Engine PCM Vs EPC output / transmission line pressure signal problem

1996 Tahoe - purchased it without engine. Replaced engine with GM longblock, and immediately found that transmission was bad (no forward gears).

Swapped in rebuilt 4L60e from well respected performance transmission shop. Transmission shifts up & down as it should, but shifts at high RPM are slow / sloppy / sometimes flare.

Transmission rebuilder scanned and found that the EPC force motor commanded amps range from 1.1 at idle to 0.48 at WOT, not down to the ~0.1 needed for firmer shifts and proper line pressure. To trick the computer, we unplugged the TPS connector so the the PCM defaults to max line pressure (~0.1 amps). Confirmed the amp reading was 0.1 and then drove the truck, MUCH improved shifting at WOT, shifts like it should.

I have checked the function of the TPS, MAF, and MAP by logging them during driving - all are reporting the correct values and function correctly. I get an occasional TPS code while driving, I've swapped out that sensor and checked it numerous times, the volt output is correct and smooth. I don't think the TPS is bad.
What else could cause the EPC / force motor commanded amps to never go below 0.48?

What is the TPS code number that you are getting? Are you clearing it?

ECT sensor (by the thermostat) can cause the VCM to act up if it’s getting a bogus temperature, but I couldn’t say if it uses that to adjust line pressure. Check with a scanner, see if it’s reporting some oddball temperature.


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Code is P0221 - TPS low voltage. I have the frozen PID data from when the code occurs, and the TPS always shows a value (8% the last time it happened). I would expect to see 0% if the voltage truly dropped out of range.
When the code trips, the PCM starts sending the ~0.1 amp signal to the transmission. It will continue for a couple of drive cycles, then revert back to "normal" (varying the EPC signal, but never less than 0.48 amp). If I clear the code, it immediately goes back to "normal"


I checked the function of the ECT sender by the Tstat - ~2.5V / 167F which agrees with my dash as well. Not sure why the truck runs a little cool, but both senders agree.

To close the one out - I did fix the issue. The problem was the PCM itself. Swapped the PCM and the force motor signal worked correctly. I also not longer have the TPS code, and the idle is more stable.