I just want some thoughts on installing an auxiliary tranny cooler on my 89 3/4 ton Sub with Turbo-400. I sometimes pull trailer with it and noticed my engine temp gauge creep up to around 210 degrees when going up six percent grades (GM replacement 350 with 20,000 miles on it) I have the factory oil cooler and don't run the ac when doing these hills, but thought to maybe install tranny cooler before factory cooler (on radiator) so as to cool tranny fluid before it sees the radiator/cooler. What do you guys (or gals) think? Thanks

I would install a bigger radiator(more cores). More cooling.

210 degrees F is an optimal engine temperature, not too high and not too low - about just right.

Transmissions do like auxiliary coolers. No need to put a bigger radiator in it.

I would expect the engine to run hotter, at a controlled level, under the uphill load you describe. Doesn't automatically mean the trans is getting too hot. Only way to know for sure is to temporarily install a trans temp gauge and keep an eye on it next time you do the big pull.

That said, I would recommend a trans cooler for any big GM SUV. I find the factory "heavy duty" trans cooling included in the towing packages to be marginal. The 700R4 in my '90 Suburban lasted only 85,000 miles and I hardly ever towed with it. A 700R4 is not as strong as your 400 but I think it should have lasted longer. I discovered that the HD "trans cooler" was just a flattened pipe running through the hot water across the bottom of the radiator.

When the rebuilt trans was installed, I added an auxiliary trans cooler the same size and configuration as the engine oil cooler, next to it on the front of the radiator. Second trans went 215,000 miles.

I also recommend Red Line D4 synthetic ATF for added insurance. If you care about the vehicle and/or plan to keep it, Mobil 1 ATF is the MINIMUM requirement.

I agree with sub006 about the synthetic tranny fluid, my first 700R4 ran on dino oil and decided to die at the most inconvenient time. The current rebuilt unit is running on Mobil 1 with a tranny cooler and so far so good. The tranny was installed with dino fluid and then converted to synthetic which made a noticable difference in drivability.

One other tip - when I installed the tranny cooler there were two places where the cooling lines ran near the radiator frame creating a possible wear point so I took an extra couple inches of transmission line, split it and put it over the top of the line carrying the fluid. I then zip tied the split line in place giving some additional protectant to the fluid lines, likely saving me grief down the road and probably a long walk if the hose should happen to get a hole in it. Good luck.

I've got a 92 with 225000 miles on the original 700R4 tranny. Of course the saving grace is that it has towed maybe 7 time in its life. The truck used to be my dads and I bought it off him, it was good shape and extra clean, and I started towing Boy Scout Trailers with it when we go camping. She pulls fine but she sprung a couple of leaks in the tranny case this last year. Looks like its time to give the tranny a re-birth. It has the original tranny and engine oil radiator cooling system.

What about puting a deep tranny oil pan on it? I've seen them for 700's and the old 400's that would add about 2 1/2 qts to the fluid. Is that good enough to run on the original radiator tranny cooling?

Thanks for all of your replies. I ended up putting the aux. cooler before the factory (radiator) cooler to keep the temps from being too crazy. Towed trailer this weekend and it appears to be good. My T-400 has the factory deep oil pan on it. I now need to get more power to the ground!