91 K1500 no brake lights help

[ChevyGuy572]

I have a 91 k1500 silverado manual transmission, the brake lights just randomly stopped working, I have running, blinkers, and reverse lights, but no brake. I think it's a wiring issue, I've replaced both switches but it seems like they work backwards from what they should. They are both in the ON position when the pedal is not pushed, and off when the pedal is pushed in. Anyone know how it works?

 

[Erik the Awful]

If you look at the pedal, it rests against the switch, and when you press the pedal the push button on the switch is released and the contacts inside close.

Are you saying that when you press the brake pedal the switch opens and you lose continuity through it? And that with the pedal depressed you have continuity?

 

[AK49BWL]

I feel like you may have gotten the wrong switch on one of them... There should be one normally open and one normally closed, and when you press the pedal they'll swap states.

 

[ChevyGuy572]

Yes, I am saying that the circuit opens when the pedal is pushed. I have figured out that there was a ground clip that came loose but I'm still searching for a short because the brake lights only work 10% of the time.

 

[Road Trip]

I have a 91 k1500 silverado manual transmission, the brake lights just randomly stopped working, I have running, blinkers, and reverse lights, but no brake. I think it's a wiring issue, I've replaced both switches but it seems like they work backwards from what they should. They are both in the ON position when the pedal is not pushed, and off when the pedal is pushed in. Anyone know how it works?

Greetings ChevyGuy572,

Welcome to the GMT400 forum! Questions about the brake light circuits come up from time to time,
along with the brake switch itself. In this first reply we'll get into the details of the "Stoplamp/TCC" switch
that's in the Factory Service Manual. And in the 2nd reply we'll discuss the rest of the circuit.

****

I started by looking through the '91 FSM, and some of the drawings were not the General's best work.
Since the '88-'93 years all use the same brake switch part number, on a hunch I cruised through the '93
manuals and the wiring diagrams were easier to follow. (No doubt due to feedback from dealer mechanics?)

Alright, in order to set up the Big Picture of the circuit, let's first take a look at a detail of this brake switch in circuit:

(Excerpt from '93 FSM with added notes.)

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Takeaways from this:

1) The switch for the brake lights is NO. (Normally Open) Depressing the brake pedal closes the switch and sends power
towards the brake lights (via the turn signal switch) on the white wire connected to Pin D.

2) Meanwhile, the adjoining switch for the TCC (Torque Converter Clutch) control function is NC. (Normally Closed)
At rest, power from circuit #439 (the ECM/IGN 10A fuse "Hot in Start or Run") is sent through the switch to the
PCM's pin E13. Depressing the brake pedal opens/removes this power from the pin, communicating this change of state
to the computer.

3) Connector C271 is keyed so that it cannot be installed backwards on the assembly line. (Look for extruded
section next to Pin D.) Keep this in mind when we look at a physical switch to help orient yourself in the following pics.

Here's a picture of the '88-'93 brake switch, AC Delco part number D806A:

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So if you compare this pic to the wiring diagram we now have something we can meter against.

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Here's where I got the C286 info for the 2 mystery pins near the top. And a quick check of the wiring diagrams
confirmed my suspicion that the signal wire from C286 heads to the Cruise Control Module.

And here's a cross reference between the AC Delco part number and the original GM #:

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(credit: ebay auction)

Hopefully the above clears up any mystery surrounding how the brake switch (aka: Stoplamp/TCC switch) is set up to work in your '91 K1500.

In my next reply we'll figure out the rest of the electrical path from this brake switch to the rear stop lamps.

 

[Road Trip]

Yes, I am saying that the circuit opens when the pedal is pushed. I have figured out that there was a ground clip that came loose but I'm still searching for a short because the brake lights only work 10% of the time.

Once you verify that you have a 100% reliable brake switch in place, let's figure out the path that the
power intended for the brake lights out back has to travel. Here's the destination for the output
side of the (Stoplamp) brake switch. (White wire, circuit #17)

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(Section of wiring diagram showing Turn Signal Switch detail. Brake circuit big picture to follow.)

So first locate Circuit #17 coming into the Turn Signal Switch on pin P. For now, let's not modulate
the brake lights with either Left or Right turn signalling and that the Hazard Flashers are also off.

So with power on Pin #17 in, we will have power out on Pin M (circuit #18, Yellow wire to LH stop lamp)
and also Pin N. (circuit #19, Dark Green wire to RH stop lamp.)

Pulling it all together, here's the Big Picture of the "Stoplamps" circuit, from brake pedal to brake lights out back, all on 1 sheet:

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Now let's take the wiring diagram functional perspective and try to turn it into a physical layout we can follow
in the real world:

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I'd like to stop here for a moment and recap. Essentially in the previous reply I've dragged us
all through the implementation specifics of the brake switch, followed by zooming in on the
Turn Signal Switch and discussing how the 'stoplamp' signal navigates through that.

But putting on my troubleshooter hat, I want to figure out if the brake lights are only working
10% of the time is because of a flaky switch up front, loose ground out back, or something inbetween?
Common sense me thinks that any part of the circuit that is forced to work outside gets checked over first.

If that all checks out, then it's time to figure out if it's a flaky brake switch, tired connections
inside the Turn Signal Switch, or something loose in the C100 > C101 > C400 path. And of course
the cleanliness of ground G410.

And just to give you an idea of what the factory troubleshooting flow look like for the
'Stoplamps Do Not Operate':

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I did the above mostly to introduce myself to the intricacies that the power from the brake pedal switch
(courtesy of the Stop/Haz fuse) must navigate in order to get those stop lamps out back to illuminate.

I hope that helps you with figuring out a fix. And if you don't already have access to a set of factory service
manuals for your machine, then don't hesitate to give yourself the good stuff. ('88+ Service Manuals) Best of
all, the price is right.

Best of luck getting those stop lights back to 100% reliability. And let us know what the fix turns out to be.

Cheers --

 

[ChevyGuy572]

Ty, I got them working, I found a shorted out electrical connector.