Hey guys. Take a look at the speedometer pointer to see if there's a little stop peg the needle is against. By the late '80s / early '90s everyone went away from the too-reliable cable-driven speedometers and switched to electronic ones driven by a not-so-reliable computer. Turns out they have been rather trouble-free, but what can happen is they get hit with a voltage spike during other electrical repair work. This happens most commonly when connecting a new battery.
The speedometer is driven by a "stepper" motor. It is not a spinning motor with brushes like we normally think of. A stepper motor has four electromagnetic coils that are pulsed with varying voltage and polarity to create a movable magnetic field for the pointer to follow. You can identify these by the fact that they stay where they are when the ignition switch is turned off. They don't go back to "0" until the ignition switch is turned on next time.
The problem with those voltage spikes is if it causes the pointer to go more than halfway to maximum, when the ignition switch is turned on next time, it wants to go back to "0", but it looks for the shortest way to get there, and that will be clockwise. The pointer runs up against the stop pin and can't go any further.
There's three ways to fix this. The hardest method is to disassemble the instrument cluster to the point you can push the pointer counter-clockwise by hand. An easier method is to connect a scanner that can access the instrument cluster, and run the gauge test sequence. Typically that positions every gauge to its 1/4 scale, 1/2 scale, 3/4 scale, full scale, then back to "0". The speedometer will try to follow those steps, and once moving counter-clockwise is the shortest way to the target, the pointer will go there, then follow all the other gauges back down. Some cars can have the gauge test sequence initiated by pressing certain buttons in the right order without the need for a scanner.
The easiest method is to simply drive the vehicle faster than half the maximum speedometer scale. That doesn't mean 90 mph. Speedometer faces never have "0" and the highest number in the same place. Typically 0 to 180 will cover perhaps 270 degrees of sweep. In my minivan the highest reading is 125 mph, and 45 mph is right across from it. "80" is across from "0", so according to my sad story, if the pointer got stuck on the backside of the stop peg, I'd have to go 81 mph, then the shortest way to the target for the pointer would be to go counter-clockwise, then from there it would follow the computer back to its commanded positions.
Sorry that I don't do a good job of describing this fix. It's much easier to do than to explain it.
Thursday, October 30th, 2014 AT 11:14 PM