I know that this topic has been discussed alot on this site, but I haven't quite found a match with my problem. I have a 1996 f-250 a/t 4x4 with the 7.3l in it. The truck started shifting hard in first and second and o/d light would flash, it only did it accasionally but got worse over time. I replaced the vss, the speedo started jumping eratically at about 45-55 mph, but the truck seemed to shift ok. Now it has started shifting hard again and the speedo still jumps around. Should I replace the vss again, the last one I bought came from Napa, so should I try to get one through ford, or is there anything else that could be the cause. Any help would be appreciated as I live on Kodiak island and getting vehicles worked on here is horribly expensive and usually not done correctly. Thanks.

 

Are there any codes?

 

No, I took it in to our local ford dealer and he said that there were no codes, but the truck was not acting up at that time. He said that the computer on the truck will delete the saved codes after about seven days.

 

Unplug the RABS module and see if that fixes it.

 

Complete BS

 

Where is the RABS module located, and if unplugging it seems to work do I just need to buy a new one, or just bypass it some how?

 

It is behind the glove box.
If the speedometer and the transmission works OK with it unplugged it could be a bad RABS module or a week signal being generated by the VSS on the rear end.

If you still have the same problem then you need to check the A-C voltage being generated by the VSS @ apx 30 MPH. You can use jack stands under the rear end or drive it while someone else monitors the voltage.

Here is how the Ford Shop Manual says to check the voltage on the A-C scale of your meter:
Note: Only wiring harness end of connector is to be probed. Vehicle speed control may be used as a good indicator. If it works normally, then the speedometer module is at least receiving a speed input signal and the wiring and sensor can be assumed to be good.

Connect Rotunda 88 Digital Multimeter 105-00053 or equivalent to Pin 4 (speed in +) and Pin 5 (speed in -).

Does the voltage increase smoothly and continuously from 0 to approximately 3.5 volts as vehicle speed increases from 0 to 48 km/h (0 to 30 mph)?

Yes
REPLACE the instrument cluster .

No
SERVICE wiring. CHECK for open or shorted wiring. If no problems are found, REFER to «Section 06-09A» or «Section 06-09B».

RABS plug under the hood:


RABS module location, Image of a 1992 but the location of the module is the same on a 1996, C277 could also be tapped for the voltage test:


Wiring diagram of a 1995 but the 1996 is the same:


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Subscribed, my 96 F150 is doing this.

 

Well I unplugged the RABS module and the speedo works fine, and the shifting problem went away for about two days, but last night it seemed to start shifting hard again. So I am not sure where to go from here, but maybe it is a faulty pig tail on the vss, how long is the pig tail, because on my truck it goes right into the loom.

 

Sounds like the PSOM is needing a stronger and stronger voltage from the VSS so I think I would try another PSOM.

 

Ok, what is the PSOM and where do I find it on the truck.

 

PSOM: Programmable Speedometer/Odometer Module, A module that processes vehicle speed information for use in various electronic systems. The PSOM can be programmed to accommodate various tire and axle combinations.

PSOM:







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