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?

1 Answer 1


I believe you already found the culprit. If you can 'jiggle' wires in the harness and cause any change in how the engine is running this is NOT normal. Unfortunately this is one issue that has to be diagnosed in-person as there are too many variables when it comes to wiring.

Start by checking ground wiring first. This is easiest to do because you can perform a continuity (resistance) test between the negative post of the battery and the different grounding points in the harness. Sensors such as MAP and TPS have the biggest impact on how the engine runs so I would check the wiring related to these next.

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