Brake Light issue

raykinsella · Post by **raykinsella** » Thu Sep 14, 2006 6:42 pm

Hey everybody,

On my 412 I noticed the brake lights were always on. I disconnected one of the switch harnesses and the lights went out. They went on as normal when I pressed on the brakes. So I figured it was the brake light switch, so I replaced it. Same problem! I switched the harness from the other switch onto the new one and it worked fine. So the problem is in the wiring going to the front switch. When it is plugged in the brake lights are on all the time. Any ideas?

raygreenwood · Post by **raygreenwood** » Fri Sep 15, 2006 8:44 am

Actually you should check the switches in the master cylinder with a volt meter to make sure they are disconnecting when you release the brake. What may be happening is that your brake pedal pushrod is out of adjustment or a flap valve is sticking on one circuit. This is common. The problem is that the residual pressure may be allowing the switch to stay activated instead of releasing teh pressure. The switches are sensitive. Ray

raykinsella · Post by **raykinsella** » Fri Sep 15, 2006 11:20 am

Thanks for the reply, Ray. Are you saying that even though I replaced the switch even the new one may be bad? When I put the wires from the back switch onto the new front one ( and I left the wire from the front one off ) the brake lights worked as they should. It seems to be something in the wire narness itself, as if it is shorting out somewhere. HOWEVER: while doing all this messing around I brushed a wire against the metal around the master cylinder and it sparked a little and blew the #12 fuse. So if my problem was simply a short somewhere it would be blowing that fuse, which it isn't.
In the meantime I'm driving around with just one brake light switch hooked up, and it works OK. But I would like to figure this out. Thanks!

raygreenwood · Post by **raygreenwood** » Fri Sep 15, 2006 2:20 pm

No not at all. What I am saying...is that the adjustment of your master cylinder may be off. If...when the brake is released....foot off the pedal......the pushrod still is putting any tension at all on teh piston....the brake light switch may be activated.

Bear this in mind. When the key is on....there is always power to the brake harness. It is only when you push the pedal that pressure from the cylinder makes the internal part of the switch contact.

You can have as little as 1mm too much pressure on the pedal and cause it to make the switch contact.

The push rod from the brake pedal must have no contact with teh piston whatsoever when you release teh brake. The correct pushrod to piston gap is in the Haynes manual.

This can also happen when you have an old master cylinder...that has gummed up compensation ports in one of the pistons. What happens is that it does not fully release all pressure when you let your foot off.

It could also be a short like you suspect. I kind of doubt it. But it is worth tracing each wire back to the fuse panel. Ray

SureFit Travis · Post by **SureFit Travis** » Mon Sep 18, 2006 10:52 pm

To carry on with Ray's theory, have you tried simply 'pulling up' on the brake pedal? Is the pedal coming back fully on it's own when you take your foot off, or is there a possibility that it is sticking?

I know that sounds elementary, but I'm going to stand with Ray on his theory........just thought this would be a good start........and then check the pushrod-to-master cylinder free-play/gap.

Good Luck,

Travis

raykinsella · Post by **raykinsella** » Thu Sep 21, 2006 4:28 pm

Thanks for the input, guys. Well it definitely isn't the play/contact between the brake pedal and the master cylinder. It is somehow in the wiring of the one brake light switch. When I connect only the rear ( ie closest to the bumper ) wire to either switch the brake lights turn on & off just fine. But when I connect the front wire ( closest to the driver ) to either switch the lights just stay on as soon as I turn on the key. No other electrical problems at this time.

wildthings · Post by **wildthings** » Thu Sep 21, 2006 9:29 pm

Try cracking the bleeders on both the front and rear to relieve any residual pressure and see if the lights go out. I would still guess a problem with the master cylinder over anything else. That being said have you checked the resistance across the terminals on the front brake light switch with an ohm meter? If you have infinite resistance then the problem is in the wiring and if it shows no resistance then the problem would be in the master cylinder.