The mystery of the "always on" bus lights

Alright amateur sleuths, I've got a mystery and I've run out of clues. Here's the short version: When I turn the ignition on, the brake lights come on. When the brake pedal has a light pressure applied the lights go off. When brake pedal is pressed harder the lights go on again.

Here's the facts, jack: Phred is a 1971 bus that was taken off the road about a year ago so that my friend and I could do extensive body and paint work. We are close to the end and have been reinstalling lights and trim. During this process I also replaced the front calipers on both sides of the bus as well as giving the rear drums a thorough cleaning and inspection. I bought new rear taillights as the old ones were more rust than metal. We've got everything put back together and have come across the problem with the stop lights. The turn signals are fine as are the running lights. When the ignition is on, the brake lights come on and stay on. When light pressure (resting your foot on the pedal) the lights go off. When steady pressure (normal braking) is applied then the lights go back on.

Here's what we've done: During brake installation, there was some gunk (would you believe Teflon tape, I couldn't believe it myself) in the front lines. The lines were completely done over and now the brakes work great

We checked and replaced both brake light switches on the master cylinder.

We've checked the electrical connections at the brake light switches. All three sockets (that go over the prongs of the switch) show continuity whether the brake is pressed or not.

Checked the wiring going from the brake light switches to the brake lights themselves and all is good there.

The only thing we have able to come up with after spending all day at this is that we are getting a continuous flow of current through the brake light switch. Could back pressure be a culprit? Could the problem be in the electrical connections before the brake light switch? Should we just sacrifice some BBQ to the VW gods and hope for the best? All reasonable (and maybe some not so reasonable) responses will be attempted.

I thank you and Phred thanks you.

Reply to
J Blair
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If the switch wiring hasn't been disturbed (incorrect wiring), then it could be a problem in the switches or the pressure (as you suggested).

Do this: Go to my web site and look for:"Wiring of 3-prong brake light switches ('70 and later) with warning light 1connection. "

Pull the sockets off the switches and check continuity at the switch terminals. With no residual pressure, Term #81 should go to #81a, and 82a should be open. With pressure applied, Term #81 should go to 82a and 81a should be open.

If you get conflicting readings, try loosening the switches so that they leak fluid. This will relieve and residual pressure.

If it turns out to be a residual pressure problem, check that there is freeplay at the M/C pushrod.

Speedy Jim

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Reply to
Speedy Jim

Sounds like the brake light switch is mis-wired.

However there may be a stray voltage in the circuit. With the ignition on, there should be 12v to only one contact of the switches. So check that there is not 12 volts at the other contact going to the brake lights.

If there voltage on both the contact from the fuse(normal) AND the contact to the lights(not normal), you may want to check continuity thru the switch to see if it is stuck closed(connected). However, it's always best if it is out of circuit(all wires disconnected) because it's possible to get a false continuity reading through other components in the car's circuitry. In this case, if the master cylinder brake light switches are good, they will be "open" when the brake is not depressed.

If the switches are OK(open) and you still find a stray 12 volts in the circuit to the lights, it's a process of elimination; disconnect other parts of the circuit downstream(towards the brake lights) until you find the culprit. Good luck.

Deb 71SB

Reply to
Debra Chervenka

If you actually checked for continuity at the plug, then you probably just saw that everything had some connection to ground. This would be normal with the ignition off. Some of these connections would have higher resistance than others.

What you really need to do is check the continuity of the 2 switches.

With the pedal up you should measure continuity between 2 of the lugs on each switch and the 3rd lug should connect to nothing.

With pressure on the pedal you should measure continuity between a different pair of lugs on each switch and a different lug should now be connected to nothing.

You can also do these tests:

Pull the connector off one switch. The brake lights should work properly. Do they?

Replace that connector and pull off the other one. The brake lights should work properly. Do they?

Pull off both connectors. The brake lights should not work at all. Do they?

Make sure you have some brake pedal free play.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

Well friends and neighbors, it's a lovely Sunday morning in Central Texas and I'm about to go the Church of the Immaculate Brake System. I appreciate all of the helpful suggestions and comments. You have given me some new directions for my thinking and tests. I will let you know what happens later.

Reply to
J Blair

OK time for a sandwich and an update. First I thought I would go back and rebleed the brakes so that I could eliminate that as a problem. Lo and behold one of the back brakes still had some air in the line. After taking care of that wheel I went back to test the lights again. They flickered and then went out. I still have running lights and turn signal but now the brake lights won't come on at all. I rebled the other brakes to be sure there was no gremlin in those lines either. All the other wheels turned out to be OK. Still no lights. Feeling that this might yet be a mechanical issue, I thought about adjusting the plunger into the master cylinder. As I was looking at the linkage I noticed that the wiring that led from the fusebox down to the brakelight switches was melted in a couple of places.

***SIGH***Looks like an old condition that finally gave up the ghost. I'll be fixing the wiring next and then we'll see where things stand. Master cylinder would be next is my guess. I've got the Bentley manual which is a big help though I may need a new one soon. I'm using it to within an inch of its life.
Reply to
J Blair

Just goes to show, all the advice in the world may not be exactly right on, but it can give us motivation to start digging again after feeling you've done everything!

Reply to
Debra Chervenka

well don't overlook the head light switch, I had one that was shorted making the brake lights act that way.

Mario

Reply to
Kafertoys

It's NOT a master cylinder problem.

It's NOT an air in the brake system problem.

It COULD be a problem with:

The wiring (sounds very probable)

The MC switches (possible, but unlikely)

The MC pushrod (there MUST be some play there.)

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

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