120 mph speedometer pecked at 120? (Page 1/1)
Wayne Renninger AUG 12, 01:44 PM
Any suggections?
82-T/A [At Work] AUG 12, 02:43 PM
Hi, what do you mean the speedometer is pecked at 120?

Do you mean that it's stuck, or are you saying it won't go past that point?

On my 1987 Fiero SE / V6... in my younger years with a few mods (hogged exhaust manifolds, higher-flowing cat, etc.), I could floor it on a long stretch, and with my foot fully planted into the gas pedal, the car would reach about 125-130 miles an hour, and the speedometer would go PAST the 120 mph mark.

It eventually hit a point where the car couldn't go any faster due to wind resistance and lack of horsepower, but it would sit around ~127 miles an hour... or within a 10-mph range above 120.


If when you say "pecked" you mean that it's stuck? There is a little motor that is powered (essentially) by the Vehicle Speed Sensor in the transmission. This gives the speedometer motor the signal it needs to power the odometer and speedometer.

There is also another motor that powers the tachometer.

There are two kinds depending on whether or not you're in Canada or the US... Kilometers vs Miles:




Living in South Florida, we get a lot of Canadians that come down, and then eventually junk their cars, so we get crazy things like Pontiac Firefly Turbos to Metric Pontiac Fieros. So, I snagged a 180kp/h odometer and installed it in my Fiero, and then swapped out the M motor with a K motor, haha...




If you need help rebuilding your gauges, you can send them off to J Gunsett and he will go through your gauges entirely and fix them. This is a video for my daughter's Fiero channel where he shows the entire process he goes through:
82-T/A [At Work] AUG 12, 02:45 PM
J Gunsett's contact information is at the end of the video.
Dennis LaGrua AUG 12, 03:07 PM
The Speedometer operates with a permanent magnet generator, a 2 wire VSS on the transmission that generates a sine wave signal that is sent through a buffer. to make it a square wave signal that is fed to the ECM and gauge. It's necessary not only for the speedometer but for proper engine management.
The problem with no speed reading could be a sign of a failing buffer amplifier (in ECM 87-88 or on speedo board 84-86)) , faulty VSS, or a defective speedometer mechanism. If the speedometer is completely dead, check the gauge fuse, and verify the power (VCC) and ground connections.
If the speedometer doesn't have the 12V power feed it will not operate. I would start there. If that doesn't solve the problem, then it could be a bad VSS, speedometer itself or a defective ECM. First check the the VSS the speedometer and the ECM connectors. .

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[This message has been edited by Dennis LaGrua (edited 08-12-2025).]