hexagone

Hey guys, need some much needed help diagnosing a electrical gremlin. The car is a 993 C4S with the brake pressure pump motor common to the 993 Turbos.

I was driving today and my brake pressure warning light started flickering and all my instrument gauges stopped working. The instrument gauge fuse (slot 18, 7.5A) was blown, replaced it, turned the key and blew the fuse again.

If I leave the green brake pressure switch plugged in, it will blow fuse 18 as soon as you turn the ignition key and none of the instrumentation will work. Weirdly with fuse 18 blown, the hydraulic brake pressure pump will whirr and create pressure like normal and the green pressure switch will tell it to stop accordingly.

If I leave the green brake pressure switch unplugged, it won't blow fuse 18 and my instrumentation works (the brake pressure warning light will still be illuminated though and the brake pressure motor won't run.)

I've checked some of the grounds, I don't have any replacement relays for R66 (I don't think the relays are the issue as I heard the brake pressure pump work with the blown fuse) and checked to see if the 60A ABS/Pump fuses were blown which they weren't. No brake fluid leaks/low brake fluid and there's nothing I remember that might have caused the issue. I suspect either the green switch is bad or there's a short somewhere in the circuit causing it to blow fuse 18.

Any ideas? Help much appreciated.

JEllis

Have any of the instrument lights, the bulbs themselves, been replaced..... might be worth checking if one doesnt match the others.

hexagone

None of the bulbs have been replaced but good suggestion. Checked all the other fuses in the fusebox to make sure they weren't blown. Going to measure the current across fuse 18 and try to find out where all that current that's blowing the fuse is coming from.

I've read on this thread that a 964 owner replaced the brake switch and it fixed his issue. Ordered a new switch in hopes it will fix it.

https://rennlist.com/forums/964-foru...to-ground.html

This was the thread where I learned it was my brake switch and not some short inside of my instrument gauges effecting the entire circuit

https://rennlist.com/forums/964-foru...o-cluster.html