TheSamba.com :: View topic

Mon Nov 16, 2009 4:21 am

So I'm looking at my dash and wondering why the brake warning light has never worked since I bought the car.

I go on over to Speedy Jim's website (best place for acvw electrical references) and look up how the warning light circuit is supposed to work:

Seem straight forward enough. Power routes thru the good switch to the switch that has no pressure to light up the dash indicator.

Ok, well what happens when you have just a single 3-prong switch installed?

{sorry the fuel hose got in the way so you can't clearly see the rear fitting, but you can see the brake line going into the firewall}

Note that this RHD master cylinder only has a fitting for a single switch. It is the later dual circuit system (front & rear) and as far as I can tell was made this way. I've not seen any other RHD late style master cylinders so I don't know if all of them are like this, or not.

Can anyone tell me how I get my brake warning light to work?
There might be a fitting on the top for the early type 3rd switch just for the warning light (1968-ish?), I have to dbl-check.

Mon Nov 16, 2009 5:16 am

Just a thought, but are you sure the bulb is even good? And just looking at the wireing I would say it doesn't look like having only one hooked up would make a difference.

Since I never paid attention to the warning lightbefore..... if you were say bleeding your brakes should the light come on? If so, I dont think my light works.....

Mon Nov 16, 2009 6:26 am

With only one three prong switch connected, connecting the black power wire to 81(common) and the red/black brake light wire to 82a should work.

Mon Nov 16, 2009 6:27 am

one WORKING brake light switch is all you need. if the switch is working, there is power going to the switch, and the brake lights in the rear are grounded and working then you should have brake lights. you can even wire in a two prong if your not worried about the dash warning light.

Mon Nov 16, 2009 7:23 am

Ashman,
I have a RHD '71. Similar set up. The original MC came with 2 switches, but the replacements have only 1 switch on it and that's what i have as well. As other have pointed out the switches are in parallel and will work with just one.

Couple of things to check -- first -- you are checking the light by pressing on it right? Then check the continuity of that thin red wire that goes all the way back to the light on the dash. On mine it had been cut off somewhere behind the fresh air box.

Can I ask you a favor? Can you tell me how the wires from the brake MC are routed into the trunk? i.e. they go along the firewall towards the pass side. Then where does it go? Trying to get mine routed like stock.

BN

Mon Nov 16, 2009 7:32 am

to make the dash warning light work according to this drawing you need two 3 prong switches. try plugging a 3 prong switch into the empty brake switch plug

Mon Nov 16, 2009 8:07 am

This diagram seems inaccurate -- it would suggest that if both front and rear circuits failed, the dash light would not light up.

Per Bosch designations - #81 is the input, #82a is the output on make contact, and 81a is the output on break contact. So, wiring shown above is not correct.

I don't know what the internal wiring of the switch is but they are wired in parallel and work independently.

If your brake lights are working the switch is working IMO.

Mon Nov 16, 2009 9:57 am

RA 70 wrote: Just a thought, but are you sure the bulb is even good? And just looking at the wireing I would say it doesn't look like having only one hooked up would make a difference.

Since I never paid attention to the warning lightbefore..... if you were say bleeding your brakes should the light come on? If so, I dont think my light works.....
Not sure if the bulb is good, or not... I've not tried to take the warning light assembly apart. It looks like I have to take the dash apart to get to it.

I've never had the ignition ON while bleeding the brakes so I don't know if the light will light up. I'll give it a try the next time.

Luft kühl wrote: With only one three prong switch connected, connecting the black power wire to 81(common) and the red/black brake light wire to 82a should work.
I think then the brake warning light would be ON whenever I wasn't pressing the brake pedal.

pafree wrote: one WORKING brake light switch is all you need. if the switch is working, there is power going to the switch, and the brake lights in the rear are grounded and working then you should have brake lights. you can even wire in a two prong if your not worried about the dash warning light.
I think you missed the point. The rear brake lights ARE working with just a single 3-prong switch.
It is the brake warning light on the dash that has never worked. I'm trying to see how I can get them to work w/ just the single switch mounted to the MC.

