My friend has a 1995 Mercury Mystique that was suffering from low idle problems (stalling out at stoplights, etc). He replaced the throttle position sensor, and after that the low idle problem was fixed, but now it idles a bit high, and - more importantly - the transmission isn't shifting into second gear.
Evidently this model has some issues with wires becoming brittle, and a bunch of the insulation has cracked when he messed with the wires, so we suspect an electrical problem - plus when the problem first came up, he just jiggled a bunch of wires and it (temporarily) fixed the issue.
We've gone through and taped up all the wires on the TPS, the idle air controller, and various other connectors in the engine compartment. We ran through this test and the (new) TPS works fine - smooth transition, all the voltages correct, etc. The IAC has ~12-12.5V between the pins, so we don't think that's the issue. I checked all the fuses in the under-dash fuse box and no issues there, either.
From googling the problem, these symptoms seem primarily associated with the TPS. Is there anything else that would be likely to cause the car not to shift gears like this? If so, what is the diagnostic check for that?