I was driving home today and out of the blue i would lose power to all my interior electronics every time I touched the brake pedal... The radio would turn turn off and the clock would get reset and the gauge lights would go out. It would only do this for a split second and then everything would come back, even if i was holding the brake down ( i.e. it didn't matter if i just tapped or held the brakes, they would just go out for a split second and come back). I noticed that the voltage meter in the dash would come down from its normal running position of around 13.5 volts or so to 12 for a split second and then go back. I also noticed the transmission would make a clicking sound every time the brakes were touched ( automatic ). It happend even when i was sitting still and in park. It did this all the was back home until i turned it off and started it again, now its running fine. I checked all the fuses and they are all fine so i don't know what it could be. I haven't messed with anything electrical recently and it appears to have developed over night. I'm kinda scared to drive because i don't know whats wrong and i don't want it to happen again. Any insight would be greatly appreciated.

1990 s5 n/a automatic (gxl)

 

Sounds like a short in the wiring related to the brake switch.

 

There are three plugs at the CPU. The one with just four wires is for the brake lights. Unplug it and press the brakes and see if there is a change for the better. Also, before unplugging the plug at the CPU, idle the car and honk the horn w/the lights on and see if the lights flicker like they do when the brakes were depressed. The horn circuit and the brake circuit are related. If the same condition occurs w/the horn honked then the issue lies before the brake switch but after the 20 amp Stop fuse.

 

Its been a few days and it hasnt happend since... I will take a better look at it here when i have some time and try what satch mentioned. Thanks for the replys though.