'7427 PCM not receiving brake switch input

[Daveo91]

By looking at my datalogs in detail I've discovered my PCM is not getting a signal as to when brakes are applied; logs show that brakes are never applied.

What does this affect in the tune, if anything? Does it trigger or change any modes such as decel enlean? Control the TCC? I don't notice any ill effects drivability-wise, but decel is very rich and difficult to tune in that area.

I have not done any diagnostics yet as to why there's no brake signal. But I have a hunch that it's the pedal switch. I had some problems with it several years ago where one of the two connections seemed to be going bad. But brake lights worked and cruise control was inop then (and still is) for other reasons so I didn't worry about it. I'm guessing now I'll need to worry about it. (If nothing else, I do plan to get CC working again so I'll at least need it for that) From what I can tell, there should be a single purple wire coming from the switch to the PCM?

[brian617]

Get me details about the vehicle and I can look up the wiring diagram for you.

[Daveo91]

Get me details about the vehicle and I can look up the wiring diagram for you.

Thanks! It’s ‘91 v2500 suburban, Silverado with cruise control, RWAL brakes, 4L80e, 5.7l. I recently converted to ‘7427 pcm from ‘7060. It’s possible there’s a problem in my wiring from that but I strongly suspect the switch.

[brian617]

On that year model it appears the brake switch (cruise side) is a closed loop between the cruise module, cruise switch, and that half of the brake switch.

It appears that 7747 got the 12v brake switch signal for TCC unlock at pin a12 (purple wire) from a "diode module" that received the 12v from a switch on the RWAL from "gauge/idle" fuse.

Confusing as hell, hope that helps. Those old diagrams can be tough to follow the flow.

[Daveo91]

On that year model it appears the brake switch (cruise side) is a closed loop between the cruise module, cruise switch, and that half of the brake switch.

It appears that 7747 got the 12v brake switch signal for TCC unlock at pin a12 (purple wire) from a "diode module" that received the 12v from a switch on the RWAL from "gauge/idle" fuse.

Confusing as hell, hope that helps. Those old diagrams can be tough to follow the flow.

Definitely confusing. But thanks for looking at the diagrams for me. I took the bottom dash panel off tonight, poked around, and refreshed my memory about what I've done under there. There are two brake switches, mounted on either side of the bracket. One has both an orange and white pair for the brake light system and then a gray and gray/black pair for the cruise. This is the switch that comes up if you search for "brake light switch" on Rock Auto. 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.

Back story on why the switch is disabled: it was noisy, made an annoying loud click every time brake pedal was pushed. I ordered a brake light switch to replace it, but quickly found out that was not the problem. So I was removing this other switch and the plunger fell out so I just left it there realizing that I'd probably have to replace it eventually. At that time I thought it was just for cruise, but of course I know now it provides the signal to the PCM that the brakes are applied. The weird thing is I'm not the one that jumpered the pink and purple together. That was done prior to me owning the vehicle so it's been like that for a while. I want to get it back working correctly so I need to figure out a normally closed switch for that location. I'm pretty sure the brake light switch is normally open so I wouldn't be able to use that? What about the cruise part of the regular switch? Is that normally open or normally closed?

[brian617]

I'm pretty sure the brake light switch is normally open so I wouldn't be able to use that? What about the cruise part of the regular switch? Is that normally open or normally closed?

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.

[brian617]

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]

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]

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!


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.
1721817219

[tayto]

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

[PlayingWithTBI]

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]

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.


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]

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.

[tayto]

91 suburban is a squarebody, not gmt400 platform. if you have a vacuum diaphragm near the throttle linkage, then you're other brake switch should have a vac line. I removed a dash and engine harness from a 1990 suburban a few months ago and it had same cruise setup as my 86.

[Daveo91]

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.

Hmm, I'll have to check into that. I was under the impression the vortec system I have would work. Thanks!


91 suburban is a squarebody, not gmt400 platform. if you have a vacuum diaphragm near the throttle linkage, then you're other brake switch should have a vac line. I removed a dash and engine harness from a 1990 suburban a few months ago and it had same cruise setup as my 86.

Yup, I know my 91 burb is a squarebody. Burbs, Blazers, crew cab 1 tons all stayed square until 1992. All other pickups started with GMT 400 in '88. I was just curious if they changed to the non-vacuum cruise in 88 with the first GMT 400s. My '91 definitely has vacuum cruise, but I want to eliminate it and go with the electric system. It will be much easier to mount on my non-stock intake manifold and just seems more reliable than the vacuum system.

BTW, on a different thread we were talking about my burb's DRAC/VSSB. You were right, it's mounted to the backside of the dash, under the radio. I had my radio out recently for something else and there it was. :rockon:

[brian617]

The later DRAC is capable of outputting either 2000ppm or 4000ppm.

There are some diagrams in this thread,

http://gearhead-efi.com/Fuel-Injection/showthread.php?1734-VSSB-module-pinout&highlight=drac+diagram

[Daveo91]

The later DRAC is capable of outputting either 2000ppm or 4000ppm.

There are some diagrams in this thread,

http://gearhead-efi.com/Fuel-Injection/showthread.php?1734-VSSB-module-pinout&highlight=drac+diagram

Awesome! It looks like my '94 vintage DRAC should be able to supply the 4K output then. The diagram in that thread shows that it would go to terminal C4. Thanks.