6.2 700r4 lockup 12v source

[rsparhawk]

In the process of troubleshooting why my tcc won't lockup. I've figured out the 12v side is not making it to the connector at the transmission. I have a factory service manual but I still can't figure it out. It says it goes to the rwal diode array aff of the firewall, but it's not on the rwal diagram page..... Does anyone know where the pink/black wire gets its power from? I've already checked all the fuses as well.

Thanks

 

[rsparhawk]

I forgot to include its a 1991 1500 suburban.

 

[MadOgre]

@chengny

 

[chengny]

Man, those later model wiring diagrams are a pain in the ass to follow. They jump all over the place. Anyway I think this is the way the circuit operates:

Out of the fuse block on ECMB, into the giant common pink/black circuit and then into the brake override switch. Out of the brake switch (as a purple) and through the diode set.

The function of the diode set as pertains to the TCC is that it disables the TCC if the front axle is engaged.

The purple comes out of the diode array as a purple/white and transits the firewall at the bulkhead connetor. It stays purple/white all the way to the + side of the TCC coil.

Coming out of the - side of the coil it is tan/black. Goes from the TCC coil to the control module. The control module grounds the tan/black internally - when the operating permissives are met - and the TCC pulls in.

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

If you are sure that the brake switch is good, I would jump the 420 (purple) to the 820 (purple/white). If you then have power at the positive side of the TCC coil, you have pretty much narrowed it down to either a bad diode array or a faulty front axle switch in the transfer case.

 

[rsparhawk]

Yes, those diagrams could use some improvement, I look at peterbilt wiring diagrams all day long as part of my job and they aren't half as bad at jumping around as this dang service manual.

Mine is 2wd, and I do have 12v at the brake switch and the brake switch interrupts it as it should.

So could I just bypass the diode array without any other issues?

 

[chengny]

Yeah, I can't see why not. It should be fine - especially if you do it externally to the actual diode array module. The "array" has multiple functions, but if you confine the modification to just jumping 420 to 820, the only switch you will be by-passing is "front axle engaged".

Since you don't even have 4WD and make the jumper connection outside of the actual module...I can't see that causing any problems at all.

They probably only made one version of the Diode Array module and one common harness so even trucks without 4WD got that "FWD engaged" lockout.

Another option is to not even go near the diode array (it's supposed to be under the dash but I have no idea where).

You could just **** off the purple/white lead somewhere convenient on the firewall (before it heads down to the transmission case). Then run a fresh lead straight from the purple (where it exits the brake interlock switch), out through the firewall and splice into the purple/white that you just cut.

Nice and neat, no factory wiring is disturbed and all safeties remain fully functional. That's what I would do.