Run highway speeds come to stop transmission bumps into 2 gear

It's actually putting itself into second gear. Chrysler A-604 transmissions are an electronically controlled transmission which use speed sensor to see things like vehicle speed, engine rpm, throttle position and internal transmission component speeds. It can determine if there is an issue with the transmission based on these sensors and when it does it tells the transmission to go into "limp mode" which is basically just second gear. This would allow the driver to move off the highway to the side of the road or drive it slowly for a while to a service facility. To determine the exact issue with the transmission you need to go to a shop that can read the transmission codes and decipher whether the transmission can be repaired in the vehicle or must be removed and rebuilt. It is common to have a sensor fail and cause this issue which means a reasonably low cost repair. It is also common for the trans to need complete overhaul. It IS worth it to go to a "qualified" transmission repair garage though so it can be properly diagnosed.

Good luck and hope this helps.

P.S. Did you check the fluid level first, idling in park? Had to ask.

Did you hear it hit something when you went over the bump? U could of hit the linkage to the trans and knocked it out of sink.

There may be a problem with the electrical connections to the transmission and/or vehicle speed sensor, remove all connectors and check for damaged wires and corrosion. Make sure not to break any connector locks.

Probably too late but for other users searching: You probably have the 4T65 transmission like I did. This is the vehicle "hunting" in and out of overdrive on the highway (marked "D" on your dash) and you're probably on level (or close to) ground. You have two options: drive in the gear selection "3" (more fuel consumption though) or change your speed. I used to drop down to 53 MPH and it would stop, or on the highway @ 65 for it to go away.
Seems to be common in this trans around 60 MPH. Also - start saving for another vehicle or repair. This trans will start to slam into 2nd gear from a stop after it warms up (internal solenoid fix required) OR it will shudder horribly from a stop after a highway run. My fix was to replace the solenoid in the trans (there's video on YT) by pulling the right side of the drivetrain off the van and pulling the trans cover. If you go this route - you *must* ensure no -and I mean NO- dirt gets in that tranny while the cover is off.
Good luck!

check for a pinched or crushed trans cooler line going to the radiator i have seen them crushed from running the car up against parking curbs, underneath the radiator

When you shift into park and if Park holds then the trans is gone. (Auto clutch packs burned up) Smell for burned fluid. Pull up the trans dipstick with the car running and warmed up. Make sure the level is good. Most likely a trans Overhaul is needed.

We had a 1999 Sebring that would go up through the gears and shift fine until you stopped at a stop sign. Then it would hang in first until you turned it off then back on. Then it would shift normally until you stopped again. Turned out it was a bad speed sensor (not the one for the speedo). Speed sensor is located on the transmission and several years ago was a $20.00 part. I'm not sure if this your problem or not, just thought I'd share this with you.

Good Luck

Bob

Either it one heck of a coincidence or the trans is somehow associated with or to the water temp.. The only way I know of is the transmission oil cooling. Ck the trans fluid level and onsider having the trans service.Oil change and filter and confirm or ask about the relationship of cooland and trans, if there is any. Good Luc and please let me know what you find out. Thanks Ned

code p 1870 refers to transmission component slipping
That can be from dirty oil or a clutch pack or band slipping or low oil pump pressure in the transmission
you say that it occurs on highway driving after 2 hours and that slipping will add friction heat to the transmission oil bringing on the oil temp light
take it to an accredited transmission professional service center that works on your make of vehicle , have the oil changed which require the pan removal
at the same time that will allow the adjustments to be done
when reassembled and refilled , have a pump pressure test done

this year of van had a recall for the transmission because the second gear was getting too hot. It required a dealer to install a special part that spliced into the transmission cooler lines and dumped fluid down the transmission fill plug hole and onto the second gear. Perhaps under high speed, the slice is leaking. Check to see if you have this part and see if the splice clamps are loose. If so, tightening them may help.