bnam wrote: I have a RHD '71. Similar set up. The original MC came with 2 switches, but the replacements have only 1 switch on it and that's what i have as well.
Thanks, I didn't know this. I actually thought VW cheaped out and built them w/ just one switch. I can redirect my frustration at the OEM and aftermarket parts makers !! :twisted:

bnam wrote: Couple of things to check -- first -- you are checking the light by pressing on it right? Then check the continuity of that thin red wire that goes all the way back to the light on the dash. On mine it had been cut off somewhere behind the fresh air box.
The light in the dash which should be the warning light does NOT light up when I press it. I've read/heard that pressing the light will test the bulb, but to be honest, the light does not feel like a switch at all. It doesn't move and feels like a plastic bulb cover, not a switch.
I'll check the wire lead and test to see if it is grounded. If its not, then maybe the bulb is burnt out.

bnam wrote: Can I ask you a favor? Can you tell me how the wires from the brake MC are routed into the trunk? i.e. they go along the firewall towards the pass side. Then where does it go? Trying to get mine routed like stock.
I will take a pic next time I'm in there. Someone else had a similar question as they were routing their horn wires. I believe there is a hole somewhere along the left side of the gas tank, but I'm not certain.

justtoby wrote: to make the dash warning light work according to this drawing you need two 3 prong switches. try plugging a 3 prong switch into the empty brake switch plug
If you look at the picture of the master cylinder above you will see that there is a switch screwed into the front of the MC, but where you would expect the rear switch to screw in... there is a solid flat face. No hole was ever made in the MC for a rear switch. I don't think it would be a good idea for me to drill/tap my own hole. :wink:

bnam wrote: This diagram seems inaccurate -- it would suggest that if both front and rear circuits failed, the dash light would not light up.

Per Bosch designations - #81 is the input, #82a is the output on make contact, and 81a is the output on break contact. So, wiring shown above is not correct.

I don't know what the internal wiring of the switch is but they are wired in parallel and work independently.

If your brake lights are working the switch is working IMO.
I would guess that in a single circuit failure you would get a warning light.
If both circuits failed... you wouldn't CARE if there was a light ! :shock:
Just kidding ! :D

After reading all of the above I would guess that the original VW MC had two switches and the brake warning circuit originally work, but with the current (single switch) replacement MC, I still have a non-redundant brake light system... which may not work if the front circuit fails. I also give up the brake warning system since it cannot work with a single 3-prong switch.

Next time I'm under there I'll take a multimeter and test the terminals to verify the connectivity, but I trust Speedy Jim's details. He's never been wrong !

Thanks everyone for your inputs.

Fri Nov 20, 2009 3:47 pm

Kinda dragging this back up again, but I have a brake warning light assembly that I know works. But it won't work in my car. The brake lights come on and work fine, but no warning light. What exactly should I check out? Everything on my car is stock, no aftermarket switches. My warning light assembly is the newer style, that is not a push button. I'm about 80% sure it was wired up correctly, maybe that's my problem? The one original to the car was fried and kinda melted, dunno if that has anything to do with it.

Sun Nov 22, 2009 4:42 am

The brake warning light on dash has two function.

1. Bulb testing to see bulb is not blown
2. Brake failure warning.

Up to 1971, you should push the red light in while ignition is on to see your bulb is working. If it's made after 1971 it has a self testing circuit, which is connected paralel to charging light on speedo. So when ignition is on brake warning light goes on, and after engine starts then it goes off. First make sure if your bulb is okay.

Your tandem master cylinder has been changed once, because the wiring you have is for double sender unit, but the new tandem has only one sender, and for the other sender there is no hole to install second one. So you should machine another hole same thread size or change tandem cylinder with two sender version. You also need two senders with 3 prong connectors.

The sender you have may also failed and not making your brake lights work on rear.

Sun Nov 22, 2009 2:35 pm

Replaced my MC and both brake light switches on my '70 std a few weeks ago. Didn't find out 'til today that my previously non-working brake test light is now working !!! Don't even know why I pressed it...thought for sure the bulb was out. Both my old switches must have been bad...

Sun Nov 22, 2009 7:02 pm

I found some photos of when I had the original brake warning light in place, and where all of the wires go. Even properly wired up, the light won't come on (after '71 not push-button unit). My brake lights work though...so shouldn't my switches still be good? I traced the wires, and as far as I could tell, they all work. What two wires should I check for voltage?