924Board.org

[pockygt]

1981 931, the brake lights don't work. The bulbs work, the switch works, the sockets are clean, the plug going to the light panels/cluster is clean, and I get some voltage at the brake light switch plug. I can't figure out what is up with my lights not coming on.

Is there anything else obvious that I should check? Or should I just re-wire the lights up? I hope it doesn't come to that
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1981 924 Turbo (931)

[Stampedetrail]

Try the ground connection. Really!
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1977.5 924 "Martini" head / New Old Stock rebuild
1985.5 944 Why? Because I can.

[Ozzie]

using the assistant or block of wood, keep the pedal pressed and check volts at the light sockets.
Bad earths will cause problems.
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Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[Mike924]

Do you have a multimeter/DVM/DMM/voltmeter? It's never easy tracking these soerts of problems without one.

How did you determine the switch was working? Are you getting the 12 V feed at the bulb holder? At the switch? All the lights at the rear share a common earth connection, IIRC. So, if this is the problem, all the lights will be affected.
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1985 Porsche 924 'Lux', Kalahari Beige (my ex)
1993 Porsche 968 Coupe, Midnight Blue, 6 spd

'There is no substitute for a little grease under your fingernails.' - Chrenan, 924board.org

[pockygt]

I do have a DMM. I determined the switch was working because i measured infininate ohms when not on the pedal, 0 when the pedal was pressed.

when measuring the various places for voltage, what kind of readings should I get? 12v at any/all of them?
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1981 924 Turbo (931)

[Smoothie]

Yes, you should find approx. 12V at all of the re/bk wires when the pedal is pressed. -But be aware of a little trick the wiring can pull on you - you can have a poor/partial connection that allows 12V to pass and register on your meter while not allowing sufficient current through to light the bulbs fully - so a test lamp would be better for testing in this case.

To test the grounding, you could do an ohms (resistance) test from the ground side of each of the bulb holders (should be brown wires) to the ground point at the rear (inside the luggage compartment in the area between center of rear and the left rear light)(- that'd be ground point #2 (ref. Haynes manual pgs. 294, 293, and 257)) If you get 0 ohms, the ground is good. Infinite ohms would indicate a bad ground - clean that ground point and all the wire end connectors attached to it. -Actually, clean that ground point either way - it's too easy and might solve some unknown or soon-to-be problems.

If the ground was found to be good, you might have burnt/oxidized pins in the back of the fuse-relay board blocking the 12V+ side of the circuit. The pins associated with the brake lights are F20 and A16 - that's pin 20 of connector F and pin 16 of connector A. Also check 6-pole connector T6a that should be hanging in the area of the fuse-relay board (this info is from Haynes pgs. 294 and 257 and the arrangement of the connectors A-F is shown on pg. 237).
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"..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

[pockygt]

Okay,

At the connector on top of the brake pedal, i removed the plug and tested that plug for voltage and got .01/.02V.

The grounds appear to work, as the entire cluster works, and the grounds back there are very clean already.

I will certainly check the relays mentioned. Do I have to check behind the block, or can i pull the relay out to check that way?

And if all of that fails, can i just run a +12V to the switch and put it on it's own seperate fuse? If yes, where do the wires coming off the plug run? on mine, there is one connection with one wire coming off of it, and one connector with two wires.
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1981 924 Turbo (931)

[Mike924]

[Smoothie]

Check your fuse #6.

Do you have a copy of the Haynes manual?
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"..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

[pockygt]

fuse 6 is good, and I do have a Haynes manual. Thank you for the page numbers, because I went through the whole flow diagrams and went "WTF? Where's my year??"

As a temp fix, i ran a +12V to the pedal switch and connected the half of the plug that went to the lights. I feel so much safer now.
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1981 924 Turbo (931)

[Smoothie]

Yah, brake lights do come in handy - nice fix.

To continue troubleshooting the original problem and assuming you were reading that .01/.02V from the re/wt wires at the brake pedal switch, just check other points upstream from there for voltage. Next I'd check the voltage right at the fuse from one end of it then the other - if voltage is good at the fuse, then the problem is back the other way, so check D21 on the fuse-relay panel (yes, it would have to be disconnected, some relays unplugged and the panel removed to get a good look at and clean any oxidized/burnt pins**). If voltage differs from one side of the fuse to the other, then the fuse would be the problem (even if it doesn't appear to be burnt out) or the fuse holder might need cleaning-tightening.


**-(if doing this, number all the relays with the numbers that appear on the panel when each relay is removed and letter the connectors as they're shown in the Haynes (bottom of pg.237 (that'd be "A" through "F" from right to left if laying on your back looking up at them)) and note where all of the "G" connector wires go on back of the panel and label all the "H" wires (the G's go to specific G terminals, while the H's can go on any H terminal they fit on (disconnect the battery first - especially before touching any H's because they're hot with 12V+ all the time))))
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"..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