924Board.org

Chrenan · Joined: 15 Jan 2003 Posts: 3903 Location: Canada

Hello Everyone,

I was changing a tail light lens this evening and I learned my brake lights don't work (don't worry, I haven't been on the road with this problem).

The indicator, marker and reverse lights all work fine. However, the left and right brake light won't come on when I step on the brake. The bulbs are fine, I swapped them out with ones I know work. I cleaned the connection plates to the bulbs as well, no luck.

I find it funny that both brake lights don't work. Could it be a problem with the brake light pressure switches on the master cylinder? If so is this something an amateur can diagnose and fix?

Could it be a fuse problem? The Haynes manual is none to helpful on this issue.

Any tips and tricks from te pros would be appreciated...

Thanks,

Chrenan
_________________
1987 951 - M193 Version for Japan

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2745 Location: Sydney, Australia

Check fuse 6. If that's blown your headlights won't pop up.

It's unlikely to be the master cylinder pressure switches. Any problem there should cause the brake warning light to come on. The switches are joined before they go to the brake lights so they'd both have to fail for the lights not to work.

Drag out the Haynes and the multimeter and look up the wiring diagram for the '79/80. It's a very simple circuit. Measure the voltage (with the brake pedal pressed) at both the red/white and black/red wires coming from the master cylinder switches. If you get 12v there check at the connector near the left rear lights which connects a black/red wire to two red/black wires then at the lights.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

kabok · Joined: 25 Jan 2003 Posts: 179 Location: Ft. Collins, CO

I would check the fuse before looking at the brake light switch. Here's a way to find if the light switch is the problem. Take the light switch connections off and figure out which one is the live and which one is the by pass ground. If you connect these two with a wire, the brake lights should come on. I am not sure which pin numbers are which. Look at Haynes for this.

Lizard · Posted: Fri May 02, 2003 3:26 am Post subject:

check to make sure there isn't alot of corrosion around the bulbs not allowing electricity to pass through
_________________
3 928s,

Chrenan · Joined: 15 Jan 2003 Posts: 3903 Location: Canada

Should I be looking at Fuse 6? Peter suggests if this fuse is a problem then the headlights won't raise, but the headlights go up and down and turn on with no problems...
_________________
1987 951 - M193 Version for Japan

Roger · Joined: 06 Jan 2003 Posts: 1235 Location: Cordova, TN

First check with a know good bulb. Second check to make sure you have 12 volts and a good ground at the bulb holder. Then check your fuses.
_________________
1981 924 NA

Some people are like Slinkies. Not really good for anything, but you
still can't help but smile when you see one tumble down the stairs.

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2745 Location: Sydney, Australia

the brake lights are powered via fuse 6 same as the headlight motor. The power to fuse 6 and then on to the master cylinder switches is unswitched (not switched on by the ignition key) so put a brick on the brake pedal and start checking for voltages.

One of those $2 auto wiring test lights would be quickest. The ones with the pin that pierces the wire insulation and the alligator clip for earth.

I'd suggest starting at the master cylinder. and working towards the rear until the light goes out. Testing with a light is best because with corroded connectors a multimeter can still read 12 - 13v but there's not enough current (amps) to actually light a globe.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

numbbers · Posted: Fri May 02, 2003 10:50 am Post subject:

Well Peter, my brake lights quite working on my '80 and it turned out to be the pressure switch on my master cylinder. The brake warning light never came on.
_________________
1980 924 Turbo

J1NX3D · Joined: 06 Feb 2003 Posts: 1333 Location: New Zealand

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2745 Location: Sydney, Australia

OK. That was my reading of the wiring diagrams - that a master cylinder switch failure would light the brake-check light (unless both switches failed). Learn something every day.

_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

Chrenan · Joined: 15 Jan 2003 Posts: 3903 Location: Canada

Thanks for the help everyone,

I figured it out last night. Got out the multimeter and had two hours of fun with the 924 wiring. Turned out the little wire underneath the emergency brake handle was disconnected from its switch. Without this the brake warning light on the instrument panel and the brake lights would not work. Reconnected the wire and all is well. In the process I also scraped down all the grounds and connectors I came across and checked and swapped all bulbs where needed and fixed all four corner lights as well. Now the car lights up like a Christmas tree.

During my adventures in wiring land I noticed the PO had inserted a 25 A fuse in a slot that should have had a 8 A fuse. I swapped it for an 8 A fuse. What is the danger of having a 25 A fuse in there instead (probably a bit of a dumb question).

On another note, was the 924 wiring setup designed at the peak of Oktoberfest?

Chrenan
_________________
1987 951 - M193 Version for Japan

Smoothie · Posted: Sat May 03, 2003 1:18 am Post subject:

Fuse ratings and wire sizes are chosen based on the amp draw of the components on a circuit and the fuses are there to protect the wiring from going up in flames in case of a short circuit or failing component that might cause an increased amp draw. If nothing happened to fry while you had the 25A fuse in there, then there's no problem.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

numbbers · Posted: Sat May 03, 2003 9:43 am Post subject:

Was the 924 wiring setup designed at the peak of Oktoberfest?

Well, the Germans design great mechanical things, but they are not known for their electronics. Ever own an Grundig stereo?[/quote]
_________________
1980 924 Turbo

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2745 Location: Sydney, Australia

congratulations! The hardest part about fixing electrical problems is actually starting.

Really, German wiring is not that bad, especially when you compare it to Italian or British wiring of the same period. The only wiring that's any good after 20 years is Japanese because they totally over-engineer and keep it simple.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

HahnW · Joined: 03 Nov 2002 Posts: 11 Location: Poulsbo, WA

My first post in about 2 years, but I have the same problem.

I determined that both pressure switches are faulty (bridged the contacts on the plug with a paper clip and the brake lights came right on), but I am afraid to remove the switches in case the brake fluid comes spewing out and I will have to bleed the system.

How exactly do you replace the brake light pressure switches?
Do you need to bleed the brakes afterwards?

Also, where do I buy new pressure switches?
_________________
Onyx Green Metallic 1980 924NA
5-Bolt, 5-Speed