i have replaced the speed sensor and the wire up to the fire wall. what would make it jump around.

 

Maybe a bad ground? IDK just a guess.

 

Perhaps the tone ring on the carrier is damaged or has come off the shoulder upon which it is supposed to sit?

 

On an '81, the speed sensor wiring has nothing to do with the speedo, and the sensor itself is just a mechanical link from one section of the cable to the other. If the speedo is jumping, either the speedo head is bad, or the cable needs to be cleaned & lubed, or replaced.

 

:o
Doh! I didn't notice the model year

 

Me too! Way to keep us honest Steve.

 

My truck is a 95 and I have checked the rear ring

 

And only jumps around 60

 

I had a 91 and my speedo started to jump a little at first then alot and it ended up being a short some where, never found out where, and had to have the whole speedo replaced

 

I don't see how a short would do ANYTHING to this type of speedometer. There's NO, again NO electronics involved. Well, the light bulbs are pretty close, still do nothing. There is a cable (mechanical cable, not electric cable) that has a square shaped end that fits into the back of the speedometer. When the speedometer is out you can even turn it by hand and the needle will move. Where's the electronics in that? Nowhere. The cable that I mentioned goes from the speedometer to the transmission, it goes into the transmission, where it's turned by a gear. No electronics.

Since there are literally only 3 things invloved, the speedometer, the cable, and the gear, repair shouldn't be all that bad. Checking the gear is relatively easy, and cleaning the cable is somewhat easy too. When\If you need to get a new speedometer, make sure the donor truck's drivetrain matches yours. I upgraded mine from a 5.8l (mine is a 5.0) and the speedometer is off a little, my guess is the donor truck had different gears than my truck.

 

Fastford's 1995 speedometer does have electronics involved.

 

If you want to try and correct your speedometer, you can get that GEAR with a different number of teeth on it.

 

Yea, I know I can get another gear, but I plan\hope to get bigger tires soon, so I'd end up switching it back anyways. My bad, I didn't see the post about it being a 95, I went off his name so I thought it was an 81.

 

So did Steve, but look at post #7..........

Need to read the whole thread before you jump in that forcefully!

 

I figured it would be something like that... PUT IT IN YOUR SIGNATURE so it always shows with every post. You can omit 19xx and Ford F150 - we assume that. Put the engine, trans, 2/4WD, trim level, miles, mods, damage, repairs... Things that are specific to YOUR truck and have some bearing on questions you'll be asking here.

Then read this whole caption & the links in it:

Actually, I was trying to give him a subtle hint to create a signature by posting info about an '81.No, that's a cable-drive, too. '92 was the first year for an electronic speedo, as that caption details.