Brake lights not working - '80 911SC

[sigep174]

My brake lights have stopped working on my '80 911SC

Despite the fact that my taillights and turn signals both work, I went ahead and replaced the associated bullet fuse. Didn’t help.

Also, it was suggested to me that I should try adding some more brake fluid if it was low (it was – now have to see if there is a caliper leak) because the switches are initiated hydraulically. Added and didn't resolve the issue.

What else is there to check before replacing the brake light switches?

And if I have to replace them, is there a way to do it without removing the master cylinder?

[bsimonson]

You should be able to measure the voltage at the switches with a volt meter to verify that you have voltage coming in. If you don't the switches might very well be ok and the problem is with the supply. If you do and they pass the voltage on when the pedal is pressed your problem is elsewhere too.

I don't have the wiring diagrams in front of me right now so I can't explain exactly how to measure but maybe you can find them posted somewhere (of if you have the Bently manual).

Good luck!

/b

[bsimonson]

Found a wiring diagram in one of my older posts, http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_Part3-1.jpg

Measure the voltage between ground and the green/blue wire going into the switches. That should be 12 V with the ignition on but the pedal untouched. If you have voltage there you can go ahead and disconnect both of the read/yellow wires from the switches and measure on any of the switch connectors (where the rd/ye wire was connected) if you get 12V when the pedal is pressed at least one of the switches is ok and the problem is somewhere else.

Keep us posted!

/b

[sigep174]

I have 11.50+ volts at the ground and blue/green.

I have tested the red/yellow while someone was depressing the brake pedal and got nothing.

The problem is I don't know if my testing methodology was sound. All three wires are hardwired into the same plug. So I had to leave the plug attached and try and pickup some volt readings from the backside like I did with the inbound current (i.e. blue/green).

Is there any reason why this shouldn't have worked if the current was being passed?

Thanks...

[Carrera3.2LG50]

If it turns out that you need to replace the brake light switches, you can do it with the master cylinder in place. Get a 22mm deep offset box end wrench and the offset will allow you to clear the brake lines to remove the switches.
(My car is an '87 and my switches were 22mm - were they 22mm in '80 as well?)

[sigep174]

Originally Posted by Carrera3.2LG50

If it turns out that you need to replace the brake light switches, you can do it with the master cylinder in place. Get a 22mm deep offset box end wrench and the offset will allow you to clear the brake lines to remove the switches.
(My car is an '87 and my switches were 22mm - were they 22mm in '80 as well?)

How much fluid run-off can I expect? Should I completely drain the brake system prior to removing the switches?

I couldn't get my calipers in there to accurately judge...so I will have to do some research on the exact size needed.

[bsimonson]

Originally Posted by sigep174

I have 11.50+ volts at the ground and blue/green.

I have tested the red/yellow while someone was depressing the brake pedal and got nothing.

The problem is I don't know if my testing methodology was sound. All three wires are hardwired into the same plug. So I had to leave the plug attached and try and pickup some volt readings from the backside like I did with the inbound current (i.e. blue/green).

Is there any reason why this shouldn't have worked if the current was being passed?

Thanks...

Well, I guess if the red/yellow wire is grounded somewhere other than through the brake lights you should not see any voltage there.

If you disconnect the plug and run a separate wire directly from a 12V source to the connector for the blue/green on the switch you should be able to measure on the connector for the red/yellow wire when the pedal is depressed.

/b

[Carrera3.2LG50]

Originally Posted by sigep174

How much fluid run-off can I expect? Should I completely drain the brake system prior to removing the switches?

When I did it, I put a couple shop rags under the master cylinder and had everything within arms reach while sitting in the trunk. I did one switch at a time and as soon as the old one was out I immediately threaded the new one in and only lost a few drops in the process.

When I was done, the fluid level in the master cylinder looked fine to me and the brakes also worked fine - no bleeding or draining necessary.

- Paul

[tcar]

Originally Posted by Carrera3.2LG50

If it turns out that you need to replace the brake light switches, you can do it with the master cylinder in place. Get a 22mm deep offset box end wrench and the offset will allow you to clear the brake lines to remove the switches.
(My car is an '87 and my switches were 22mm - were they 22mm in '80 as well?)

This is almost ALWAYS the reason brake lights don't work.

Follow directions in the Bentley Manual.

Fill the switches w/ brake fluid before screwing it in (just a few drops).

You need to hold the pedal down while you do it, as I recall. (I used a 2x4 and a rag between the seat and the pedal).

Also: Buy the switches from PELICAN... cheaper than VW Dealer, P-Dealer, or most other places... by a bunch.

Do them both while you're "in there", even tho probably only one is bad.

[tcar]

Originally Posted by Carrera3.2LG50

If it turns out that you need to replace the brake light switches, you can do it with the master cylinder in place. Get a 22mm deep offset box end wrench and the offset will allow you to clear the brake lines to remove the switches.
(My car is an '87 and my switches were 22mm - were they 22mm in '80 as well?)

This is almost ALWAYS the reason brake light don't work.

Follow directions in the Bentley Manual.

Fill the switch w/ brake fluid before screwing it in (just a few drops).

You need to hold the pedal down while you do it, as I recall. Keeps air out of the system. (I used a 2x4 and a rag between the seat and the pedal).

[sigep174]

Thanks for the info guys...had people in this weekend, so didn't get to be online much.

Will keep you posted on my progress.

1st thing is to order the switches.

[Windsor4273]

Twice the switches were at fault for my lack of brake light issue. It is an easy job, with the right wrench. Don't tighten too much, though, as you will crack the master cylinder.... I know because.....

[sigep174]

Got them fixed! It was indeed the brake light switches. The hardest part for me was getting the correct sized wrenches for the job. Because of where they were I couldn't get my calipers in there. When I got the replacements in I found out that I needed 24mm. As it turns out my local Sears did not have a 24mm deep-off set box-end wrench, so I tried 15/16 SAE and it fit exactly. I used a 15/16 deepwell socket to get the closest to the front, but had to use the deep offset for the back. No other way to get it done.

Thanks for all your help guys...

Also, there was a little leaking after removing the old ones, but nothing terrible.

[Raulg]

Did you have to have the brake pedal depressed?
I noticed my brake lights only come on when I press the pedal all the way down.
Would this be also due to worn out switches?

[carrera32911]

Originally Posted by Raulg

Did you have to have the brake pedal depressed?
I noticed my brake lights only come on when I press the pedal all the way down.
Would this be also due to worn out switches?

This is the exact same problem I'm having, the brakes lights appear to work if depressed hard. With that said, I could press with less force (but enough to slow the car) and the lights wouldn't come on.

Still a switch issue?

[Roby466]

That is normally exactly how the switches go bad... they need a whole lot of pressure to work. Replace both as you only need one for the system to work properly so if it is not working now, both are bad.
I have to do the same on my car...