I was just noticed today while driving that when I get to about 50mph that the speedometer jumps up and down? Has anybody seen this before? Help will be grately appreciated.

 

There's a sensor on the rear differential that the speedometer runs off of. It could be dirty or have a bad connection. There's a tone ring inside the rear end for this sensor.

If that's all fine, it's probably the speedometer head itself that needs to be replaced.

 

Do a search under "bad psom" thats a very very popular problem and has been talked about a lot.

 

Ok guys

I replaced the senor on the rear diff it seem to make the problem worse. I did have the rear diff rebuilt about a 2 months ago. So I am stuck!! I have read where it may be the token ring. Is there any way in troubleshooting the module. I used my snap on scanner and it did not pick any codes up. Another thing can I just to the PSOM (module) by itself of do I need to get the entire cluster, and is it really hard to change out?

Thanks
nmmountainguy

 

You can replace just the PSOM. Pull the instrument cluster, take the screws out on the front and you'll see how each section of the cluster pops out separately. Don't worry about getting one from a truck with the same tranny either, that cover plate can be switched too.

 

If you look in the tech articles section of the website, you will find an article about chaning to a tach, that will show you how to dissassemble the cluster. You can buy the entire cluster from ford it's about 350 bucks, or you can get just the PSOM (programmable speedometer/odometer module) and swap it into ur truck (off ebay or junk yard). But you'll have to have it calibrated at a dealership for mileage and tire size so you'll have to check on how muhc that costs.

To check the tone ring...just take off the rear axle cover and check the teeth on the ring. If theres any wear or broken teeth it needs to be replaced. Has the needle bouced since they rebuilt it? It's possible they damaged it when re-assembling it.

 

Is it difficult to replace?