speedometer not working

[Tony Mattingly]

The speedomenter is not working on my 94 6.5 diesel 4X4 with a manual 4 speed w/overdrive (5 actual gears)
I just bought the truck and the transmission had recently been re-built. According to the guy I purchased the truck from, It speedometer did work untill the trans was re-done.
I checked the conections under the truck they seemed fine.
I replaced the the speed sensor with a used one and it still does not work. (I know, the used one could be bad too).
I looked at the fuses but did not see one marked for the speedometer. The one marked for guages was OK.
Any suggestions?

[BobND]

On a 4X4, the speed sensor will be near the back of the TRANSFER CASE.

[Tony Mattingly]

Originally posted by BobND:
On a 4X4, the speed sensor will be near the back of the TRANSFER CASE. Yes, I replaced that already but it was with a used one. It is possible, that one could be bad too, but I have my doubts.

[BobND]

Could the harness going forward and up have been damaged, or a connector at the firewall jarred loose when they did the tranny?

IIRC, these sensors are a coil of wire mounted around a magnetic core that generates a varying frequency (and varying AC voltage) when the toothed wheel passes them. They just have a 2-wire connector, right"

Did you check between the terminals of the sensor for continuity with an ohmmeter?

Also, one could verify the sensor is outputting a signal by jacking up the back to free the wheels, and then looking with an AC voltmeter, or a 'scope for signal while it's idling in gear. (Be carefull!)

I'll bet, though, that something happened to the harness between the sensor and the firewall.

[Tony Mattingly]

You know, it is simply amazing what a fuse will do. duh! :rolleyes:
I just bought the truck and the guy said that all of a sudden the speedometer quit working. After purchasing 1 used speed sensor for $20 and on new one for $54 I decided to double check the fuses again. Well, I noticed that the space that was marked as 4WD did not have a fuse in it. I though since it was manual transmission that it did not require a fuse. Thank goodness I put one in anyway and there you go, the speedometer is working. The guy said that "the speedometer just quit working" well yea, if it don't have a fuse, I don't guess it will. He left out the chapter about pulling the fuse. :mad:

[ZZ]

This is the hardest thing about buying used vehicles. Who knows what has been done to the wiring? I know I've had to do some rework on mine that I don't think anyone else would easily figure out.