speedo juumps around

[chippy]

i got my trk down home finly and my speedo jumps around all the time, my guess is the speed sensor on the rear diff., and i ink that is causing my cruise not to work due to a broken speed singnal

[JSchuricht]

Could be one or more of several things. The simplest is to swap the speed sensor in the rear diff, it's about $25 and takes all of 30 seconds to swap. It could also be a wire, the PSOM or the exciter ring in the diff.

I was working on this exact issue on a friend's 96 not too long ago. Turned out to be a chunk off of the ring gear that chipped the pinion and mangled the exciter.

If you end up needing to test the PSOM (Programmable Speedometer/Odometer) and are decent with electronics, I can help you in setting up a cheep test bench.
http://anscv.com/images/Truck/96%20350/P1020052.MOV

[TexasAggie09]

Definitely check that speed sensor first...that causes a lot of issues including jumpy speedo, brake light staying on, cruise not working, etc. It's easy and cheap enough to try first. Good luck!

[chippy]

yeah i going to get a sensor i pulled it and cleaned it off and the diff has 167k on it and has had norn. service so it probly just a sensor

[superduty4x4]

If its not the VSS in the rear end its most likely the PSOM (speedometer). You can either get yours rebuilt or buy another cluster from a junkyard. But since the PSOM stores mileage, whatever miles the donor rig had will show on your rig if you go with a used part.

[chippy]

Originally Posted by superduty4x4

If its not the VSS in the rear end its most likely the PSOM (speedometer). You can either get yours rebuilt or buy another cluster from a junkyard. But since the PSOM stores mileage, whatever miles the donor rig had will show on your rig if you go with a used part.

cool thanks i never figured on the ax. speedo but hey if im lucky its just the sen.