Hi all. First time poster here. I looked through the other threads and didn’t see one on this particular topic, so here goes…
I recently bought a 2007 F-350 with the 6.0. It’s a crew cab with automatic trans and 73,000 miles.
I just got back from an 800 mile trip over the weekend pulling our 10K Lb travel trailer. The roads were all interstate-type 4 lane highways. My concern is how the trans was acting. I also have a 1997 F-350 with the 7.3 and an automatic and I had a 1995 Supercab F-250 PSD before that, so I’m using those as a reference.
The trans on the new F-350 doesn’t seem to be locking the torque converter. I set the cruise control at 65 and use the Tow/Haul mode. If the truck needed to downshift (out of 5<SUP>th</SUP>) to make it up a hill, the trans wouldn’t always hold a steady RPM. The engine speed would vary by several hundred RPM but the truck’s MPH would stay steady or maybe vary by 1 MPH up or down. This tells me that the TC isn’t locked. The trans temp never moved from the middle of the gauge, but the engine fan came on several times. Also, the trans wouldn’t always shift back to 5<SUP>th</SUP> after cresting a hill or even on a slight down-hill – it stayed in 4<SUP>th</SUP> at about 2,200 RPM. If I clicked the cruise down by 1 MPH, it would usually drop back into 5<SUP>th </SUP>and 1,800 RPM.
On my older trucks, once the TC was locked, it stayed locked even if the truck downshifted from 4th to 3<SUP>rd</SUP>. It stayed locked until I lifted up on the accelerator.
I’m not sure if it’s related or not, but the truck got crappy MPG – about 8.5 average for the 800 miles. My ’97, towing the same trailer on the same trip last year, got almost 11 MPG. My guess is that if the TC on the new truck isn’t locking, there is a bunch of wasted energy being spent on transmission heat.

Thanks for any ideas!

 

You're right, the converter is not locking. It's not broke, that's the way it was designed. I fought that as hard as I could, but lost. I wanted it locked A LOT more, but the manglement above me didn't. That's one of the big reasons I don't work there any more.

The ONLY way to fix this is an aftermarket tuner. No, I can't recommend one.