The speedometer on my 95 F150 does not work right. Sometimes when on the highway, it will bounce up to 60 or 70 and drop back to zero in a split second. While at lower speeds, it usually doesn't work at all. I was told this may be some metal spring that wraps around the needle behind the instrument cluster. Does this seem right? Anybody replaced one of these metal springs before? Could the Vehicle Speed Sensor also be at fault?

 

I believe I have found out that the speedometer is electronically controlled and receives a signal from the rear abs differential sensor. Does anybody have any diagnostic information on how to test this? I would assume I do not have the metal spring behind the speedometer that some other person suggested was the problem.

 

Welcome to FTE!

Try a search for "bad PSOM" you'll find all the info you need. It comes down to 3 things, the rear axle being in good shape, a clean speed sensor (on top of diff), and sometimes it comes down to a bad PSOM. The PSOM is programmable speedometer/odometer module. It's the head unit in the dash. They're known to go bad, but when they go bad they don't give those types of symptoms, so I don't believe you have a bad PSOM, but if you look up those threads...it'll tell you what other things you need to do. I personally have told probably 20 people everything there is to know about answering your question...which is why I suggest to try the search

Take the speed sensor off the top of the rear diff and clean it, re-install it (don't bother replacing it, it doesn't really go bad). Then if still no luck, pop the cover off the rear diff and check the components inside. Other threads u pull up on the search will give more info.

 

Hello Mustang,
I'm new to this board and would like to ask about your explanation of the rear sensor on the axle you speak about. My van is on a motorhome chassis and has 7K miles on a 1999 with analog dash. All of a sudden my speedometer quit working but the trip odometer and regular odometer both work. Checking underneath i noticed there is a rear sensor and I removed the plug wire and checked it and it looks good, I also noticed a sensor on the side of the trans case which looks like a speedo sensor also. Which one is for what purpose? and what would be your best guess as to what the trouble can be. Thanks for any suggestions you may have........Orest402

 

...Welcome Orest402, to Ford Truck Enthusiasts! Please read the Guidelines, it will help you to navigate through and understand this site.

We are happy you have chosen the best source for Fords!

Enjoy FTE....and
JOIN CLUB FTE -SUPPORT THE FORUMS!
…..See you on the boards.

 

My speedo didn't work well when I bought it a year ago, but it has gotton slightly worse. Sounds like the same symptoms as your...nothing, then it will spike up to speed momentarily. Anyways, while working on my transfer case, I noticed hardly any of the teeth are left on the speedo gear (the one on the tailshaft). If you pull the cable/sensor out of the tailhousing, you can inspect the small plastic gear on the end of the cable. Feel in there for the other gear. If it's anything like the condition mine's in, you'll feel it. I expect it to work after I get'r back in, but I can't confirm this is the only problem yet. Makes sense to me though.

 

Hey, thanks for the input........is this the gear in the sensor in the rear pumpkin on top? I can't imagine that a gear would be worn at 7K miles but stranger things have happened...............Perplexed...

 

No, I'm referring to the one in the transfer case tailhousing.

 

After searching through old posts, I have gathered some more information. These PSOM's are known for going bad in these trucks. It appears that the 92-96 trucks used the rear anti-lock brake sensor as an input to the PSOM. This is located on top of the rear differential. This sensor is a magnetic pulse generator. These sensors do not normally go bad. It is possible that some crap has built up on the sensor preventing a clear signal from going to the PSOM. You could try cleaning the sensor. It is also possible that the reluctor ring inside the differential is causing the problem. I also understand that the air gap between the sensor and reluctor ring needs to be correct also, which is .015-.020 thousands. If your ABS light is on, then more than likely it is not the PSOM causing the problem. If you have lots of play in the differential due to worn out bearings and such, this could affect the reluctor rings position which would affect the signal to the PSOM. I also read that one member here had a bouncing speedo problem, and when he pulled the differential cover, found the fluid was low. He refilled the differential and fixed the bouncing speedo. Again, this is information I gathered from old posts, please make any corrections or suggestions.