I had my alternator go out and while trying to limp it home and avoid a towing bill the speedometer and odometer went out. It threw out the 0500 code saying it was the speed sensor, which is actually the abs sensor on this model. I replaced it with OEM and still not working. Checked all my fuses, looked for wiring problems, and even tried switching out the GEM, but still nothing. I'm hoping to stay away from a shop but I am running out of options any suggestions would be helpful.

 

Continuity check the two wires from the ABS sensor to the GEM (RABS trucks) or from the ABS sensor to the 4WABS Module (4WABS trucks).

 

Thanks so far i've got the rear wiring harness checked now I just need to check from where the wiring harness comes into the cab to the GEM.

 

So I figured out that the ABS module was fried. Replaced it and the check engine light went off but speedo and odo still not working. Do I just need a drive cycle or am I missing something else?

 

Check dash fuse 14 and underhood fuses 3 and 7. If the 4WABS Module is not getting power, the speedo/odo will not work.

The above assumes that your truck has 4WABS. If it has RABS, then the VSS signal is corrected in the GEM and the RABS Module would not be involved with the speedometer in any way.

Which ABS system is it? Both were available in 1998.

 

It's the RABS but doesn't that signal run through there first bfore going to the GEM or is it the other way around. Also could the motors that drive the speedo and odo be fried and still give me a speed sensor code?

 

In the 98~00 RABS Rangers, the RABS Module speed input and the GEM speed input are in parallel from the rear axle sensor. The speedometer is driven by the corrected VSS output on the gray/black wire from the GEM.

The P0500 speed sensor code is generated by the PCM. The PCM should receive the same corrected VSS pulse that the speedo/odo gets from the GEM.

 

So far I have continuity from the rear axle to the GEM and continuity from GEM to speedo. I have checked and rechecked related fuses. All the other systems run by the GEM are fine and truck still runs fine so I wouldn't think it to be the PCM either. Having said all that I am now at a loss. Thanks for everything so far you've helped me to figure out what its not. Also by the way the speedo does work when I manually apply power to it via AA battery.

 

Since you have already swapped out the GEM and the axle sensor, you could try monitoring voltage on the gray/black. IIRC, my voltmeter shows about a 0~12v square pulse at very low speeds but this quickly becomes a midrange value as road speed goes up and the oscillation frequency gets too fast for the meter to track.

If you are not seeing any voltage on the gray/black when moving, try disconnecting the GY/BK from the speedometer, speed control servo and PCM. I'd do that one "user" at a time to see if one of them is dragging it down to zero.


Also, be sure that there is no continuity to ground on either of the two wires from the sensor to the GEM and RABS Module or on the gray/black from the GEM to speedometer, speed control and PCM. You would need to disconnect everything on each those wires to make the continuity-to-ground checks.