in and out of overdrive

Hi guys, this is my first post,I hope you can help me. I have a 1994 Nissan pick up 4X4, 3.0 motor. My truck keeps going in and out of overdrive, I could have the cruse set on 65 and everything is find the trucks in overdrive about 2500 RPM then I`ll start to go down a little hill and the truck will downshift and jump up to about 3700 RPM, then once it starts to accelerate it goes back in overdrive and 2500 RPM again, till I start to go down hill then she`ll downshift again. It`s not just at 65 MPH but anytime I`m in overdrive it will go in and out. Any ideas whats wrong with my truck?
Thanks
Joe

There's two times I can think of when it should go into a lower gear. 1. when accellerating; 2. if it needs to slow down when in cruise control (i.e. when going down a hill)

Are these two things your case?

Automatics will always do things you don't want them to do. They do not know your driving style or conditions or in general what you want. That's why they have automatics where you can easily go though the gears now. More thoughtfully designed cars have had them for decade(s) already. Trucks are not in this catagory incase your wondering :grinno:

Jimmy, thanks for getting back to me, but this trans just started doing this about a month ago. It would never downshift when I was doing 65 MPH before. And you said it should downshift when you accellerate, I know it should,if I`m doing 40 MPH the truck used to be in overdrive,now its in 3rd gear, and like you said when you accellerate it should downshift. But now when I accellerate it goes into overdrive. It seems like the trans is doing the opposite now. I just had a motor put in around the same time this started, the motor was out of a 1992 Pathfinder with 72000 miles.Any more ideas.
Thanks
Joe

I suspect one of the sensors is malfunctioning, likely the throttle position sensor. The ECM cannot function properly because it is missing this critical piece of data.

Come on guys, somebody, anybody, HELP!!!!!. Does anybody know how to adjust the TPS????
Thanks
Joe

Come on guys, somebody, anybody, HELP!!!!!. Does anybody know how to adjust the TPS????
Thanks
Joe

ya I know how to adjust them, but they tend to be non adjustable nowadays - pretty much plug-n-play.

To adjust you need a DVOM and backprobe the output signal with repsect to ground and move the TPS (if movable) until the read voltage (typically around .5v) reads the idle spec. A lot of times you can use a scan tool to set it if the ECM returns TPS voltage and not just throttle angle.