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The GTO TH-400 used with the His/Hers option allowed for manual 1-2 & 2-3 shifts (and I know this is not a GTO TH-400). In Drive, the trans acted as any other trans in drive. The transmissions wer coded PB & PT having the manual feature. The trans would not shift out of 1st until you manually shifted to 2nd, there was no RPM, modulator pressure, or governor, override it shifted when you did and you could damage/blow up your engine. The same goes for the 2nd to 3rd shift. Had to be made manually - would not upshift otherwise. Put it in Drive, it went through all the gears and downshifted "as usual." Your trans kits do this. Installed the Trans-Go kit in my brother's tranny and it would manually shift 1-2, 2-3, and automatically when in Drive.

Here is some info from the Pontiac manual on the TH-400. Note it is the 'Detent" that causes the downshifts using the electric switch found at the gas pedal under the dash. I am no expert, but it appears there is the electrically operated detent switch within the trans that does this job. Maybe the detent switch is bad? I would first do as suggested and follow the power from wherever it pulls off of originally (fuse box?), then to the switch. Activate the switch and test for power going out the other connection. If this is good, then keep the switch open, and test for power at the plug on the trans. Looks like you can pull the plug (?), so test both sides of it to make sure there is not a break/malfunction between the two tabs (you know how some parts are reproduced today) If that is good, then it may be an internal problem?

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