'7427 PCM not receiving brake switch input

**brian617** · 10-28-2021

Originally Posted by Daveo91

But then the other switch (the top one in the pic below) should have a pink/black and purple pair connected to it with the purple going to the PCM (E13 on '7427, A12 on '7060/'7747). But this switch is non-functional - plunger is gone - and the purple and pink are jumpered together. This explains why my PCM thinks the brakes are never applied. I had tuner pro data logging on and I tested this by removing the jumper - brakes registered on. So....I know I need to replace this switch, but I don't know what it's called. Haven't been able to find it on Rock Auto or anywhere else.

In the diagrams it's listed as ABS brake switch.

**tayto** · 10-28-2021

The switch with the vacuum line going to it is for cruise. the brake switch has a NO set of contacts for the brake lights and NC contacts for TCC.

if you're not pressing the brake pedal in that picture i think the switch might not be adjusted correctly. it should not be contacting the switch like the cruise switch.

**Daveo91** · 10-29-2021

Originally Posted by brian617

Pretty easy to check with DVOM. The old wiring diagrams don't show normal state of the switch like the newer stuff does. Its probably spelled out the the service manual somewhere, but that would take some digging. DVOM much faster.

Yep, that's what I ended up doing. Found the brand new switch that I bought a couple years ago and didn't use. Tested continuity on both sets of connectors: with button pressed (i.e. brakes not applied) there is no continuity on contacts for brake lights and there is continuity on the contacts for the cruise. So I ended up using this extra switch to replace the broken PCM/TCC switch. Used the barrel of the old switch as a sleeve since of course they were different diameters. After cutting off a tiny flange on the connector that goes to the PCM, it fit right in the socket for the cruise connector. Did a quick check with TP: my PCM now recognizes when brakes are applied! Thanks for the help!

Originally Posted by tayto

The switch with the vacuum line going to it is for cruise. the brake switch has a NO set of contacts for the brake lights and NC contacts for TCC.

if you're not pressing the brake pedal in that picture i think the switch might not be adjusted correctly. it should not be contacting the switch like the cruise switch.

I was not pressing the brake pedal in that pic. For both switches, the button is continuously pressed with brakes not applied. Applying the brakes "unpresses" the buttons of the switches. Everything working now in regards to brake lights (they worked fine before) and brake signal to PCM. All the wires for cruise are hooked up and should be getting correct signal, but neither of the switches has a vacuum switch incorporated. I do plan to get cruise working eventually, but I have all the hardware and wiring from a '97 that does not use a vacuum-based CC. I'm pretty sure I'll be able to adapt that and eliminate the vacuum operated cruise.

Here's my PCM brake switch adapted from a regular combo brake light/CC switch. Attached pic is with both switches installed, brake pedal pressed.
IMG_2493.jpg IMG_2494.jpg

**tayto** · 10-29-2021

guess i remembered backwards, sorry. my 86 gmc Cruise has a switch with a vac hose, maybe yours is newer style?

**PlayingWithTBI** · 10-29-2021

Originally Posted by tayto

guess i remembered backwards, sorry. my 86 gmc Cruise has a switch with a vac hose, maybe yours is newer style?

I think the old square body trucks had vacuum cruise but the GMT400s switched to electric controls, at least my 88 is.

**Daveo91** · 10-30-2021

Originally Posted by tayto

guess i remembered backwards, sorry. my 86 gmc Cruise has a switch with a vac hose, maybe yours is newer style?

No worries, the “normally open/closed nomenclature” is a bit confusing for a switch that in the normal state has the button pushed. I think my factory cruise system does use vacuum tho - the switch originally did have a vacuum line attached to it.it’s still under the dash, plugged.

Originally Posted by PlayingWithTBI

I think the old square body trucks had vacuum cruise but the GMT400s switched to electric controls, at least my 88 is.

sounds about right. So there’s no vacuum at all on the GMT400 cruise system? Uses an electric control to open and hold the throttle? That’s the system I want to convert to but I have the stuff from a vortec truck.

**PlayingWithTBI** · 10-30-2021

Originally Posted by Daveo91

No worries, the “normally open/closed nomenclature” is a bit confusing for a switch that in the normal state has the button pushed. I think my factory cruise system does use vacuum tho - the switch originally did have a vacuum line attached to it.it’s still under the dash, plugged.

sounds about right. So there’s no vacuum at all on the GMT400 cruise system? Uses an electric control to open and hold the throttle? That’s the system I want to convert to but I have the stuff from a vortec truck.

On the "Normally Open" terminology in this case it would be normally open held closed. The switch springs to open when released.

IDK if the 96+ cruise gets the same signal as the 88 - 95. I think 96+ is 4,000 PPM where 88 - 95 VSS is 2,000 PPM? I don't know about 95, it was an odd year.