>  After an hour on
>the highway, however, tcase temps began to climb to over 250. 
>this was at approximatly 60 MPH on an 80 degree day.  Driving 65
>plus say temps skyrocket to over 290. 

I just went on a long trip and got a great idea of temps in my transfer case.

Driving around 65-70 it will go to 230-240 after a hour or so. Driving at 75-80 it for about 20 min. It will go to 250-255. This is the max I have seen it. This was confirmed with another Hummer with a different gauge and sending unit. At least the 65-70 temp as John does not drive it 75-80. So with the sythetic fluid this seems like an ok temp. Would like to see it lower but that is easy just slow down but yes would like to keep it around 200 max and looks like some other things should be done to do it. FYI - Sending unit in the drain plug with it countersunk on the back side to get fluid all around the sender.

Tim 97 Yellow OpenTop (Bumble Vee)
Survivor of Rubicon 97

Rich,

Here goes on the fluid temps. This is from my own initial findings, prior to adding an external cooler for the tcase. The tranny will usually run about 180 degrees. This temp will be somewhat higher during city driving and will drop abit on the highway (due to the flow of air over the cooler). The tcase will usually stay below 180 if not driven too hard. After an hour on the highway, however, tcase temps began to climb to over 250. This was at approximatly 60 MPH on an 80 degree day. Driving 65 plus say temps skyrocket to over 290.

It is my opinion that you should try at all costs to keep your ATF (tcase and trans) below 200 degrees. The working life of ATF at temps above this point is severely shortened.

Hope this gives you a ballpark,

Vincent