Q: ford f350 transmission shifts up and down on 1990 Ford F-350

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
hello i have a f350 and when im just cruising throught own at like second gear the transmission will shift up then back down and keep doing it till the trucks shift to drive what could be causeing that
(1) Answer
You likely have an E4OD automatic transmission. (there was a C6 available, but it's a 3 speed only with no overdrive).

I suspect you have an issue with your FIPL sensor (Fuel Injector Pump Lever). This sensor is a potentiometer, much like a throttle position sensor, that influences transmission shift points on the Diesel powered vehicle. It can be adjusted, and probably needs to be.

You need to find someone with experience on these older Diesel trucks. There was a limited time frame when FIPL sensors were in play, and many new techs don't know what they are.