first off a little info on the truck it's a 1994 f150 with a 302 and a manual trans. So recently the speedometer has started bouncing around and has gotten worse to the point now where as soon as I start driving it doesn't read at all or wildly bounces all over the place no rhyme or reason.. It has a newer speedo cable and no other issues with the gauge cluster so I don't think it's that big no sure if related or not but around the same time the speedo problem started the ABS light came on on the dash.. I have yet to pull codes but will add those when I do just looking to get this fixed soon because it is my daily driver

thanks, Jake

 

When you replaced the cable, did you apply a small amount of white lithium grease or suitable alternative to the ends?

If not, give that a shot and report back!

 

I didn't think a 1994 had a speedometer cable.

 

If it does have a cable, the you gotta lube it. And when they do run dry, often the plastic drive gear gets worn, adding to the jiggle. They are cheap and available. Also make make sure the speedometer end is properly seated.

Funny though, as stated above, the '92 is about when Ford went to the electronic speedo. The VSS (sensor) on top of the differential can cause this as well if it fails. A bad VSS can also trigger the ABS light since it's part of that system too.

 

1992 and newer trucks have an electronic speedometer/odometer. There is no cable.

 

More than likely he has bad bearings in the rear end or junk stuck to the VSS or bad wiring under the truck.
The VSS may have lost its magnetism but unlikely.

 

Yes you guys are correct sorry I'm getting my trucks mixed up my brother we put a speedo cable in.. this one is electronic.. what is the VSS and how can I check it what am I looking for

 

OP mentioned having replaced the cable, which assuming he did the work himself, would have allowed him to verify that he has a mechanical cable.

I know my '96 is all by wire and that the only place it bounces is slightly between 55-60. I'll need to check and clean the sensor at some point.

 

VSS - vehicle speed sensor. Electromagnetic sensor in the diff, it basically "counts" the teeth on a ring attached to the diff ring gear called the tone ring. The PSOM (speedo module in the dash) counts those pulses and points the speedometer needle accordingly, and also sends the speed signal to the PCM. A bad VSS, or crud in the diff fluid or crud between the notches on the tone ring, can cause what you're seeing. If the diff fluid is old anyway, it would be worth removing the diff cover and draining it, this will give you a chance to inspect the tone ring. If it just needs cleaning up, it would save you the cost of a VSS.

Good thing you have a manual; on automatic trucks, that sensor also provides the speed signal used to determine when to shift the tranny. With that kind of malfunction, you'd be having nasty shifting issues, if......

 

The VSS itself very seldom goes bad but gets changed out a lot.
You have a bad signal on the VSS wires at the RABS module so yo know the problem is in the wires going from the RABS module to the rear end sensor, the VSS plug or has junk stuck to it or the tone wheel is opening the gap between itself and the VSS.

There is ways to test the signal but we already know it is bad so no point there. You can check the coil in the VSS with an ohm meter to see if it has open winding or shorted windings.



Here is a wiring diagram for a 1995 but a 1994 is the same:

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