Having problems with speedometre again, replaced the speed sensor in the rear end already and still no good. Someone mentioned that there is possibly another speed sensor in the transmission. Anyone got any ideas if this is so and where it is and what it looks like.
Thanks.

 

does that year truck have 4wabs? if so the abs control module may be the issue.

or the PSOM is failing. is your ABS light blinking, have you pulled codes.

 

My truck is a 1996, and i believe I have only rear abs. The abs light is not coming on or flashing and I have scanned for codes and nothing. What happens is when driving on freeway sometimes the speedo needle starts bouncing then goes down to zero. When thus happens i notice the transmission shifts down a gear cause the rpms increase. So what i do when this happens us completely let of the gas and the truck starts to slow down. When i get to about 45-50 mph, the needle kicks back in and the tranny will shift up again.
Any idea?

 

F-series pickups did not get 4WABS until after 1996.

In 1996, you do not have another speed sensor in the transmission so the problem is either the VSS, the wiring to it, or the PSOM in the dash.

 

Note... the computer gets the speed input from the PSOM not the VSS directly.

 

Either the speedometer module itself is failing, or there is crud built up on the tone-ring in the differential that the VSS works off of.

 

Where's the best place to start?

 

You might as well change the rear axle oil if it hasn't been changed in a while, and while you are there you can check out the teeth on the tone ring. You're looking for the toothed ring like this. The teeth should be clean and well defined.

 

I had the rear axle seal replaced 6 months ago and the mechanic cleaned up the rear end before sealing it and filling back up with oil and I asked him yesterday if everything looked ok in there and he said it was fine. I've replaced the speed sensor 3 times so that's not it, which only leaves the speedometer module. When replacing this what is it I'm actually looking for? Do I have to remove the whole dash and where's the best place to buy.

 

you need to trace the wiring, ive seen where the wires end up rubbing thru and causing similar issues.