'92 Topaz: Downshifting to 1st at 55MPH

My '92 Topaz, while on the highway (usually at 55/60 MPH), downshifted from 3rd to 2nd to 1st. With each downshift, the speedometer dropped about 20 MPH, even though the speed of the car had not changed.

After leaving the highway, the speedometer read zero, the odometer stopped clocking miles, and the car wouldn't shift above 1st.

The next morning, the car upshifted again, but while on the highway, it downshifted to 1st again with the same symptoms.

Deciding not to risk the highway anymore, I kept the car to smaller streets, but after a few miles or so, the car would downshift again and the speedometer would cease to function.

I don't think it's the speed sensor, and I've heard word about the "throttle pressure rod," but the Chilton's has been unhelpful (I'm an extreme novice). If anyone knows what the source of this problem might be, and the location of the parts under the hood, I would be very grateful.

Not the speed sensor? There's some wrong signal somewhere, if your speedo drops 20mph but you keep going the same speed. Hooking it up to a diagnostic tester would be a good idea.

The reason why I think it's not the speed sensor is twofold. Another guy with a similar problem (a '92 tempo) replaced the sensor, but the same thing kept happening (that thread was where a "throttle pressure rod" was mentioned).

Also, the transmission downshifts first, the engine revs, and then the speedometer slowly drops about 20MPH.

However, I do agree, there's some miscommunication under the hood. Think I'm gonna cave in and bring it somewhere for a look-see.

Much thanks for your response, though. It doesn't go unappreciated.

Sounds like it's a downshifting problem with the trans. When you say, "Engine revs then slowly goes down 20mph" that sounds like a normal downshift condition. Good luck finding the problem; I'd think putting it on a diagnostic computer and treadmill would find the problem.