August 13, 2010 5:04 PM   Subscribe

I am far from home and my truck is acting up. The speedometer has started flailing around wildly and every so often we'll feel the kick of the transmission (automatic) shifting or falling out of and into gear. I think the problem is electronic as every so often the speedometer starts behaving normally and the transmission settles down and behaves normally.

Bonus: The ABS light has also come on and stays on when the speedometer is behaving itself. My wife was driving when this started and she isn't sure if it came on when the other started or if it came on a bit earlier.

The truck is a '95 Ford F-250 with a 7.5 liter engine and about a zillion miles on it.
posted by Kid Charlemagne to Travel & Transportation (3 answers total)
Probably a bad vehicle speed sensor or worn out speedometer gear. Since the vehicle speed is important information for the ABS also, the malfunction light is on for that system too. It probably sees a big difference between the VSS and the wheel speed sensors and recognized the fault.
Early cars had a cable that attached the speedometer gauge to the speedometer gear in the trans. Now, the gear remains but the cable has typically been replaced with a hall effect sensor or AC signal generator. The gear itself is a crappy plastic gear that meshes with a metal gear on the transmission output shaft. If the plastic gear is damaged, sometimes the teeth mesh and the correct vehicle speed is reported and other times it flips out. It should be an easy fix if this is the cause.
posted by Jon-o at 5:14 PM on August 13, 2010

I had this exact same thing happen to me in my 94' f150. Turned out to be a loose battery cable that would loose contact and re-engage. Tightened them up and it never happened again. Good luck to you!
posted by Esefa at 5:20 PM on August 13, 2010

Couldn't hurt to double check the battery connections first, as esefa suggests. I've also had the car do crazy shit like that when the battery's contacts got corroded. And one time when the alternator went dead.

If the voltage is low, the computer sees the wrong speed and tells the transmission to downshift or pull out of lockup. Or, the lockup can't draw enough amps and breaks free or slips, causing the computer to go nuts.

One way to check to see which is which would be to check the odometer and make sure it is clicking off the right amount of miles. Because I'm almost sure the odometer only runs off of the speedometer sensor. (this, unfortunately, depends on knowing how far out of spec it was to begin with. But if it's more than 10% low, or isn't consistently off, that's a pretty sure bet it is the speedometer sensor/cable.)
posted by gjc at 7:26 PM on August 13, 2010

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