Hi all, I've got an 09 3500 duramax 4WD SRW and so far really love the truck. It has an in dash trans temp gauge which is cool (forgive the pun) but my question is what's a normal operating temperature ? The owners manual doesn't really specify. So far the temps range from 130 on the freeway at 70 to 150 /160 around town, I think it got to 170 the other day. I'll be towing 10,000lb trailer with this so what should I expect to see when I'm cruising at 60 mph or pulling a grade on a hiway?. Most importantly, what temperature is too high (boiling?) and I should pull over?
Do these trannies run hot? Thanks for any insight/advice.

Dave in HB

I dont pull any trailers, but my temp on a warm day is around 180-190. Too high would be over 225. Then it would be time to pull over.

I have a pre-LMM truck, but the Allison is the same 6spd tranny as you have. The procedure for a "hot" check in the DMax supplement to the owner's manual lists the normal operating range as 160-200F for a "hot" fluid check.

Another general rule of thumb if not towing or hauling a heavy load is that the tranny will typically turn about 100F hotter than the ambient air temperature. It sounds like hocus pocus, but in almost three years of driving my truck, in varying temps, it holds pretty true. If it is very hot and you are in stop/go city traffic it may run hotter than the 100F rule or if you are towing that does not hold as true.

I have towed 8000-9000 lbs up/down Colo mtn highways in the summers and have never seen over 210F on my guage. If it climbs over 220 and stays there without hvy towing or extremely hot ambient temps, you may have an issue. I believe the "red" line is at 230F and you might run up over 220 if towing heavy, uphill, in hot weather; but it likely will start dropping on the downhill side.

These things are built to work and I see people all the time worried about temps in the upper 100's F or even 200, 210, etc. The red line is there for a reason and even that probably has a factor of safety designed into it.

I agree with Jfarr .

The typical temps seen by most drivers, even when loaded, are far below the danger range. Short spikes up to 250 deg F are not an issue; prolonged temps above 225 deg F should be avoided. Always stay out of the red zone!

If you use the tow-haul feature, and keep the fluid level appropriate for the heat range, you'll likely never have an issue.

Same truck as you here, just different year.

I've never seen it over 165 unloaded.

more like 135-160 for normal freeway op. conditions.