[Biffster]

I've been lurking this forum since I bought the truck and am always amazed at the amount of extremely useful information posted here.
I hope that maybe I can contribute something now.

I noticed that the transmission was running hot after an hour or so at 70+ MPH. It would be stable around 190 degrees F before that.
After the temp started to rise it would continue until I slowed down. It would be accompanied by the expected hard downshifting and whine with a hot transmission.

I took it to the dealer twice for this issue. Twice they kept it for a week and said it was fine now and they couldn't reproduce the problem (I did give them specific instructions guaranteed to cause the problem). I later found out that they didn't even try. They just drove it around town and never on a highway. Eventually they told me that I'd have to overheat (destroy) the transmission to get any work done on it, even though I bought an extended warranty with the truck.
I took a photo of the gauges while running at 255 degrees F and gave it to the dealer along with an email I received from ZF stating that "There is a cooler bypass valve in the cooler line, which could be restricted. Make sure you do not drive under those heated conditions, because the transmission is in danger of overheating".

Essentially I took a truck (under warranty) to the dealer with proof of a problem and told them what part to change. That wasn't enough. They did nothing. FCA backed up the dealer when I called them.

Enough whining. With the current quarantine situation, I started to run out of maintenance items to take care of on our vehicles.
I really didn't want to touch the transmission on the truck as it was under warranty, but the dealer basically told me that the warranty was worthless so I decided to do it.
You need to be able to trust your vehicle on long trips.

I bought the pan/filter (Mopar 68225344AA), 7 litres of ZF Lifeguard 8 fluid, a replacement Transmission Heater (Mopar 68463479AA) and a Transmission Line Disconnect tool (Mopar 9546).
The heater took a few hours to do, mostly due to pre and post work (remove front driveshaft, disconnect shift cable, disconnect trans cooler line and coolant line mounts...).
The hardest part was getting the coolant lines disconnected from the top of the Trans Heater. Just a tough area to reach for a fat guy under a truck.
I did not drain the coolant. Seems like a way for someone to sell more coolant. Instead I used 2 sets of pinch pliers on the rubber coolant lines in the engine compartment that lead back to the transmission. Much less mess.

I did the pan and fluid replacement the next day. That took a couple of hours. Not a big job. Make sure to torque the drain plug on the pan before installing. It's located directly over the exhaust and there's not enough clearance for my stubby Allen wrenches to get at.
Fluid came out very clean to my surprise. Didn't look terribly different from the new fluid I put back in.

When topping the fluid off I did something probably controversial. All the documentation I could find stated that the vehicle should be level when checking the fluid level.
This transmission is used in many makes and models. All have the transmission mounted at different angles front to rear. Not a big difference but big enough when comparing a BMW 7 Series and a Ram 1500, coupled with the fact that the fill plug is at the rear of the case, not in the middle where mounting angle would not be as critical.
I asked ZF whether to have the vehicle level or the transmission and the response I received was not nearly specific enough. Basically "level vehicle and don't worry about the details". Seems like a non-answer for a precision piece of equipment.

Now I've taken the vehicle on the route that previously guaranteed 255 degree F trans temps - 2 hours at 75 MPH. Stayed rock solid at 188 this time. I also noticed that it climbed to operating temperature more slowly.
Here are "Before" and "After" pics -

 

[Robin Haskell]

I have the identical problem with my 2016 Rebel. My pictures of the dash are "identical". 250 degrees at highway speeds in the 70's. If I take an exit and leave the freeway for a few minutes it drops back down to 185 - 190. Sometimes I'll jump back on the freeway and it will climb back up to the high temps and sometimes it will remain at around 188 for 30 to 40 minutes before it starts climbing again. I now have 77,000 miles on this truck/transmission. (Side Note: My transmission was replaced at 22,000 miles because the housing was machined wrong and they could stop it from leaking. Quick question.... You swapped out the heater Mopar Part # 68463479AA but did not swap out the cooler valve.... Or does the RAM Rebel with this ZF transmission not have a cooler valve.... I'm not a car mechanic so excuse the question if it doesn't apply.... Thank You....

 

[Biffster]

Hi Robin,

It's kinda funny you asked that. I did purchase a new 3 way coolant valve, just wanting to make sure when I was done with the project that I wouldn't need to mess with it again.
I removed the air filter box to get at the location and was surprised to see I didn't have that valve at all. In fact, I traced all the coolant lines just to be sure. It's just not there on my truck.
I should have updated the post as well. After the work I did I took the loaded truck on a 1200+ mile round trip, much of it through the Kentucky "hills" with the outside temps in the 90's.
No matter how much I tried, couldn't get the transmission temp to break 200. I cannot believe the huge difference.

I don't know if the original problem was the heater block or the fill level of the transmission. It did take at least a quart more with the pan level versus the truck frame level and I know the dealer was checking the fluid with the frame level.

Either way, it's 8 months later and many highway-speed miles and the trans has been rock solid.
Hope this helps!

Biffster

 

[Snowman53]

I have a similar problem to yours. I have a 2015 Ram 1500 4x4 Big Horn Crew Cab with a 5.7 Hemi and 8spd transmission. I have about 140,000 miles on the vehicle and have an extended warranty on the drive train. The Friday before Memorial Day weekend I took it to the dealer with what I thought were a few minor issues - a small coolant leak, the famous Hemi tick, and a transmission that whined in reverse. They spent a couple hours with the vehicle and informed me that I needed a new water pump, exhaust manifold bolt replacement, and a new transmission. They needed the warranty rep to review the case to determine if the dealer would repair the trans or install a rebuilt. After about a week they told me they needed to install a rebuilt. First they had to find one. They finally found one and returned my vehicle to me after having it 25 days. 2 days later I pulled a 6x10 box trailer about 50 miles with no issues. After dropping the trailer off I headed home travelling about 70-75 MPH. After about 20 miles the trans warning light came on. I pulled off the road to let it cool while my wife called the dealer. They told me to let it cool down and to bring it in for them to check. I dropped it off that evening and they called me back the next day and said everything was OK. They had driven it about 30 miles and nothing happened so it was "fixed". I told the customer service rep that I wasn't so sure and that I was going to try to replicate the issue. I drove it about 20 miles at 70-75 MPH and the same thing happened. This time I shot video of the gauges with my phone. I called and told the customer service rep what had happened and told him that I was bringing it back and that he needed his transmission guy to do some research. The next day I took the truck back to the dealer. He said that they would have to drive it to repeat the problem. I mentioned that I had done some research on line and that people were having problems with the trans heater. I was heading out of town for vacation so I told them they had a week to figure it out. He called me 3 or 4 days later and said that they had talked to a factory rep and determined that it was probably a bad heater and that it needed replaced. It was not covered by warranty and it would cost $650. If it is an internal part of the transmission, why would it not be covered by warranty. Plus, he said it probably was the heater. So I guess I am paying $650 for them to experiment with my truck. Sorry for all the venting. I have had the use of my truck for about 3 days since before Memorial Day. I have had to make other arrangements for 2 extended out of town trips. It is great that you were able to fix your truck. Like you, I have done many repairs on many vehicles in my 68 years laying on my back in the gravel driveway but I am not sure I want to attempt this one. Can you give me an idea on the cost of the parts on this repair? it will help me determine whether I want to try this or hand over the $650. Thanks.