I run 100-115 constant, have been now for some time. My trans seems to be fine.

 

If you are using the factory cluster gauge to monitor trans temp, the 100 degree peg should actually be 160. The needle on the trucks I have watched never budges until 160 even though it is labeled 100, 210, 260.

The 5L40/5L50 trannys from '04 to midyear '05 used a cooler line thermostat until GM realized they sucked and never looked back. Yet.

 

Newer trucks have it in the digital readout. Not sure what year they went to that option. Guessing 07

 

I'm running the same set up as you guys. Strait to the 40K and back, no bypass. It also runs 115 after driving for some time. haha No problems here

 

I think if you are using the factory bunch gauge to watch trans temperature,then 100 degree peg should actually be 160. The needle on the trucks never budges until 160 even though it is labeled 100,210,260.

 

I use my lil scan gauge to monitor the ECT, TFT, and IAT..
ive been running the Trucool 40k for 4-5 years now? Trans got up to 102* this morning right before i got on the interstate, converter locked and it dropped down to 95 and stayed there till i got off the interstate.

even in the heat of the summer it barely ever gets over 120. if im getting gas and letting it idle it will hit 140-145..

its still rocking

 

So last night during an extended ~75 mph highway run in 40° ambient ATF temp never exceeded 110°.
Everything seemed to work just fine.

 

As a general rule of thumb NO you can not over cool a transmission, and bigger is ALWAYS better as far as coolers go..I also run a straight 40k with zero issues...Now a engine on the other hand can be overcooled

 

No such thing..... My last 4L60E was a Caprice 9C1 and from factory the cooler did not have the preheater going through the radiator. It just went directly to a fairly large exchanger behind the grill. It was my winter beater for a couple years and it regularily stays in the -20F or colder ( down to around -50F) here. I sold the car with around 120000 miles on it and it was still rocking it's original 4L60E.

My 80E equipped trucks all have gauges and they rarely move off the bottom line on the scale in the winter ( the gauges are accurate verified with my EFILive). All stock cooling system on both.

Just drive it.

 

Just food for thought. Truck is usually never parked outside but due to some recent event's I had my truck parked outside for two days in a parking lot and this morning I went out to start it and drive back to the house. Outside temp has been below 0 for a while and the trans temp read in the negative teens this morning. I put the keys in and let it idle for about an hour (got sidetracked and idled longer than planned). Trans temp was 140 when I came back and quickly cooled down to around 106 after a few minutes of driving. Shifted great (didn't romp on it to much). Will be doing this over the next few weeks sadly.. might be time to switch out my truck for the wife's ride.