Skunkworks

I have an 82 US 928 (auto).

The Stop Lamp warning indicator is lit on the dash (does not go out). I have no brake lights. Simultaneous to this occurring the MC started to fail. I have replaced the MC and have a firm pedal. I have checked and cleaned all connections at the brake lights and tested -- the lights are good. Still no brake lights.

I expect the pressure switches could be faulty, but I do not know a way to diagnose. I'd like to drive the car down from Atlanta to Sebring in March (but not without brake lights). Any help is greatly appreciated.

David

jpitman2

Does the dash show 'Brk press' or similar? If so this can be caused by either of the brak light switches failing, or not firing together - BTDT. You need to disconnect the battery briefly to reset this warning light. If in Any doubt, replace both together (cheap items - Audi/VW badged - seen on ebay ). I replaced mine one at a time without needing to do any bleeding. Seal the top cap with cling film to prevent air entry, remove and replace 1 at a time (24mm hex?) DO NOT overtighten. getting the wiring plugs back in is the hardest part IMHO. A bit of pedal work usually removes any air that gets into the lines, along with alowing to stand overnight. As a temporary measure to get brake lights, you could disconnect the rear switch, and not get the warning.
jp 83 Euro S AT 53k

Skunkworks

The dash does not show "Brk Pr" or similar. As well, I have disconnected the battery ground for more than an hour to reset. No luck! Only the Stop Lamp warning on the dash. Is there a way to check the pressure switches? Also, where do you recommed purchasing the pressure switches?

I'll try disconnecting the rear switch to see if I get the lights.

Thanks,

David

Skunkworks

Just tried disconnecting the rear MC pressure switch. No joy... still no brake lights.

David

jpitman2

Check out this thread -
https://rennlist.com/forums/928-foru...ing-light.html
- part no there for the VW switch, also they can be had from the usual suspects. Also notes from WallyP which I used when I had an issue....Check that there is power at the switch connectors...
jp 83 Euro S AT 53k

dr bob

Start with easy stuff, like the fuse. There are two switches, either of which will cause the brake lights to work if the fuse and wiring are good. The fuses are a bit temperamental as they age. Sometimes it just takes a twist of the fuse, sometimes a bit more, like cleaning the holder tabs a bit and fitting a new bright shiny fuse instead of that corroded one.

Fuse 7 is the one for brake light power. That fuse is opowered from the X bus. Lots of things come through the X-bus like fans and fog lights, so if those are working the X-bus has power. From fuse 7 in the CE panel, power flows out at terminal F6, through two switches in parallel, then back into the CE panel at F7. That point is jumpered internally to S6, which goes to the bulb control unit, and also externally to the cruise control unit. The brake light bulbs are fed from the bulb control unit and a couple connectors in the tool panel area to the sockets at the lamp assy's.

chrly924s

I got new pressure switches at Autozone for about $12ea. Have someone watch brake lights while brakes are applied. Then jiggle the ignition switch in the 'run' position to see if they come on. Bad contact in switch can cause trouble w/lights, windows.

dr bob

Ignition switch drives X-bus relay. If fogs/wipers/etc work, X relay is working. Check for power at the fuse holder; should be live both sides with key in 'run' position, brake pedal pressed. Start with the simple stuff!

Skunkworks

According to the fuse chart in the manual, the brake lights and cruise control are both on the circuit controlled by fuse #10 (8 amps). With the ignition on, I have power on both sides of the fuse (the brake pedal does not have to be pressed). Where do I go from here?

David
82 928 Auto

jpitman2

Check for +12V on the connectors to the switches at the m/cyl. Shorting that to one of the other pins should give you brake lights. Both connectors MUST have +12V on one connector. The wiring diagrams should show that the switch at rest has 2 pins connected, and when operated, the connection moves to the 3rd pin.
jp 83 Euro S AT 53k

Skunkworks

I checked for power at the MC pressure switches (with the ignition on). Neither one has power at any of the pins. When I introduced power to one terminal, I was able to light the brake lights; power to another teminal caused the Brake Pressure light to come on at the dash. From this, it seems I am not getting power to the switches but the wiring to the stop lamps and dash warning system is correct. I do have power across the fuse. Any ideas?

Thanks,

David
82 928 Auto

jpitman2

You will now need to trace the supply line from the fuse board to the switches. Curious that both connectors are out - dont recall where the two +12V lines split, it might be close to the m/cyl? On my car when I had the issue (Brk Pres light), I found the connectors had been twisted up too much and the wires were very tightly entwined, as though the switch had been fitted with the connector already on it.
If you can locate where the wires run from the switch to the fuse board, you may readily find the break, or maybe looking on the back of the fuse board will show something? Pull the fuse, and verify with a meter that there is no connection from the fuse board to the connectors. Otherwise , here is what i have done in the past. See if you can beg/borrow/steal a cable tracing device as used by phone or network cablers - Fox'n'hounds is one brand. disconnect car battery. connect a lead from the tracer to a pin in the connector that should have +12V on it, set it to generate tone - this sends a warble note down the wire. The tool has a 'listening' device that picks up the tone by induction - run this along the wires until the tone stops - it may not stop suddenly, but peter out a bit - move listener back and forth to try to localize the failure point.
HTH
jp 83 Euro S AT 53k

WallyP

Locate Terminal F6.

This will be on Plug F - plugs are alpha, left to right, so F is the 6th plug from the left end. Terminals on plugs are arranged

4 8
3 7
2 6
1 5

There will be 1, 2 or 3 black wires in F6. With ignition switch on, check for power on the black wires. One wire goes directly to the brake light switches, one supplies power to the cruise control box, and one supples power to the Lamp Control Unit.

If there is no power on F6, the problem is in the fuse connector or in the gray wire from the fuse connector to the socket for Plug F.

If there is power on F6, the problem is a broken wire between F6 and the switches.

I'm betting on the fuse connector.

Skunkworks

Solved! Utilizing the directions from Wally, there was no power on F6. I then took a look on the back of the panel to check the gray wire from fuse connector to the socket on F6. Holy Cow! Almost all the wires (dozens) were gray on the back of the panel and it looked like a wiring nightmare. However, I was able to trace the wire from F6 and for whatever reason, the wire was not connected to Fuse 10 as indicated in the manual. The wire went to Fuse 7 (which was listed as unused). I simply moved the fuse from 10 to 7 and voila!

Thanks so much for the help! I now have brake lights!

David

ROG100

I have been trying to solve a problem on my 82 after I cleaned all the fuse holders and replaced all the fuses with new.
The fuse chart in the manual states that fuse #10 covers the brake lights and the fuse #7 is NOT used.
I left out fuse #7 - I read that Dr. Bob says that the #7 is for the brake lights and sure enough put an 8 amp fuse into #7 and my brake lights work again.