Occasional brake light failure - cylinder switch?

Twice now I've been approached at traffic ligths by the driver of the car behind me, "Your brake lights aren't working". But when I get home they seemed fine.

Checked them over today, cleaned the earth, bulbs, switch contacts etc. But once or twice they did indeed only come on when the engine was running and pedal pressed hard (they usually come on even without engine running).

Is this a normal failure mode for the brake light switch on the master cylinder or should I look elsewhere?

many thanks

Also check the wiring isn't rubbing off the body where a grommit is missing etc.

icosahedron wrote: Since the brake lights still work with one faulty switch, some people then disable the brake warning light.

The brake warning light hasn't been disabled. It comes on with the handbrake.

icosahedron wrote: An '89 model should have two brake light switches fitted to the master cylinder.

Ah ha, the Haynes manual (see below, click for bigger pic) shows two switches in parallel, presumably for reliability. Since the brake lights are not working properly I guess both of them are faulty. Should I change them both?

It's easy to get to the one near the front, what about the one at the back?




Edited by pootle - 10 Jun 12 at 11:58

icosahedron wrote: Edit: Right, please ignore my posts.

:)

Looking at the diagram I'm pretty sure it works this way:  The switches are normally open. When the pedal is pressed they close and the lights come on. If either switch fails open circuit the other will still operate the brake lights. If either switch fails closed the brake lights will be on permanently.

In my case one of  the switches must have failed open circuit years ago without me knowing (since the brake lights will still work). Now the other switch is starting to fail the same way so the brake lights are starting to play up.

I'll buy a couple and try to change them both.

Cheers for the advice.


Edited by pootle - 10 Jun 12 at 21:39