So here's the scoop, playing with a buddies 91 v3500 w/ 6.2l 4l80e

Last year it lost 4th gear and lockup, few months later started shifting irratically, then eventually had just 2nd gear...

It had some issues like a broken/leaking pass through connector at the trans, was throwing codes for shift solenoids and speed sensors. So the pan had to come off for the harness so we swapped all the solenoids and both speed sensors at the same time.

Amongst the codes was 21, tps high. Checked the tps, totally fubar, voltage jumping all over the place, so replaced the tps...

Cleared codes, went for a run and it didnt want to shift (extremely delayed). Once it came out of first gear it shifted 123 perfect, no 4 or lockup. Checked codes, 21 again, voltage at tps was only 4.45@wot

Fk sakes... So set it way down, 3.5@wot, still get the same code and same shifting. Checked the voltage at the PCM/tcm same deal. I've heard of bad alternators causing ac fluctuations and wreaking havoc, wasn't the alt...

Now for the funny part, checking the tps voltage ac power, I get roughly 8v ac and 3.5v dc @ wot

I was getting this ac voltage with the engine off or running...


Wtf...

PCM causing the ac power?