Hello everyone! Another electrical question (what would I do with my 928 if I didn't have these problems to chase - drive it??) Symptoms:
1. Brake lights work ONLY when car is cold/cool. When car reaches operating temperature (1-3 miles down the road) brake lights stop working.
2. If car is allowed to cool for 1/2 hour or so, brake lights will work again - until car warms up.
3. Master Cylinder leaks at random - It may go a month without losing fluid then one day need topping up.
I have unplugged and cleaned the 2 switches on the MC and the fuse is good. What really baffles me is that the lights work fine when I first start the car, so what changes when the car warms up? Thanks in advance for the help.

Alan
1979 Auto 135K

 

Do you get a brake light warning on the dash when this happens? Do the lights fail, or just trip the warning? Without more info, I'm voting for high resistance. I would venture it's not so much that the car (proper) is reaching operating temperature, but that the use of the brake lights is heating the already corroded circuit and causing the fault.

Tried pulling and cleaning the light sockets and assembly? I had a brake warning that would happen only a minute or so after I turned on the lights - turned out to be a corroded side marker connection.

 

1) Get a 12 volt tester. The best kind for this testing looks like an old-fashioned ice-pick with a clear handle and an attached wire.
2) Drive the car until the brake lights stop working.
3) With the ignition switch on, check for 12 vdc on both sides of fuse #10. Test on the fuse holder, not the fuse. If there is no power on either side, there is a bad connection behind the fuse panel, or the ignition switch is bad.
4) If there is power at the fuse, with the ignition switch on, test for 12 vdc at the black wire at the brake light switches. If there is no power here, there is a bad connection on the black wire, possibly at F6. (Plug F on the bottom of the fuse panel - A is on the left, W on the right)
5) If there is power at the black wire, check for 12 vdc at the black/red wire with the ignition switch on and the brake pressed firmly. If there is no power here, the switches are both bad.
6) If there is power at the black/red wire at the switch, find the lamp control unit - I think behind the glove box. Check for 12 vdc at the black/red wire at terminal 9 with the ignition switch on and the brake pressed firmly. If there is no power, there is a bad connection at the lamp control unit or the lamp control unit is bad. This is the last single point of failure that would kill both brake lights.

Most likely failures are at the fuse or at the lamp control unit.

 

Alan,
If I'm reading this right ...you aren't blowing fuses?
Give us the scoop on the side markers...do they work or not? Do they shut down with the Brake lights? All the above is sage advise.
If you start cleaning sockets don't forget those sidemarker light sockets and, and the leads -female spade connectors-. Check those lens seal grooves. If the seals are bad order new ones -they actually serve a purpose. Be judicious when tightening lens covers -takes very little to crack them and water/crap can get in-.
Spend a afternoon cleaning sockets and grounds, use greenpad or maybe 400 grit -wet/dry-sandpaper.
Do the check(s) recommended by Wally, if you are reduced to tears and have to replace the Lamp Control Unit. the units are the same at least till 84' but there are different mount brackets , do not discard your old bracket or screws! The brackets all mount to the LCU but won't mount to the under dash mount point. For sure there is a difference between the 79 and 82 brackets though the LCU is the same. If it comes down to the LCU let me know...do not start yanking on the feed wire connector as not only does the bundle connector detach from the LCU there is a cap on top of the connector that will release the 12 individual wires - where do they go?- which turns a quick 20 minute swap into a project that could go on for day's. How do I know you might ask.....
BTDT! I have a spare we can haggle over, tested out Identical to the original.
My brake /marker light turned out to be the work of some demented autobody guy's who ran DRYWALL screws into the rear light assembly's to remount them after rear quarter panel replacement - they had physically ripped them out (sheer genuis)-.
After a while the threads wore thru to the conductor on several wires and when I'd hit a bump they would short and I'd blow a fuse.
Put NOTHING past a PO and some of them are pretty good at camouflage work. Look for splices especially one's with gobs of black electrical tape - for that mtter gobs of masking tape-.
Keep us posted.
HTH

 

Thanks for the advice!
James: All side lights/parking lights work perfectly. When the brake lights stop working I get a stop light warning on the dash.

Wally: I tested per your instructions and there is power at both sides of the fuse and and at the black wire at brake light switches BUT no power to black/red wire at switch. So that would mean both switches are bad. Do they usually fail at the same time? How much are the switches vs. a new MC?

John: I did clean the sockets and grounds with no improvement. Thanks for the info about the LCU; hopefully I won't need your spare. Sounds like you and I had the same PO!

I really appreciate the help.
Alan

 

Master Cylinder, P/N 928.355.011.04 $130.86

Brake Light Switch, P/N 113.945.515.G $23.60 ea.

All plus shipping.

There are reports of cheaper switches available, but there are also a couple of reports of them failing.

 

Alan,
Not sure of this, and I think you were alluding to same but... In another post someone mentioned BOTH switches came with the M/C.
This is a little far out - what again? - but,
if you pull both switches and spray them out at the fluid side with a Carb/Brake cleaner -you'll need a small tube like on the WD-40 cans for the nozzel-. It won't take much so watch the eyeballs for kickback pray.
Let it soak awhile, blow out again, soak for a minute or two then invert and tap on a couple of paper towels, cardboard...
I did this after replacing both of mine and some nasty stuff came out when I tapped it.
This might actually be a cure.
If you read the recent post's on removal of these puppies it's rather straight forward with the happiest of sHARk owners being those with small hands -I was not happy-.
Give it a shot, who knows? Besides you have a couple day wait for the new stuff if you order. and it will familiarize you with the the removal/installation procedure. 2nd time should only be 1/2 hour.
BTW, FWIW, there is a VW stamp on my original switches - same Part Number though.

 

Wally: Just the info I needed. I'll order the switches tomorrow.

John - cleaning the switches is a good idea; I'll give it a try while I wait for the new ones.

I'll let you know what happens.

Alan
1979 Auto 135K

 

Just wanted to let you guys know the outcome: I replaced the 2 switches on the MC and this solved the problem. The brake lights now work properly. (I tried cleaning the old switches with brake cleaner but it did not work).

Thanks everyone for the help!

Alan
1979 Auto 135K