Brake warning light coming on

[rcjunky]

I recently bought a 1970 Beetle and the right rear tail light was not working properly, like the wires were not going to the correct bulbs. I got the lights working correctly now, but now the brake warning light in the center of the dash lights up when applying the brakes. If I take the tail light bulb out of the socket the dash light does not come on. Tried a different bulb and still the same. So I would think its getting some type of back feed from the wiring going to the socket. Any ideas what to look for or how to fix?
Thanks

[FreeBug]

On the rear light, do not simply content yourself with making it work. Check the wiring, which wire goes where from the colors and a diagram.

The brake warning light comes on when there is a pressure problem in the master cylinder, normally. Try looking there.

[rcjunky]

Thanks for the answer, I understand what your saying. That's what I was starting to look, the brake pressure. Its just weird that the warning light didn't come on until I got all the functions on the tail light working correctly. Then also when I remove the tail/stop light bulb out of the socket, the dash light does not come on. When I unhook the rear brake pressure switch from the master cylinder it does not light. Then with just the rear master cylinder pressure switch plugged in the dash light glows dim when the brakes are applied. But with both pressure switches plugged in the light is bright. But it just seems like its getting something off of that bulb socket since it does not light when no bulb is in the socket.

[baldessariclan]

Check and clean all connections and grounding points at the the taillights, and etc. — with poor connections / grounding at those locations, you can get all sorts of weird effects as the voltage seeks alternate paths through the brake system wiring and circuitry...
_________________
1971 Standard Beetle — fairly stock / driver

baldessariclan -- often in error, never in doubt...

[KTPhil]

[rcjunky]

Thanks for that info, I'll look at the grounding conections. The pedal feels firm and stops great. It has the dual 3 pole switches on it.

[Dark Earth]

Here's a link ...

https://www.thesamba.com/vw/forum/viewtopic.php?p=8119664
_________________
My Build: '69 Baja - Dark Earth Version

~I'm almost done. I just lack finishing up.~

[Luft kühl]

[ashman40]

It sounds to me like you have your brake switches or taillights wired incorrectly.

Normally when you press on the brakes it closes BOTH brake switches allowing current to flow from the fuse box, thru the brake switches, thru the brake lamps to ground. Current flows towards the rear of the car.

If your case, it sounds like current is flowing FROM the taillights backwards to the brake switches. Or current coming into the brake switches are being incorrectly routed to the brake warning lamp.

Double check the wiring at both taillights. Here is a pic of the taillight wiring from Speedy Jim's site:

Note that the parking lights and brake lights are two filaments in the SAME (dual-filament) bulb. Make sure you have the correct 1157 bulb installed in the middle position of the taillights. You would not be the first to discover they incorrectly have a single-filament 1156 bulb installed in the dual-filament socket. Look in the socket and at the bottom of the bulb; the number of contacts should be the same. Also, this bulb will only fit into the socket one way. The prongs at the bottom of the bulb are offset. Look at the grooves in the socket and the prongs on the bulb. Make sure they fit correctly.

Here is a pic from Speedy Jim that shows how the brake switches work:

Use your VM and a jumper wire to test all the wires at the two brake switch plugs when they are completely disconnected from both switches and when only one or the other is connected.

With the ignition ON, there should be constant 12v at the black #82a INPUT wires at both plugs. Make certain the black wire is connected to the correct brake switch terminal (sometimes the wires are installed in the wrong position). Unlike the 2-prong pre-69 switches, THE WIRES ARE NOT INTERCHANGEABLE. Each wire must be installed in the correct position on the switch. Follow the diagram above.

Test both the #81 and #81a wires at the plugs. These are OUTPUT wires and should NOT normally have voltage. Repeat the test with the parking lights ON. If voltage is comes in on the #81 wire it could mean bad grounds at the taillights.

Using a jumper wire, connect the #82a and #81 wires at the plug. This simulates stepping on the brakes. With the ignition ON the brake lights should turn ON. Test this individually for both brake switch plug.

With both plugs disconnected, using a jumper wire, connect the #82a and #81a wires at one of the plugs. This forcibly powers the brake warning lamp. With the ignition ON the brake warning lamp should be ON. Test this individually at both brake switch plugs.

You can simulate a brake switch failure by disconnecting just one plug and using a jumper wire to connect the #81 and #81a female connectors in the disconnected plug. Step on the brakes and the brake warning lamp turns ON when the rear brakes turn ON. The brake warning lamp becomes a 3rd brake lamp. Repeat the test on the other plug.

Looking at the diagram you and see that with one switch disconnected and no jumper wires there is no way current is supposed to make it to the brake warning lamp over the #81a wire. The only way current normally reaches the brake warning lamp is with current flowing thru one switch and backwards thru the failed switch. If you disconnect one of the plugs from the switch there should NEVER be voltage on the #81a wire. Try to identify how your brake warning lamp wire (#81a) is powered?


Also look for stray wires. I recall someone finding one of the wires inside their brake switch plug/boot had become frayed and was shorting between wires. This requires opening up the plugs and closely looking at all the wires.
_________________
AshMan40
---------------------------
'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}