I have a supercharged 06 and was having transmission problems as well. I bypassed the stock coolers and put two denser side by side coolers on the front. Working great. I haven't seen the trans temp even reach 200 yet and that used to be the normal operating temp. I honestly think the stock unit (part of the radiator) isn't that efficient and likely needed replacing as it is 6+ years old.

 

Haven't messed with it since the last big saga, but I'm leaning in that direction. I think I'm going to run a secondary trans cooler inline with the new one that was replaced with the larger radiator and see what happens.

 

I don't know about others but my stock cooler (the radiator) was causing the problem because of lack of proper flow. With the stock cooler (inline with an added cooler) I was running temps of 200 even when not towing. Bypassing the stock cooler and going through two aftermarket ones with high flows has my temps at 125 under the same conditions. I had done multiple flushes on the system with the stock cooler but just couldn't get as much flow through it.

 

Thanks for the info. I had to take My H2 home the other day because it started overheating the trans. I dropped my trailer and let the trans cool but when I started heading home it would overheat every 10 miles. I got home and took my wife's H2, picked up the trailer and continued on my trip. I noticed that when going over the mountain, her trans temp would stay at just under 200 when mine usually overheated on the same mountain. So hopefully, I didn't cook my trans completely. It performed ok but I have this feeling that the cooler is probably clogging up with clutch plates breaking down. I also have codes showing up on my computer that there is slippage. If I have to get a new tranny, I'll just get the Allison 6 speed and the Duramax it bolts to. Either way, I'll definitely bypass the stock cooler and go with the set up you have. Thanks