I'll start with the short version.
At 20,000lbs my '99 F350 7.3L/ZF6 will hit 230-deg on the manual transmission temp gauge at 75mph within 40 miles.
Empty, it takes 100 miles to get in that ballpark.
At 230-deg, I pull over to the side of the road, and grab both cooler lines at the radiator.
Both sides are cold. Not warm and hot, not hot and hot, cold and cold.
50mph in 5th gear at 20,000lbs keeps the temperature around 195. 55mph and it creeps up.
Bad bump/drive? Bad pressure relief?
I confirmed the cooler is clear - disconnecting the lines from the transmission, I was able to blow through the circuit with nothing more than pucker power.
I also got a squirt of fluid out of the ZF6 when I let the clutch out, but can't really say much more about it (I was in the truck, wife was keeping an eye on the ATF puddle)
Adding an auxiliary cooler did nothing to change the situation.
I did not completely bypass the factory cooler though. But as noted, confirmed I can blow through the cooler.
Now the back story.
2nd owner of an e99 7.3L F350. 183k miles.
I loaded my gooseneck with a Scout II 4x4 and hauled it to Spokane for a car show. Transmission saw 230 or so. Driving empty, it had never moved within the first 100 miles, and had never gone above 150. This was the first tow.
230 or so going over the hills to Spokane. It had me worried, but it was OK.
I changed the fluid when I got home - it was probably due.
Left for Colorado, same load, with another 3,000lbs of pop-up camper.
Transmission approached 230 the first day. Second day, saw it again.
While pulling a modest grade at 70mph with the cruise on, I heard a pop/smack. I chalked it up to a very bad thrown-rock, because what else could it be?
Well, it was the throwing bearing taking a bump, and/or one of the springs from the clutch disc coming out, and/or a few of the other things that got bounced around inside the bellhousing in the ensuing melee.
450 miles of no-clutch was ahead of me. Aside from the transmission running hot, no big deal.
Transmission ran hot. And hotter. Saw 240 and I was concerned, but needed to get to my destination. Saw 250, at which point the gauge numbering stops - that must be a bad number since they didn't bother to mark the gauge after that.
I can confirm that north of 250, and by 260 in particular, the shifter rattles pretty good.
So I had the clutch replaced, and had the ZF6 opened up and found it was smoked, and a reman was installed.
Transmission shop confirmed the cooler was in good shape and free flowing, and talked me out of an auxiliary cooler. They said I'd be fine.
Thus, on the return trip, 43 miles out, I pulled over to let the transmission cool, and after another jaunt, I was installing an auxiliary cooler in the CarQuest parking lot, which ultimately did nothing for me.
I drove home in 5th gear, 2000rpm, 50mph. 12.3mpg wasn't too bad, but the two days of driving took the fun out of it.
Confirmed on multiple occasions that I had cold cooler lines, both sides.
After I got home, I went for a 75mph drive 100 miles away, and watched the transmission temperature climb - hotter than the original ZF6 ever got driving empty.
My conclusion - the pump drive key isn't installed correctly, or the pressure relief on the pump output is too low.
Of course, the nearest service shop for the warranty work is 250 miles away..
I'm not sure why the original ZF6 ran hot - part of me wants to blame it on the transmission being damaged before I got my hands on it (~3000 miles ago) and that contributed to the heat buildup, but I'm not sure.
I never stopped to check the cooler lines on the original transmission, that I can remember. I remember checking that the transmission was too hot to touch, but I don't recall checking the lines.
If I need to be concerned about the ZF6s temperatures when loaded to GCWR, then I'll go back to insisting I run an aux cooler - once the pump is confirmed to be working well.
What else am I missing? Is this fairly common?
My friends run 80+ in their ZF6 equipped trucks pulling comparable loads and do this all day.
The SD was nice with the quieter cab, air conditioning, and the 12mpg loaded.. but I was yearning for my '74 IH Travelette I'd left at home.. sure, 6mpg hurts the pocket book, but the old T-35 stick shift wouldn't have complained, and I could've bought a lot more fuel for the cost of the ZF6.
Both trucks pull the same. The T444E just gets 2x the fuel economy of the old SV392.