'01 GMC Yukon XL 2500, 4L85E with the HD towing/cooling so I've already got one air-to-liquid cooler in additon to the radiator "cooler". But now that temps are in the 90s, I'm seeing trans temp as high as 205, sometimes when I'm not even towing.
So a buddy has a B&M Supercooler 70264 laying around that I want to install. Any thoughts, tips, tricks before I go ripping my grill off and fabricating a mount for it and the OEM piece?

I'd have a good tech drive it with a top-notch scan tool attached. It might not be going into lockup- that will result in hi temps. Had it happen to me.

I bought the B&M and mounted it in series with the OEM external that was on the truck. Did little. Ended up being a ground issue that was one of GM's silent recalls. They even had a $50 replacement harness that solved the problem.

One suggestion- remove the radiator-tank cooler from the system. On hot days, the coolant can be 210+. You may be heating, not cooling your ATF.

Was the harness the reason it wasn't going into lockup? Was it not going into lockup at all? I know that's part of the problem specially with the local driving-mine doesn't like to lockup until 50mph...and most of my speed limits are 45mph I have to tow in 3rd to get it to lockup when I pull the boat. I need to get around to changing the ECU to force the lockup.
RE: taking the rad cooler out of the loop...agreed, I just need to see how much a PITA it will be to plug the rad tank and adapt to the lines. I guess I could just cut off the QC fittings and cap/crimp them, then clamp the hose over the lines.

Yes, it was the ground problem. I also had a problem with an O2 sensor going bad, setting a code and not allowing lockup. Don't confuse overdrive with lockup. You can feel it go into OD. You can't really tell if it's locking.

On the lines- I only use flared steel. I've had double-clamped rubber lines come loose on me. It's a real hassle but worth the peace-of-mind. Mine came off on I-70 in Central Indiana in January- late one night. I felt it acting funny and smelled the fluid- caught it in time to not smoke the trans. I even managed to get the line back on- in a snowstorm holding a mini-maglite in my teeth. It was the 4 mile walk to get fluid that really sucked- especially the return trip carrying 8 quarts of ATF.

Agreed, can't "feel" it...but 3 shifts and then a couple hundred rpm drop is what I look for.

So did you cut/flare the existing lines or make all new ones? So far I haven't been able to make a good flare to save my life...

Ib think the rpm drop you see is the OD.

The secret to flaring is a good flaring tool. AutoZone will loan you one for free. You can put unions into the existing lines- get a bender when you get the flaring tool.

1-2, 2-3, 3-OD...then lockup. That I'm pretty sure about

Agreed on the good tool...combined with a fair bit of successful practice.

BTW, thanks for chiming in...much abliged

Yeah- 3 shifts, not 3 gear ranges.