527572
Brake Light Switch

I have a DTC 526 brake light switch circuit open. I tested the old brake light switch and found it was not switching the 12 volts from fuse 282 to 0 volts on pin 2 of the brake light switch connector when the brake pedal was depressed. The brake lights stay on at all times. I replaced the brake light switch and the brake lights are still staying on whenever the key is on or engine running. 

I rechecked the circuits and all relevant fuses are good and I have 12 volts on pins 1, 2, and 3 with the brake pedal released. When the brake pedal is depressed, now pin 2 (white/red) goes to 0v as it should. 

The problem is I have 8 volts on pin 4 (red/black) with the pedal released, and it goes to 12 volts when the pedal is depressed. If I disconnect the electrical connector from the brake light switch, the red/black wire has 8 volts on it. This wire goes directly to pin 25 of the engine control module. So it appears the ECM is supplying 8 volts to this wire at all times, and when the brake pedal is depressed this wire goes to 12 volts from fuse 14.

My question is, shouldn't this red/black wire to ECM pin 25 have 0 volts with the brake pedal released, and 12 volts with the pedal depressed? Ive inspected the wiring all the way to the ECM and there doesn't seem to be a short to voltage anywhere. I haven't inspected the electrical connector at the ECM yet because I'd have to drill rivets on the connector bracket to do that. Any help will be much appreciated.

Obie Technician
Spokane, Washington
Obie Default

There is a procedure to install those switches. If not correctly done the brake lights will stay on. Check the installation and if the switch is aftermarket get an OE and try again.

Default Ð Bounty Awarded
Justin Technician
Concord, California
Justin Default

My first thought was to check how you installed it (make sure adjusted proper) then maybe check to see if a tail lamp filament is shorted to brake lamp circuit

Default Ð Bounty Awarded
Victor Owner/Technician
Doral, Florida
Victor Default

Check for shorted wiring or incorrect bulb.

Default Ð Bounty Awarded
Bill Owner/Technician
Jackson, Michigan
Bill Default

Possible switch instillation error (reference service info for switch install) or shorted/incorrect bulbs

Default Ð Bounty Awarded
Alex Technician
Denver, Colorado
Alex Resolution

Thanks to all who replied. I checked pin 15 of T32B at the vehicle electrical system control module and found 0 volts when I should have been seeing the same reading I had at the brake switch connector. I found the red/black wire broken about 6 inches away from T32B. This red/black wire from the brake switch splits and goes to the electrical system module and the ECM. The 8 volts I was seeing…

Resolution Ð Bounty Awarded