Just bought a '92 F350 (460) a couple of weeks ago and had a chance to try out the cruise control. When activated on a fairly level location, the cruise control oscillated quickly from accelerate to decelerate. It wouldn't settle at all, just constant changing speeds, up and down. 188k miles. Is the cruise control mechanical and just needs cleaning and lubricating? The truck had been sitting for at least a couple of years unused.

Jeff '92 F350, '64 F100, both CrewCabs

 

The vss in my truck caused the same thing as the gears went bad because it didn't keep speed with the driveshaft yours doesn't have gears but it could get the same problem

 

Researching the VSS reveals a couple of things. They change the VSS to a DSS (Differential Speed Sensor) that is located on the rear differential to accommodate the anti-lock brake sensing. This same sensor also is used for the speedometer, which is fairly accurate judged against adjacent traffic. So, I don't think it is the DSS, but thanks for pointing me in a direction, from which I learned some new things!

Jeff '92 F350 CrewCab and '64 F100 CrewCab

 

The speed control system in your truck is vacuum operated with an electronic interface. There is a dump valve on top of the brake pedal assembly and a vacuum reservoir behind the driver side headlight. That reservoir was open to atmosphere on my old 1992 F350. If the speedometer does not fluctuate I would suspect a vacuum leak, bad servo or binding cables.

 

The differential mounted sensor was used for the RABS system only until 1991. In 1992 Ford used the same sensor to provide the same vehicle speed output to the RABS system then buffered it with the PSOM to use with the electronic speedometer, speed control and vehicle speed input to the computer (PCM).

 

You might find this helpful:

1992 F Series Vacuum Speed Control:
31-1


31-2

 

It very well could be the PSOM sending out bad speed info.

The speed control hunting was the 1st problem of a PSOM failing, then it got to the point where the E4OD tranny's shifting was affected and code set in the ECU. All the while the speedo needle never wavered once.

Got to love those leaking capacitors in the PSOM's pc boards......