Hello folks,
since yesterday I have problems with the idle of my 2.5l Fiero. I think that this problem can have many causes. However, it is noticeable that the speedometer failed at the same time. The rest of the instruments still work but the speed is no longer displayed. Does anyone know a cause that could explain both problems. Personally, I thought more of an electrical problem? I could be wrong.

VSS on Transmission drives the speedo. Then speedo output speed "data" to ECM and Cruise if car have that options.
VSS could be bad or crap wiring to them or speedo itself.

VSS should output low AC volt then car is jack up so you can turn a wheel by hand.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Most wiring failures occur at the connector. Unplug the VSS connector (Yellow and Purple wires) and see if one has been pulled out of it's metal terminal.