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2008 Lincoln Navigator downshifts to 3rd gear and wrench light comes on

We have a 2016 Keystone Hideout 26RLS. We tow it with a 2008 Lincoln Navigator. The Navigator has the towing package (9000 lbs.) While hauling our TT on highway at 65 MPH (within weight limits), the transmission downshifted to 3rd gear and would not come out. The wrench light came on dashboard. When I shut down Navigator, wrench light went off and then everything returned to normal. Transmission shop rebuilt tranny, installed new torque converter and heavy duty discs. Also put in new alternator and condenser. After rebuild, the problem returned and the mechanic road tested and could not find a problem. Mechanic cleared all codes and test drove with TT with no problems. Next time out, exact problem came back. Took it back to transmission shop and they installed a new torque converter and replaced a suspected valve and updated computer. Test drove with TT problem returned, wrench light on dash and downshift. Same solution. Pull off; shut down; everything returns to normal. If not towing the TT, the car runs perfectly. At a loss. Any ideas?

Have you tried towing with the overdrive turned off ?

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Yes we have.

If you had a replacement trans that would have provided a clue if problems persisted. But you have original trans with some new parts. But not ALL new parts. Does trans perform poorly immediately? After 5miles of towing? Or does it take longer?

It would really help our guesses if you told us the transmission error codes. I’d like to know what the car thinks is wrong enough to throw a wrench light.

Sure sounds like it might be overheating because the torque converter clutch is not locking up when you tow and creating lots of extra heat. Based on your description of what the trans shop is doing, they may be thinking the same thing.

Car works perfectly without towing TT. Once we start towing and get on the highway, we only make it 1 or 2 exits … so approximately 5-15 miles. We pull over, turn off engine, then wrench light goes off and car is back to normal. We don’t go back on the highway. Turn around and go home using back roads. Car is fine at the lower speeds towing the TT. Just seems to do it’s crazy thing while towing at higher speed.

Actually, transmission shop is at a loss. They want to put in a new computer. However, we are going to pick up the navigator, hook up the TT, and have them ride with us with their scanner to see if it throws the wrench light and downshifts. We’ve done this once before but worth a try again before spending any more money.

Write down the code that throws that wrench light. Ask the mechanic for it. There will be one. Post here and we will try and help

wonder what trans fluid temps are? So, you are saying issue is towing related only? I wonder if trans had issue for along time and only appeared when you towed? No tow, you may have never found out?

I’m going to approach this backwards. Years ago I had a Pontiac with a torque converter that wouldn’t unlock when the engine was hot enough. The car would die at stop lights unless the heater was running. Check your cooling system and make sure it’s performing properly. Excess heat could be interfering with the transmission cooler doing its job.

Do you have a gas or air filter that might need to be changed, towing causes extra demand and perhaps one is not up to snuff.