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Sat Nov 16, 2013 10:09 am

I cannot figure this one out.

With the key in and turned to the first stop the brake lights come on. If I step on the brake pedal the lights will dim near the end of the pedal throw and then at the end the brake lights will come on full power.

I disconnected the black 12v brake wire at the fuse panel and the lights did not work-check ok.

Thinking it is one of the switches I first disconnected the connections to the front brake light switch and the brakes worked as they were supposed to. Then I plugged the front switch back in and unplugged the rear switch and the brakes again worked as they were supposed to. Then if I plug them both in at the same time the lights are on all of the time. This did not eliminate either switch as the culprit.

What is the next step? I am baffled!

Sat Nov 16, 2013 6:05 pm

Any ideas what to look at? The wire connectors are a white three plug socket with a black boot-I was thinking of changing the front connector to the rear and see if that did anything.

Right now I am running with the rear connector unplugged so the lights work properly.

Sat Nov 16, 2013 6:53 pm

Check to see if you've got 12V going through both switches on the master cylinder. You shouldn't. Pull back the rubber boot to test. If one switch (the rear one?) does, swap it out.

Sun Nov 17, 2013 9:31 pm

Sounds like a bad ground, if they're both affected.... Or is that a short?? :?

Sun Nov 17, 2013 10:08 pm

GI Joe wrote: Sounds like a bad ground, if they're both affected.... Or is that a short?? :?
A bad ground would cause no lights. Basically an open circuit. A short will usually cause a blown fuse. Basically a zero resistance circuit. The light bulbs offer some level of resistance.

From what's been described it sounds like a defective brake switch that has a permanently made condition. Basically a continual flow of electricity that lights the brake lights.

Looking at the schematic drawings shows that dual master cylinder has two hydraulic activated switches in parallel with each other. The latest report says removing the rear wiring connector from the MC clears the problem. That indicates the problem is being caused the rear switch.

JMHO

Mon Nov 18, 2013 7:43 am

hobie16 wrote: GI Joe wrote: Sounds like a bad ground, if they're both affected.... Or is that a short?? :?
A bad ground would cause no lights. Basically an open circuit. A short will usually cause a blown fuse. Basically a zero resistance circuit. The light bulbs offer some level of resistance.

From what's been described it sounds like a defective brake switch that has a permanently made condition. Basically a continual flow of electricity that lights the brake lights.

Looking at the schematic drawings shows that dual master cylinder has two hydraulic activated switches in parallel with each other. The latest report says removing the rear wiring connector from the MC clears the problem. That indicates the problem is being caused the rear switch.

JMHO

"Thinking it is one of the switches I first disconnected the connections to the front brake light switch and the brakes worked as they were supposed to."

He isolated both switches, and had positive results with either switch plugged in, but not both...

Or am I reading this wrong?

I agree on the "bad ground", but a dodgy ground will cause strange indications...

Mon Nov 18, 2013 10:04 am

Yes, I pulled the connections to the front switch first and the lights worked properly. So then I pulled the connections to the rear switch and reconnected the front and they worked properly again. This did not eliminate either switch. When I connected both switches the brake lights came on all the time again so I just disconnected the rear switch connections so it is working properly in the mean time.

This is what I plan to do (what would you suggest?):
1. Pull the connections off both switches and check continuity of each switch using an OHM meter.
2. Hook both switches up then remove the red wire at the brake warning light at the dash to eliminate that as a source of the problem.

This has me stumped!

Mon Nov 18, 2013 2:42 pm

74 Thing wrote: Yes, I pulled the connections to the front switch first and the lights worked properly. So then I pulled the connections to the rear switch and reconnected the front and they worked properly again. This did not eliminate either switch. When I connected both switches the brake lights came on all the time again so I just disconnected the rear switch connections so it is working properly in the mean time.

This is what I plan to do (what would you suggest?):
1. Pull the connections off both switches and check continuity of each switch using an OHM meter.
2. Hook both switches up then remove the red wire at the brake warning light at the dash to eliminate that as a source of the problem.

This has me stumped!

I misread your description before... :oops:

How about connecting the front circuit to the rear switch(alone) and vise-versa... See if either of those configurations make a difference.

Fri Nov 22, 2013 8:56 pm

Can someone take a good working brake switch and check the ohms across the terminals that are opposite one another?

I got a reading in the 7.5k range for ohms across the terminals with nothing hooked up.

