ltbordo

which one of the fuses or relays controls the brakes lights? all other lights and signals are working.

Does does anyone have a good reference/list of what all the relays control and what type of relays go into which sockets? Or are they all interchangeable?

john

Ed Scherer

How timely. (<-- Click)

ltbordo

thanks alan!

WallyP

Up thru 1983, the brake lights are operated by twin pressure switches on the bottom of the brake master cylinder. From 1984 - up, the brake lights are operated by a simple plunger switch mounted on the brake pedal under the dash.

The earlier brake light switches do double-duty, operating both the brake lights and the brake pressure warning signal. These switches can cause some problems.

The switches are identical - one in each hydraulic circuit. They are single-pole, double-throw switches. This means that you have only one circuit (single pole) thru each switch, and that the switch chooses one of two outputs (double throw).

Each switch has three connections: power, central warning computer, and brake light.

Not Activated
There is a power feed into each switch. When the switch is not activated (no pressure in the brake circuit) the power isn't hooked to anything.

When the switch is not activated, the central warning computer and the brake lights are connected inside the switch. Nothing happens, since there is no power applied on either circuit.

Activated
When the switch is activated by brake hydraulic pressure in one of the circuits, the central warning computer connection is broken, and the brake lights are hooked to the power feed. This removes the central warning computer from the circuit, and puts power on the brake lights.

(This might be easier to visualize if you think of three wires: one for 12 vdc, one to the brake lights and one to the warning computer. The lead that goes to the brake lights is connected to the one to the warning computer until brake pressure pushes it off of that wire and over to the one going to power.)

All three leads go to both switches.

When you apply the brakes, both switches should activate. Both switches disconnect the warning computer and connect the power to the brake lights. The brake lights come on, and no signal is sent to the warning computer, so it is happy.

If only one switch activates (either no pressure on one side of the brakes, or a bad switch), then things change.

On the switch that is not activated, the brake light circuit is still hooked to the warning computer.

On the switch that activates, the brake light circuit is hooked to 12 vdc.

The brake lights come on.

In addition, power flows on the brake light circuit back to the non-activated switch. Since in this switch the brake light circuit and warning computer are hooked together, 12 vdc is applied to the warning computer circuit. This set the Brake Pressure Warning light.

If you have a bad brake light switch, I strongly suggest that you replace both switches. (P/N 113 945 515 G) Replacement of the switches usually requires bleeding the brake system, so this is a good time to flush the brake and clutch systems.

neilh

Fuse 10 according to owners manual and Alan's diagram. Interestingly the 82 wiring diagram and Porsche own training manual says it should be fuse 7, and fuse 10 is not used. In my 81 fuse 7 is in, 10 is spare, and my brake lights work!

Five-8

Had the same issue last year...all lights working except brake lights...lights would only come on with an extreme amount of pressure on the pedal.
Replaced both pressure switches under the master...this was the fix for me as Wally is suggesting.Not an expensive fix, but getting at them is a bit ummm...difficult.

Cheers,

James Bailey

Just so everyone KNOWS....few relays are the same especially on the older cars and randomly moving them can EASILY burn up wiring and possibly the car !! When in doubt stop and figure out what goes where. Trial and error could be a disaster....

Alan

I will check these fuse 7/10 details - thanks

neilh

Alan, also see my post on your main CEPanel thread with screen shot of CE version info.

Alan

Yes thats useful info - I have snatched the relevant stuff - wish I had the same detail up through 1995 for the new panels - anyone?

Alan

ltbordo

neilh, my 1982 is like your 81.

turns out my 1982 has some fuses that are shown on the 1983 diagram....ie brake lights are on 7 vice 10, works fine now. thanks to everyone!

neilh

Great, glad you had an easy fix.

axis of evil

just found this thread after about a month of trying to get my brake lights working,
I thought I might have re-wired something wrong when I dumped the early twin sensor M/s, replaced with Mk 4 Supra M/S and servo and re-wired switch to pedal.
thanks for the pointer NEILH, My problem all along has been the fuse 7 and 10 confusion.
my owners handbook 1982 ROW shows fuse 7-" unused", brake lights on fuse 10.
I'd cleaned all the contacts (some weeks prior to noticing that the brake lights were not working) and reinstalled with fuses sized as per owners handbook, leaving #7 empty
Just put a fuse into number 7,
Working again!

SeanR

Timely thread, I've got one '81 and an '82 here that the brake lights have failed on. The '82 I replaced the switches two years ago and find it odd that they would have failed so quickly. Going to run some tests on both cars first and if it is the switches, well the brakes were due to be bled anyway right?

longleader

Sean, the switches on my 83 went out after 1 year. Changed them with the MC in place and problem fixed.