my speedo has been acting very strange lately..... sometimes its reading at least 20 mph off, sometimes 10, but sometimes its right on,,,,, does anyone know what the problem could be? thanks

 

Tell us more about your truck. Year, model, tranny etc.

The speedo is driven one of several methods but without the above info we are just guessing.

Jim Henderson

 

it is a 91 f-150 with the E4OD

 

My speedo was jumping around. I changed the pickup sensor in the
rearend. It got worse, so I put the old one back in. I then disconnected
the rear abs box behind the glove compartment and it works know. I
do not care if I do not have rear abs. It locked the rear tires up when it was connected. Now I can use the cruise control again.
I know this is not the right way, but it worked for me.
My truck is a 1993 f150. If your speedo is the same setup you can try it.
You can do a search for speedo. There are other members with the same
problem.

 

That '91 likely has an all mechanical speedo and I'd say the speedo itself is on the way out, time to go shopping for a replacement cluster to pull the speedo out of.

 

Go digital w / a new 3 1/4 inch full face gauge. The speed sensor at the back of the trany should be O.K. It's just a magnet spinning around. I went all digital w / my 89.

 

thanks guys i will prob have to start looking for a new speedo then

 

I have had a problem with my mechanical speedometer jumping and not reading properly. I have seen this problem in the past growing up in my Dad's shop. Remember, here the speedometer works through a flex shaft that is connected to a gear at the tail end of the transmission. the flex shaft itself is dry and binding inside the the cable housing. disconnect the speedo from the instructment cluster and spray some lube down the shaft.

 

Maybe too late, but if you haven't bought anything yet...

I am not sure when the electronic speedos came in, my 94 F250 does have electronic. Don't know your model year.

Crawl under the truck for a minute or two and look at the top of the differential.

If you see a walnut size thingie on top with a couple of wires on it then you probably have an electronic speedo. Usually erratic speed reading with the electronic speedo are from a bad speed sensor(VSS). Replacing that and maybe checking the inside of the diff for metal fragments might be the cure. Sometimes when the VSS dies, junk gets spread on the ring that sends a signal to the VSS, so then you have to clean the inside of the diff. Usually this isn't needed since the VSS just dies or gets corroded connections.

If you have a mechanical speedo then I would suspect a sticky cable or bad speedo or gear in that order. The lube down the cable is a useful fix in many cases, cheaper and slightly easier than a new cable.

Good Luck,

Jim Henderson