Speedometer only works when cold?

[Sam TII]

Hey Everyone,

I have a '98 B4000 and my speedometer only works when the truck is cold and then it only works for about 2 or 3 minutes, then it starts to bounce and eventually gives up. I tested the resistance on the VSS on top of the rear diff and it was within spec, but just for fun I replaced it with a new one from Autozone (tested that one too, also within spec) and the issue continues exactly as it was. Has anyone else run into this issue where it only works when cold? I've checked the wiring under the truck and haven't noticed any breaks or corrosion or other damage. Please let me know if you've got any thoughts on anything else to test.

Thanks!
Sam

 

[Earl43P]

Welcome to The Ranger Station!

There is a large square plug under the driver seat.
It may be corroded inside.

Other than that, we need more info about your truck.

4wd uses the 4WABS module to condition the VSS signal for use by the speedo.
2wd uses the GEM to do that.

RD/PK and LG/BK wires from the VSS got to either the GEM or 4WABS module.
GY/BK wire delivers the speedo signal from there.

 

[Sam TII]

Welcome to The Ranger Station!

There is a large square plug under the driver seat.
It may be corroded inside.

Other than that, we need more info about your truck.

4wd uses the 4WABS module to condition the VSS signal for use by the speedo.
2wd uses the GEM to do that.

RD/PK and LG/BK wires from the VSS got to either the GEM or 4WABS module.
GY/BK wire delivers the speedo signal from there.

Earl thanks for the reply, I'll check the plug under the seat tonight and let you know what I find. My truck is a 4x4 with a 5spd.

 

[Sam TII]

Checked the plugs under the drivers seat on the underside of the cab, no corrosion seems to be present, but the same behavior continues, I get about two miles every morning and then the speedo quits, doesn't work on the drive home but works again the next morning.

Would changing my gear oil or something help? It seems to be pretty clearly related to when the truck warms up. No ABS light on (but the ABS light passes the key on test). Any other thoughts?