I've got a new-to-me 2006 F350 5.4L.

I hauled my 5th wheel trailer for the first time last week and I had some issues. I estimate that it's about 7000lb when loaded up, so well within the truck's capacity.

I brought the trailer home (100km) and it was fine.

When I had it all loaded up and we were heading out, about 5 minutes in the tow-haul light started flashing and the tranny started shifting hard and generally not working well.

Turned around, un-hitched and took it into a tranny shop where they replaced the TC solenoid.

The next day I hauled the trailer about 400km (250 miles) and everything was fine, until just when I parked, the tow-haul light stared flashing again.

It was still running fine, but I took it into the nearest dealership and they ran the codes and tried to diagnose the problem. The codes were showing multiple solenoid errors, which they said was very unlikely...probably an electrical issue instead. They checked what they could (within the scope of a basic diagnosis) but didn't find any issues.

A few days later, as I was hauling the trailer home, the trans temp gauge (which had remained normal through all of this) would spike and fall. I wouldn't say that it was erratic, as it usually followed an increase in RPM and a downshift while going up hills. But it would go from normal to high in a few seconds, then back down to normal a few seconds later.

So what do you think is going on?

I've seen a post or two about the temp sending unit (and possibly the wiring harness) being a problem. Is that likely?

 

Yes, that's likely. I'd know a lot more if you had the numbers of the codes that were in the computer.

 

P0710
P0758
P0976

 

May I suggest an OBD Bluetooth dongle/transmitter and Torque App. Gives you ability to monitor tranny temperature (among many other parameters) in real time. I never EVER tow with my rig anymore without it up and running.

 

Those two codes indicate electrical circuit problems. P0710 is the electrical circuit for the temp sensor, and P0758 is the electrical circuit code for shift solenoid B. When two or more electrical codes show up at the same time I always suspect a damaged wiring harness.

This one also refers to an electrical problem with shift solenoid B. I really don't understand the difference between these codes, but they all point to wiring issues.