When I checked for volts at the black/red wire when one switch was plugged in I did not get a reading like I am supposed to, but when both switches are connected the black/red wire had a reading of nearly 8 volts on both switches and of course the brake lights came on.

I also disconnected the dash light and that did not do anything so it eliminated that as a possibility.

I am thinking it may be the switches, but it seems strange both would be bad at the same time.

I hate electrcal troubleshooting. Still running with the rear switch disconnected so the lights work properly.

Sat Nov 23, 2013 6:20 am

Those switches are so cheap ( I have them new for around $10) I just replace them with good OE German units any time there is a question of that system’s correct functioning(s)

Sat Nov 23, 2013 7:32 pm

Is it possible that one of the switches is wired backwards? Each would work fine in isolation, but when connected in parallel, one switch's hot side would always be connected to the other switch's ground when there's no pressure, right?

(I hate trying to figure out electrical troubleshooting!)

-Larry

Sun Nov 24, 2013 10:11 am

WOW. Talk about confusing??@#$ Start here:

VW Thing Dual Brake Warning /Stop Light System with three prong dual brake switches

For safety the VW Thing has a dual braking systems
System one brakes the front wheels.
System two brakes the rear wheels.

Each system has it own brake fluid pressure. The dash mounted, brake warning switch light should come on when the ignition switch is first turned to the on position. When the car is started the light should go out. While the car is running, the warning light should illuminate if the pressure in drops. This indicates a failure such as a leak or break in one of the two braking systems. When one system fails, the other system still operates safely, hence the safety feature.

The Warning/Stop Light Wiring
Positive power comes from the fuse block on terminal #2. The black wire goes to the outside terminal of both brake switches and terminal #15 on the dash mounted brake warning light.

A red wire goes across the center terminal of both brake switches as then on to terminal # K on the dash mounted brake warning light.

Terminal #31 on the dash mounted brake warning light (as always in German cars ) has a brown wire and is a ground.

The black wire with the red bands goes from the other side of the brake switch to the bulb sockets on both tail light sockets on the brake light positive terminal.

Both brake lights sockets are grounded to the body in the rear of the car via a brown wire.

Terminal 61 on the dash mounted brake warning light via a blue wire goes to terminal #61 on the voltage regulator (on cars with generators) then through a wire connector to the generator warning light (position K2) on the speedometer.

(Borrowed from the RoMTOC Thing Data CD -permissions granted haha)

Sun Nov 24, 2013 12:03 pm

kubelmann wrote:
System one brakes the driver’s front wheel and the passenger rear wheel
System two brakes the passenger’s front wheel and the driver’s rear wheel.
Are you sure it's not

System one brakes the front wheels.
System two brakes the rear wheels.

Sun Nov 24, 2013 3:22 pm

I thought the front brake switch in the master cylinder was for the front brakes and the rear switch for the rear brakes.

The wires are not crossed-they are in the white plastic three connector push on from the factory.

I was thinking of getting some good new switches and plug those in and if the lights do not come on I will know it is the switches so I will have to remove the original ones and replace them. If the lights do still come on then I know it is not the switches and it is something in the wiring.

Sun Nov 24, 2013 5:00 pm

You are correct. I was having a bit of a brain lock and was using a Pict of one of our off road cars that is not-stock brake set-up to write the description. Front and Rear are the two systems and the warning light comes on when the pressure drops on either of the circuits. Thanks for pointing out my confusion so that we have this correctly described here.

Tue Nov 11, 2014 6:14 pm

A year later I am still having this problem. I was driving around with the rear brake light unplugged and it worked properly-then it stopped working all together.

Then plugged in both the front and rear brake lights and the same thing happened as origally posted-the brake lights were on all of the time. I checked with a multimeter on the brake light wire connector that is located under the brake fluid res in the passenger compartment and sure enough with both brake light switches wired up current flows to the rear brake lights.

Then I unplugged the front connector to the brake light switch and everything is running properly again.

Any suggestions??? Grounds??? This electrical gremlin really has me stumped.

Wed Nov 12, 2014 12:34 pm

This is just a shot in the dark. Have a look at this discussion of brake pedal free play adjustment, and this discussion of brake system pressurization.

Sat Nov 22, 2014 11:59 pm

It ended up being the front brake light switch.

Sun Nov 23, 2014 2:07 am

Only took a year to figure out!! :wink:
Glad ya got it sorted....