Automatic transmission gurus?

[WrenchMonkey]

The 4L85E trans in my 02 Burb has started to run hot, when towing the same 6000# trailer I've pulled for years without issue.

The weird thing is, it seems to be strictly speed-based. Up to 60 mph, I'm fine. Up and down hills, stop and go traffic, no issues at all. Stays about 200-210.

Above 60, even on flat ground (~2200 rpms I think) the gauge will slowly creep up. 220, 230, 250. Once it's hot, I have to get back below 50 for it to cool. And it does, cools right off. Between 50 and 60, it'll maintain temp, neither heat nor cool.

It does have a tranny cooler, and I could just swap that for a bigger one. But it seems like there's an underlying cause. One, I never had this problem before. And two, it's not just the transmission working hard. Because surely it works harder (and should generate more heat) pulling a mile-long hill at 50, than flat or even downhill at 70.

I know eventually the torque convertor should "lock in," but I don't know exactly how that works or if that could be my issue...

Any advice?

[WrenchMonkey]

Oh, and for what it's worth, the trans was just rebuilt about 2.5 years ago. It probably doesn't have 5,000 miles in that time...

[blazerbrad]

First thing I would check is the temperature gauge itself. Is this the factory gauge? I know this era Chevy trucks were notorious for the stepper motors on the gauges going bad and giving you all types of weird readings. I've replaced most of of them on my '03 Silverado.

You can verify the torque converter is locking if you see a couple hundred drop in RPMs once you reach cruising speeds. A lot of people mistake that drop for it hitting another gear.

It may be surprising how much more work is required to pull a barn door (Suburban) pulling a trailer down the freeway at 70 mph as compared 50 mph, though I agree with your thinking comparing pulling a grade at 50 vs. flat road at 70 mph. Aerodynamic drag (pushing against the wind) is almost doubled when speeding up from 50 to 70 mph. Part of the temperature increase could also be caused by what gear the transmission is running or if the converter is locked in those circumstances. Also keep in mind the engine temperature is playing a role also....the trans cooler will usually run close to whatever the engine is running because of the location (assuming the trans cooler is piggybacked against the engine radiator).

[Ky Mudhunter]

The gauge shouldn't be a problem in the '02 models. They don't use a stepper motor like the '03-'07 model trucks.

[blazerbrad]

The gauge shouldn't be a problem in the '02 models. They don't use a stepper motor like the '03-'07 model trucks.

Good call...wasn't aware the '02 models were different than the '03 and up models on that.

[WrenchMonkey]

Oh, I understand the air resistance. And my Burb looks like a bullet train compared to the doorless XJ on the trailer. Like a brick a parachute had a baby.

I guess the TC not locking is the likeliest suspect.

Engine temp never budges above 165, and there's nothing "new" from the many times I towed (up to 85+) without an issue.

I guess I'll try changing the fluid and filter just because, and brace myself for more...