My '99 Grand 3.3L with 165k has orignal tranny and I replaced the radiator, changed the tranny filter and fluid with ATF-4. Test drove it around the block and it was slipping and had to pull over a few times as it would not go forward. The tranny was running fine and now it's throwing codes P1782 and P0700. It will shift into reverse but slipping in forward or moving slowly when I move a few feet in and out of garage. I'm afraid to leave the garage since the forward is shaky at best. I checked the fluid level and it was high. Dropped the pan and took out about a quart. Added 1/2 quart and it was reading dry. Added a few more ounces and it then read really high. I've been checking the level lately and it's never been this difficult to get consistent level on the stick.

Any suggestions as to what is going on and whether there's any hope of saving van from grave yard? My wife drives it only a mile to work and back and would hate to junk it.
Since I don't really want to go to a dealer I'm thinking about throwing some parts at it from junkyard. If I replace the control solenoid will I have to get the van to a dealer for a tranny relearn? Should I replace any of the sensors? Your help is greatly appreciated!

 

Sounds to me like you have an air pocket in the
transmission cooler (inside the radiator) and/or lines.

Things to check for:
* Did you install new cooler hoses?
* Make sure you didn't swap their installed positions otherwise you'll have problems.
* Did you verify that there are no restrictions in the cooler before installing the radiator?
* If you used anything other than ATF+4 then drain-flush it out and refill. Probably should install a new filter while you're at it as you can ensure that the current one hasn't fallen down into the pan...this will result in loss of pressure as no fluid will be picked up. Same story happens when the o-ring on the filter is left out or doubled up (new one along with old)

There's also a chance that the front pump failed due to air pocket (last straw syndrome). I've seen this happen to other brand high mileage transmissions and even engines.

If you look at the transmission in the area near the solenoid pack you'll spot some threaded plugs. These are ports to install a test gauge to check pressures for diagnosis.

If you have a factory shop manual along with an appropriate gauge and know which end of a screwdriver to stick in your nose then you can proceed now that I've given you some clues.

Good luck and please report back.

 

Forgot to mention that while you were getting greasy that you may have disturbed the wiring harness to the speed sensors and solenoid.

Remove the connectors and inspect for wear, damage, corrosion etc. and repair as needed.