I’ve got a new to me GMC Sierra and I’m curious what the the normal operating temperature of the transmission is? Does anyone know a range? I can’t find anything in stone and it’s not in the owners manual. I can see the temp on a display but a lot of good that does me when I don’t know what normal is.

180-190 is normal depending on how it’s being used at the time....I wouldn’t like to see it get higher than 200

It’s kind of a loaded question. Several factors ambient temp, hwy, stop and go towing... Most nowadays run on the warm side some upwards of 200 degrees in a non towing situation. Most all of them have thermal valves in the system that block fluid from entering the radiator or cooler until it gets up to temp then it’s running through the radiator which is sitting at around 200. I wouldn’t worry unless it’s creeping over 200 cruising unloaded

Thanks guys! Y’all are way better than google. My temp seems to stay between 175 and 190 which is normal.

1shot, am I understanding you correctly that the transmission cooling system doesn’t cool the fluid until it reaches the 200 mark?

Yes.. I can’t recall right off what temp the thermal valve is designed to open up on yours but I can look it up in the service manual in the morning if you just gotta know. But yes the fluid will not enter the actual cooler until the that temp is reached. In some system's there is a manifold in between the radiator and the cooler which allows fluid into the radiator and block the cooler which will actually aid in warming the trans fluid faster as the coolant temp rises faster than the trans.

Just on another note your old school ATF fluid or most any conventional hydraulic fluid starts to break down and oxidizing around 160 these newer synthetics are way better and don’t start until around 220

What year did U get?

Dr. B

Right on and thanks again 1shot. I’m just learning my way around a new vehicle and like to know how everything is supposed to run.

Doc, I got a 2018

How’s the shifting in the transmission?

Dr. B

Mine is a 2018 Sierra and my transmission temperature runs around 190.

Typically the automatic transmission running temp should be about 100 degrees above ambient.
Anything very much above that deserves some attention.

It’s good doc. 5.3L V8 with 6 speed trans and everything is smooth.

Be sure to delete that AFM garbage immediately, if equipped.

+1 on deleting and. Buy a range disabeler for $200.