when my truck is going in and out of overdrive at 45-50 mph is the computer out or the control solenoid? its a 98.5 dodge cummins.

 

huh? it wont stay in overdrive? jumps in an out?

 

That is near the speed that the torque converter locks up in most of our trucks. If it locks up the slightest bump will inadvertently cause your foot to change pressure on the accelerator pedal and cause the tranny to unlock . If you maintain the same speed it will lock up again.
This is my theory on this condition and I may just be all wet.

 

Usually it is around were the OD kicks in.. OR like you said Charlie he could be feeling the lock up going in and out

 

Well every now and then it's kicks out of overdrive someone thought it could be the computer or the solenoid. Is the tranny computers expensive

 

you'll want to determine if its dropping the actual OD gear (+500 rpm), or the TC Lockup (+300 rpm). different years and versions of pcm programming shift these in/out in different sequences.

it could be the solenoid.
it could be a TC Lockup clutch thing.
i doubt its the PCM. (no separate trans computer).

could be simple things like:
bad throttle position sensor (TPS) <<---- common
bad brake light switch.
improper TV cable adjustment.
transmission overheat

a real time scanner can identify whether the PCM is commanding the trans to drop OD, or whether its dropping it on its own.

w/o any other info, change fluid/filter, drill out check valve, and change TPS.

 

Swap in a stick. That should fix it.

 

"What should I do.." - suggest naming the thread based on the technical issue. It will allow those that are knowledgeable to see the thread and actually review and respond.

If driving around 45-50 like in town, I suggest turning OD off to stop it. This is on the edge of the rpm/pressure differential used to control shifts.