Dodge Caravan 1990 doesn't shift after car gets warm

Hello All

I just got a Dodge Caravan 1990, 3.3L 6cyl and my problem is that it will not shift into higher gears once it has warmed up. When I first start it (cold) it will shift just fine, no noise, no hard shifting nothing wrong at all, then after a couple of miles (1 to 2) it will lose the gear and I'll be driving in 2nd from then on. If I shut the car off and back on while it is still warm it will not go into any higher gears either. This is the same all the time, which makes me think it is a electrical problem, but what electrical part could cause this? Or am I wrong and is it not electrical?
I am great full for any suggestions anyone may have, thank you.

Cheapest thing to do is change the tranny fluid and filter if it has one.

Cheapest thing to do is change the tranny fluid and filter if it has one.

Thanks, that is something that has already been done, and the fluid level is fine when checked. Also added some Trans X but that did nothing at all.

You're stuck going to a tranny shop as there is nothing you can check if you don't know that transmission. It is an OBD system with no feedback to the PCM/ECM. It could be a solenoid issue, but you require special tools to test line pressures and such.