I've got a 94 F-250, 7.3 IDI, E4OD tranny. When I hit bumps and stuff the speedo jumps all around. It'll drop to zero then come back up. It will be really wacky. Last week or so since it has gotten colder the speedo will act up more and more, sometimes the ABS light will come on, it won't shift right. I thought it was the VSS sensor, but I replaced that and it still happens, so I don't really know what else it could be. Any thoughts?

 

Check the wires from the sensor to the guage cluster. Mine were corroded/damaged in several places. Similar symptoms, speedo would bounce when hitting bumps and would flutter over 55 mph as well. I ended up replacing the entire length of wire.

 

The truck will also randomly start shifting hard. Is there a sensor that controls that? I'm not sure that the shifting sensor would have anything to do with the speedo. It may be independent problems, I don't know.

 

There is no "shifting sensor" per se. The VSS provides the speed signal, via the PSOM (speedo module), to the tranny controller. So yes, a bad speed signal will affect shifting. Watch carefully when the speedometer drops to zero -- does the odometer go blank?

 

The odometer does not go blank. I have all other gauges perfectly fine, it is only the speedo. Also, is there only one sensor on the 94s or are there 2. I've heard a couple people say that there is one on the tranny and one on the rear pumpkin, and others say that stopped in 92, so there is only one on the pumpkin. The only sensor I can see on my tranny is the neutral safety switch, and I'm not sure that even counts as a sensor.

 

The only one is in the differential

 

^^^ And that sensor, and/or the wiring between it and the speedo, are the prime suspects. Since the odometer does not go blank, that suggests that the PSOM/speedo are okay.

 

Check the wires right at the plug to the sensor. Very common problem on OBS pickups for the wires to break near the plug. The replacement pigtail has the plug and 12-16" of new wire to splice in.

 

I’m having a speedo problem too. Same truck, except it’s manual. The speedo just stopped working altogether. Is it electronic or mechanical?

 

It's electronic. Does your odometer display a number, or is it blank?

 

It’s still showing mileage.

 

Okay, that _usually_ means the speedo module (PSOM) is good, and the problem is the sensor (VSS) in the diff, the connector or the wiring.

 

Thanks, I’ll check that out. I spent almost 20 minutes laying under the truck on the frozen ground, looking for the sender on the transfer case. Never would’ve thought to look on the rearend.

 

Yup, it's on the rear end.
Not (probably) your issue, but rust/pitting on the "tone ring" inside the diff that the sensor actually reads will cause a bouncing/jumpy speedometer signal, though it has to be pretty bad(like truck sat for years with water in the diff bad)

 

My speedo was working fine, but then just quit altogether. No bouncing or anything, just sits at zero. The odometer still shows mileage, but doesn’t move anymore.