1996 F150 4.9L Inline 6

Having trouble finding the problem with my speedometer. Seems to only happen when im going over 50mph. Here is what im experiencing:

Driving over 50mph my speedometer will do several things depending on the day/road traveled/phase of the moon. It will either gently vary my a width of 3mph, it will sharply dive 10/20/30 mph then jump back up to proper speed or it will slam to 0 mph and jump back to proper speed. I keep an OBDII reader with me for when it happens and have yet to get a code and i get no CEL. This has been going on for about a year now and here is what i have done to try to fix the problem:

Change differential oil 75w-90 synthetic
Change VSS
Change VSS harness connector
Replace wiring from VSS connector to main harness at the frame/rear gas tank.

Another symptom is it changes the way it shifts often, sometimes its smooth, other times it gives a little jolt into gear, Sometimes 1-2 sometimes 2-3.

I bought the truck from my father in law and have put a ton of work into it. I use it to pull around a 5x10 trailer that I use for my small lawn business. Here is what I have done so far:

Upgraded to double row radiator
Valve cover seal
Pushrod cover seal
Temp sensors for dash and computer
O2 Sensors all 3
Changed trans fluid and filter
Added 12x12 trans cooler
Fan clutch
Smog pump
Belt
Cleaned MAF

 

Does the RABS lamp lite also?
If so you may have bad bearings in the rear end.
If not you may have a loose component on the PC board of the PSOM.

 

No RABS light so maybe its this PSOM?

 

I found this:

https://www.ford-trucks.com/forums/1...placement.html

Looks like the same problem im having and he fixed it by replacing the PSOM and VSS

 

The VSS very seldom goes bad and if it does it would not be intermittent.
The VSS is a small AC generator but does have a moving part to generate the AC voltage.

 

After doing a lot of reading and finding a complete lack of any true answer to the problem besides swapping out RABS and PSOMs till it works im going with the theory that the VSS isnt giving a strong enough signal and the reason for this might be the gap set by the manufacturers of these sensors. So i bought a spare just in case and went to work on the one currently installed in the truck, im guessing how much i need to take off the sensor. Heres the first attempt:

Here i took a file to the bottom of the sensor just to see how much material there is between the outside of the casing to the sensor inside. Id say maybe an 16th or less. Underneath is the sensor head witch is flat while the whole containing it is round so i mixed up some epoxy and filled the voids.


After that i used a file and removed material from the skirt of the sensor. Removed about 1/16 from that and installed it in the truck for a quick test drive then removed it. There were no signs of interference from the sensor at its new depth. I have yet to do a real test drive but will update when i do.

 

I rember Ford had a TSB about eratic PSOM operation. Something along the line of what you are doing with the VSS not being sensitive to the tone ring.

 

Well i pluged in my RABS module grabbed a 1/2 socket and ratchet and spare VSS and hit the road expecting to have to pull over and install my spare. To my surprise it worked! I got on to the highway and even got up to 80 for about a mile and the speedo stayed solid.

 

Others have shaved them down to get them to work.

 

I read that someone did too but it was always third person and nobody knew if it really worked or not. Thats where i got the idea. Its not mentioned very often, most suggest replacing the PSOM or unpluging the RABS module.

 

I was looking the other day at my Ford microfiche and I noticed that Ford had different part numbers for the VSS for the same year and same truck. The part number depend on if you had RABS or not.
I take it they are saying the VSS has to generate more voltage if you have both system fed by the VSS sensor.
Maybe the one all the auto parts stores sell is for the one system and can not handle two systems as you have found out unless it is shaved down.
Maybe the trucks sold in other Countries do not have to have RABS..