E38/T43 TCC Brake signal issues

[AJohn6]

I have a 71 C10 with a 2014 L96/6L95, E38/T43 combo. It?s been running and driving for quit some time and until recently I didn?t realize I was having problems with the TCC signal. Finally put gauges in the truck and seen that i never had the TCC relay hooked up correctly.

Decided to hook up the scanner and sure enough the brake pedal signal never changed. Now with it hooked up with a 1k resistor on the pin 87a NC, 12v IGN on pin 87 NO side, and the TCC signal wire on pin 30. It works in the scanner will the engine is off but as soon as the engine starts it stays on what ever it was on while starting ie: brake on or brake off.

I?ve seen several people have had this issue and wondering if anyone has figured out what?s causing this?

[skova12]

same issue works with engine off and then stops switching with engine on, anyone able to get it sorted?

[AJohn6]

Unfortunately I have little hope of getting this worked out. You're the only person who's responded since I posted it.

[gcowart]

Did anyone ever figure this out I?m having the same issue

[AJohn6]

Did anyone ever figure this out I?m having the same issue

I never figured it out

[Alvin]

The TCC brake switch needs +12v WHILE NOT ON THE BRAKE which is pretty much opposite of how you'd expect. So tieing into a brake light wire would be the inverse of what you want and why either a separate switch like factory is needed or a relay to invert the signal.

This is the problem I see on most with this issue.


Without this wire the TCC usually will unlock at less than 2-3?% throttle.

[AJohn6]

Yeah, the TCC is wired correctly. I've done several LS swaps. The problem was that the ECU/TCM isn't actually locking up the TC. With engine off key on, the scanner shows the TCC signal switching. When I start the engine, it stayed on whatever the last thing it was on,TCC on or off, and won't change.

Doesn't particularly matter now, the truck is parked because i got an '04 Silverado 2500 to tow with

[gcowart]

Yeah, the TCC is wired correctly. I've done several LS swaps. The problem was that the ECU/TCM isn't actually locking up the TC. With engine off key on, the scanner shows the TCC signal switching. When I start the engine, it stayed on whatever the last thing it was on,TCC on or off, and won't change.

Doesn't particularly matter now, the truck is parked because i got an '04 Silverado 2500 to tow with

I tried the relay and everything else I could think of. Im thinking maybe some operating sytems just work with the relay and resistor and some dont. Id be curious to see a list of ppl that made it work and ppl that didnt with what operating system they are using.

[Alvin]

You can log TCC brake status to verify you have it wired right.

This is a very common issue in the hotrod swaps I have roll in.

[relicdew]

The TCC brake switch on a E38 and E92 computer work the opposite of a 24x PCM.

The Brake switch wire to the ECM is looking for GROUND at rest, via the resistance of your bulb. Then when the brakes are APPLIED, it sees +12V. Technically if you also remove the TCC wire from position 9 and have it OPEN, it also believes the brakes are applied. So sending it voltage ALL the time will think the brakes are applied. This wire is also used on cruise control for the vehicle and to trailer brake systems. When you provide it +12V it allows the factory shifter to release from Park as well on most applications for the shifter's interlock feature.

You can observe this function via VCM Scanner or a Tech 2. Simply take the brake switch wire that goes to position 9 and some cases to the T43, position 6 to a GROUND Source. The scanner should then say NO or released. Then when you remove the brake wire from the circuit, you'll see it say applied.
The brake switch wires from the ECM and 6L80E TCM are tied together and go to your brake switch "cold side". If you have LED bulbs, you would have to use a relay to make the computer see ground at rest and then it can be OPEN when you're on the brakes.

Brake Switch_E38.jpg

https://speartech.com/v/vspfiles/instructions/led.pdf

[dra123]

Did you ever get the brake switch figured out? I've got same problem staying latched in yes or no which ever way it was when vehicle started. I have the pull down resistor and everything is good when I only have the key on