My Speedo seems to bounce all over hell, accompanied by a small ticking sound. it does it at odd times, not everytime i drive it. it will just do it for a second, then go back to normal. This is a 96 F350, 5.8L auto 4wd. Truck has 63k orig miles, but plenty of western ny rust underneath. Any info is appreciated fellas!

 

speedo cable

 

Your '96 does not use a speedometer cable. The speedometer is electronic. The VSS sensor on top of the rear end housing generates the signal for the speedometer as well as the RABS module.

There is a box under the dash that converts the VSS signal to something usable for the speedometer to measure called a PSOM. I would start by removing the VSS then inspect/clean it. This is the most common cause of your issue.

 

Does your cruise work? The RABS has been known to cause a fluctuating speedo also, but I would change the VSS first and then go to the PSOM

 

My RABS module causes mine to have a slight bobble all the time and to fluctuate a little which is about 5 mph randomly at highway speed. I leave it unplugged, since I can't figure out whats wrong. I have tried 2 new/reman ones and 3 used ones. Some work better than others.

Try unplugging it and see if your problem goes away. I would do this test first since its free and the easiest. Its the black box in front behind the glovebox.

I agree that the VSS is generally the issue.

 

thanks alot guys i will try that as soon as this 4 feet of snow melts away that my truck is buried in. i appreciate all the help.

 

I did what Texasguy did and it worked for me. I can use the cruise control now. I first replace the vss sensor in the rear end but it made it worse. The vss was a Autozone one. So I put the original one back in.

 

the diff sensor is only 45 bucks from ford , start there! the psom is the actual speedo head itself . you could have a messed reluctor wheel in the diff as well. only times i ever had issue with my speedo is when the truck blew up the 8.8's .

 

You can get the Motorcraft VSS from Orielly for 25-30 bucks.

I'm still curious what the issue with mine is. I have tried a few different RABS modules and none work well. There is a voltage drop with it connected.

 

mine does same thing but its a 92 f250 5.8 4wd 5spd

 

If you swap the sensor and find it isn't the cause you'll have to open the differential to check the exitor ring. The teeth on that ring causes the signal on the sensor, so if it's broken the signal will make the speedo jump, if that is the case you should look at other damage or wear on the diff.