My speedometer stopped working a few months ago and I am now ready to fix the problem. I know there are two Vehicle Speed Sensors(VSS) in my 93 Trooper- One in the speedometer and a second on the transmission. Most of the time the speedometer needle is at zero, however, on occasion the needle will shoot up and fall down and sometimes even match the tachometer position. It is erratic and does not seem to follow any pattern. I was wondering if anyone has had any experience with this issue and if they found it to be the VSS in the tranny or the VSS in the speedometer. My inclination is that it is the VSS in the speedometer because of it's odd behavior, but I do not want to go and spend a bunch of money for the wrong VSS.
Any help would be appreciated.
Thanks
Mike

Why would a vehicle speed sensor be installed in a speedometer?

I think you are right that there has to be a sensor by the tranny, and some other electric component in the speedo, to translate the info, but to say one is bad based on no real proof could be expensive.

My advice would be to find out how the VSS works and how to test it. Is it mechanical / electric etc.? There might be a procedure outlined in the factory manual. Sort of like checking for resistance while it is removed, and doing something to mimic a signal to see what happens and if it sends a signal within range. The general rule of thumb is to isolate something which might be wrong, then test it by itself. If it is OK, then you know the problem is in the speedo.

I know this doesn't help much but could get you going in the right direction.

I did some leafing through the factory manual and there is a test for the speed sensor in the tranny. It requires a
VOM but I already have one. I am hoping to get a chance this weekend to pull off the VSS and test it. If the one in the tranny tests OK I will replace the speedometer.

Because the speedometer is driven off of electrical currents rather than a cable in the 2nd Gen Troops there have to be two VSS. The on in the tranny counts the rotations of the driveshaft and translates that to electrical pulses and sends it to the speedometer. The VSS in the speedometer then translates the received electrical pulses and they are reflected in the position of the needle on the speedometer. This is what I got from the manual. Also, there is no test for the VSS in speedometer.

When one of the VSS is not functioning properly it affects other things; odometer, cruise control, speedometer, speeding tickets, etc.

Mike

I realize that it isn't an Isuzu, but we recently had a similar issue with my work truck, an F250. It, too, exhibited the symptoms that you described along with rough shifting of the tranny.

The solution was replacing the VSS logic board in the dash.

hate to nit pick but if it is not a sensor that measures vehicle speed than whatever is in the dash is not a vehicle speed sensor. that wouldn't make any sense.

hate to nit pick but if it is not a sensor that measures vehicle speed than whatever is in the dash is not a vehicle speed sensor. that wouldn't make any sense. [/quote]

Just going with what the factory manual says

maybe it was translated from japanese. My fuse box has one relay labeled with yellow lettering that reads: "spirit force of motor is".

Nah, just kidding, but they can be way off sometimes.

I think the "VSS logic board" or the "VSS in the dash" is traditionally called a stepper motor. That may be just for the odometer though. Anyone have a repair manual with the actual part name?

The factory repair manual says there is a Vehicle Speed Sensor contained within the speedometer as well as a Vehicle Speed Sensor attaching into the side of the tranny.