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Brake Light Switches and Brake Warning Light Wiring Q
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Captain Spalding
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PostPosted: Fri Sep 07, 2018 11:43 am    Post subject: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

Hello all.

There have been a couple of topics recently that require knowledge of the brake warning light. I have wanted to help in those threads, but I don't have a good understanding of what's going on in the brake switches and the warning lamp.

The Brake Light Switches:

The brake switches each have three wires apiece going to them. The black wires carry current from the no. 2 fuse. The black and red wires carry current to the brake lights when the brake pedal is depressed. The red wires carry current to the brake warning light. Is this correct? Under precisely what conditions does this occur? When the brake pedal travels too far? When the pressure in the brake lines drops below a certain threshold?

The Brake Warning Lamp:
The brake warning lamp has 4 wires connected to it. The brown wire is of course the ground for the bulb. The blue wire gets current from the voltage regulator when the key is on but the engine is off, for the purpose of a bulb check. The red wire sends current to the bulb in the event of a brake failure. Correct? So, if the blue wire energizes the bulb at startup as a bulb check, and the red wire energizes the bulb in the event of a failure, what is the black wire coming from the no. 2 fuse for? Clearly I'm missing something. Rather than receiving current from the brake light switches, does the current run through the warning light bulb and down to the brake light switches, which provide and alternate ground when a failure occurs?

I know this is deep in the weeds, and it's not exactly keeping me up at night, but if anyone can shed some light on the situation, I'd be grateful.
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mondshine
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PostPosted: Fri Sep 07, 2018 1:05 pm    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

Think of it like the three way switches in your house, where the load can be switched from two locations.
If you were to flip both switches at the same time, the light would remain off.

This illustration shows the old style warning light with the push-button lamp test, but the brake light switches function the same way as in later cars (like USA Things).
Image may have been reduced in size. Click image to view fullscreen.
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Captain Spalding
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PostPosted: Sat Sep 08, 2018 1:55 am    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

Hi Gary. It took me a long time to wrap my head around the 3-way switch analogy. I had to diagram it and trace the current path for each mode.
Image may have been reduced in size. Click image to view fullscreen.


Thanks a lot for the clarification. One question still remains about the new-style brake warning light. If the blue wire illuminates the warning lamp for a bulb check, and the red wire illuminates the lamp when there's a fault in the system, what does the black wire coming from fuse 2 do?
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mondshine
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PostPosted: Sat Sep 08, 2018 10:02 am    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

I've never had the guts to dissect a working late model warning light (fear of ruining the thing).

I do know that there is a tiny circuit board inside, so I assume that Terminal 61 provides ground when the ignition is switched on but the engine is not running (in the bulb test mode), and Terminal 15 provides +12V.

Once the engine is running, I assume that Terminal 61 provides +12V to "switch" the function of the warning light over to the standard configuration where Terminal 31 provides ground, and Terminal 81a (from the brake light switches) provides +12V.

That's my guess.
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Captain Spalding
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PostPosted: Tue Sep 11, 2018 11:36 am    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

Interesting.
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mondshine
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PostPosted: Tue Sep 11, 2018 11:43 am    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

I can only assume that it was a federal regulation that caused VW to use this complex arrangement; or change from the simple push button bulb check.
I guess it's just one of life's mysteries. Wink
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PostPosted: Tue Sep 11, 2018 6:35 pm    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

Mine has never worked, never gave it much of a thought. With this newfound wealth of info, I may actually under take to fix it. Well done guys. Cliff W. in western Pa.
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dagimp
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PostPosted: Wed Sep 12, 2018 7:54 am    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

Thanks for the info! Mine has always been on... even through a Master Cylinder and front disk brake conversion! I'm doing work on the wiring now and may test while I'm in there.
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1973 Orange VW Thing, "Otto".
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4118thing
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PostPosted: Thu Sep 13, 2018 7:45 am    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

Trying to check why I donít have light with ignition on. Bulb works, but no light.
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Captain Spalding
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PostPosted: Thu Sep 13, 2018 12:28 pm    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

mondshine wrote:
I've never had the guts to dissect a working late model warning light (fear of ruining the thing).

I came across this drawing, with the explanation that follows. A tip of the hat to Mr. "Speedy" Jim Mais, whose website is a font of information on obscure VW electrics.

Image may have been reduced in size. Click image to view fullscreen.

Speedy Jim, in an email to yours truly, wrote:
Some years back I was curious about the innards so one got sacrificed for science.

Term #61 was fed from the gen regulator or, later, from the alternator.
When key is first turned, there is no voltage on #61 (engine not running) and power
flows from #15 (fuse) thru the bulb and the diode out to #61.

When the engine starts, #61 rises to 12V level and no current can flow thru the diode.
But the transistor will turn the bulb on if the dual brake switches send a signal (+12V)
to Term "K" , which is the transistor Base.
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mondshine
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PostPosted: Thu Sep 13, 2018 2:35 pm    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

It still seems like "magic" to me.
Even since I was a young boy, I have been taking stuff apart to learn how it works. That's fine for mechanical things, but my understanding of electronics is about as deep as a diode, and no more.
In any case, Captain, that was interesting to see; thanks.
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Captain Spalding
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PostPosted: Sat Sep 15, 2018 3:11 pm    Post subject: Re: Brake Light Switches and Brake Warning Light Wiring Q Reply with quote

Each of us has his own black box to fear. Mine is syncing dual carbs.

But if you ever want to get out of yours, look for a set of books by Forrest Mims. They are simple electronics books meant for young teens. I bought my first one at Radio Shack, probably in the same year that my Thing was built. Get a hold of those PDF's and within a couple of hours you will have a perfectly adequate 14-year-old's understanding of electronics. Don't worry. You'll still think it's magic after you've read up. It will just seem a little less mystifying.
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