Brake Light Switches and Brake Warning Light Wiring Q

[Captain Spalding]

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.

[mondshine]

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).

[Captain Spalding]

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.

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?

[mondshine]

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.

[Captain Spalding]

Interesting.

[mondshine]

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.

[[email protected]]

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.

[dagimp]

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.
_________________
Buck

1973 Orange VW Thing, "Otto".

[4118thing]

Trying to check why I don’t have light with ignition on. Bulb works, but no light.

[Captain Spalding]

[mondshine]

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.

[Captain Spalding]

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.