Thanks, Franklin2.
I needed that diagram too.
Up at the upper left is the circuit for the overdrive cancel switch and light ( OCS and OCIL.
That is the only circuit I see that doesn't require any external test equipment to verify whether or not the Transmission Control Module is "awake". Just turning the ignition key on should power up the TCM. The TCM should see the OCS switch , if it is pressed , then illuminate the OCIL. And turn it back off should the OCS be pressed again.
The engine need not be started or running. This test only verifies the TCM computer is getting power and has enough of its circuitry running to read the OCS switch state then toggle the OCIL lamp accordingly. If this is working, given no other pending disasters, start the engine, try driving it, monitoring it for proper solenoid sequencing.
There are only three wires going to the code reader. I think one is ground, another is to be grounded to tell the TCM to send the codes, and the third wire is the wire the codes will be placed on. As you know, the code reader has its own battery.
I went out yesterday to verify this on my 1994 IDI E4OD van. Just turning the key on powered up the instruments and the TCM and it recognized the OCS switch with the proper response.
I hate to think it's a bad TCM, as I have found them to be quite robust excepting the "electrolytic capacitor plague" of the early 1990's. And this doesn't make it malfunction this way. The TCM loses its mind as the voltage spikes that those capacitors were supposed to filter out don't get filtered out and get processed as a signal, which fouls up the computer input, which results in all sorts of erratic operation and accusations of sensor and actuator failure.
