I just bought a used 91 GMC Sierra pickup. 350eng, auto
It runs well for it's age, and I like the extended cab and full bed.
It has a few issues mostly with the wiring. For one thing, the heater
fan wires were disconnected from the speed switch, and someone used a
regular house light switch for on-off (hi speed) only. Well, ok, that
works but I'd like to find out if there is a common problem with those
original switches.

I also got a passenger side door switch that does not shut off the
light unless I duct tape a piece of cardboard on the door. I know the
old metal switches could be pulled out and lengthened. Is there a way
to do the same with these newer plastic ones? If not, how do I remove
the whole switch, does it just pull out or what?
light is for the both the regular brakes and parking brake

And finally the actual topic of this message. The red brake light is
on most of the time on the dash. The brakes work perfectly. I made
sure the parking brake was completely off, and pulled it upward rather
aggressively. The light stayed on. I'm thinking that the problem is
not the brakes, but the sensor. There is a plug on wire on top of the
master cylinder. Is that the sensor for the brakes? Is there a way
to test that sensor? Yes, the brake fluid is FULL.
OR
The truck had the original owners manual, and it says that same
brakelight is for the parking brakes. Where would I find that switch?
Maybe that one is stuck. I know to unplug that one to see if the
light goes off. I just need to find that switch.

Anyone got any ideas?

Is there anywhere online to get a wiring diagram for this truck?

Thanks to all.

LM

<letterman@invalid.com> wrote in message news:7m3ti4tqo7hq578uo483pahj97kkufkl4n@4ax.com...

Snip Other Grimlins

> And finally the actual topic of this message. The red brake light is
> on most of the time on the dash. The brakes work perfectly. I made
> sure the parking brake was completely off, and pulled it upward rather
> aggressively. The light stayed on. I'm thinking that the problem is
> not the brakes, but the sensor. There is a plug on wire on top of the
> master cylinder. Is that the sensor for the brakes? Is there a way
> to test that sensor? Yes, the brake fluid is FULL.

> Thanks to all.
>
> LM

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Make sure you emg brake is OFF. Turn off the key. Disconnect the battery for 30 seconds.
Reconnect. Turn the key on without starting. Light should go out if its OK. Sometimes
the sensor gets 'jammed' in an electronic way. Never been able to figure out what combination
of things cause it, but it's a harmless little bug that only pops up once in a great while.

But if this doesn't work, then have it looked at.

HDS