I have a 94 f-150 351W e4od i have been having problems with the speedo jumping around above 30 mph. it jumps about 20 mph up or down. it never did this untill i had the tranny rebuilt. i put a new speed sensor in the rear end. i put 4.11's in checked the tone ring and its all good. put a new instrument cluster in and still have the same problem. it also reads 5mph slower than waht i'm really going with the stock gears and then also with the 4.11's can anyone tell where to start looking next.

 

in a 94 it shouldnt have anything to do with the trans its all electic. is the speed sesor pushed all the way down?

 

yea it's down all the way

 

Are you saying it did this the first time you test-drove it after the rebuild? Who installed the trans: you or a shop?Before this symptom, or after? Did you keep the original?How exactly did you "check the tone ring"?Did you keep the original? Did you swap your speedo into the replacement cluster? Where did the replacement come from? What year truck is it for?Did you program it for your tires?Read this whole caption:

 

i already tried to reprogram the PSOM set my constant to 8.78 since i had 285/75/16 and it still bounces all over. yes it started the first drive after the tranny was rebuilt.the tranny place put a new speed sensor in.my buddy that help me with the gears said the tone ring was in good shape. not sure how he checked it.it had an aftermarket instrument cluster so i went to the junk yard and got an OEM one from another 94.

 

Hold it - an AFTERMARKET cluster??? What brand? Is that the one that bounced, or the junkyard one? Did you take the PSOM from the aftermarket cluster & put it into the junkyard cluster, or are you running the junkyard PSOM?

 

the aftermarket one was the one when it started doing it. put one from the junkyard on and same thing happened. it doesn't matter wich PSOM goes in which it still jumps around. don't know what brand it was in the truck when i bought it all it says on it is made in canada and has optiplex stamped on it. i think it's an aftermarket cluster it has white gauges with chrome all around and blue backlighting and all of the check engine lights, bright light, blinker, ect.... are all in different locations than the original cluster

is there a rectifier or something or speed control box on these trucks seems to me the speed sensor puts out ac current and the speedo runs on dc so there has to be a converter somewhere

 

Then it sounds like it's Optiplex brand.

Did you read that caption? It explains how the PSOM works.