I have a 1996 F-800 and the speedometer is way off. The faster you go the more off it is 15 mph at 65 mph. Checked it against my nav system and my expedition. The speedo is electronic. Cleaned the sensor in the tranny didnt help. Didnt know what the resistance should be to check condition of the sensor. Speedo has calibration switches on the back but they were still sealed up . The wiring looks good, didnt see any corrosion. What should I replace first?

 

Have you tried taking it to a Ford garage yet? Sometimes changing parts won't help. It is possible, that the computer needs an updated reflash. I've seen vehicles that weren't reflashed, show discrepencies in the computer logs of odometer readings (odometer shows one thing, computer shows something different). I'd start there, first, and see if they can at least tell you "what" is wrong, so that you can maybe fix it yourself. We had a 2004 Super Duty in not long ago, that our scanner wouldn't confirm injector failure. We towed it up to the Ford dealer to confirm injector failure, and we serviced the injectors ourselves. With out a scanner, and being able to read sensor PIDs, you're not going to be able to effectively diagnose "which" part(s) are acting up.

 

Do you have the OEM tire size on the truck, or are they different? With the electronic speedometers, tire size (revs per mile) affects speed reading as it does with the old mechanicals, so that things may need to be reset to the new tire size. Also, is it a single speed rear or a 2 speed. If it is a 2 speed, there is a compensation circuit for low range. Could there be a wiring gremlin that is telling the system it is in low when it is really in high?

 

I too would wonder if anything has changed recently, or has it been this way the entire time you've owned it?

 

I just got the truck and noticed it off........The tires are of the correct size...11R22.5 and there is no 2 speed rear and no computer that I am aware of. It has a 5.9 cummins with an injection pump. Just an electronic speedometer. It very well maybe a combination of both the sensor and the speedometer itself. I am just gonna use my nav system as a speedometer........the odometer reads correct as compared to the nav system. Its not that big a deal but I am one of those who hates to have something that dosent work just right.

 

1996 did not have a pcm controlled engine. You are correct with the manual pump. Chances are if nothing has been changed the sensor is bad. Speedo heads were pretty reliable but a pain to calibrate correctly with the dip switches if something has been changed.

 

I thought about that as well....I took the sensor out and it had some metal filings stuck to it since it works magneticly. I cleaned it off and replaced it in the trans. Didnt chang anything. I havent priced one from Ford which I guess is the only place to get it. I couldnt see if the exciter ring in the trans had been damaged somehow . If I knew what the specs were on the sensor I could check it with an ohm meter........I put an analog meter across the terminals and passed a piece of metal across it and got a reading when I passed it across....more or less made the needle jump. dont want to put a lot on cash into it since its almost impossible to find work for it and even more insurance is unreal! Thanks for the input.