Join Date: Feb 2010
Location: Grass Lake, MI
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I wouldn't worry about running 190-200°F sump temp if towing a trailer in stop and go traffic. Even running in the 200-220°F range isn't bad, although I wouldn't want to run there all day long. Brief spikes over that (to 240°F) are also OK. I would be uneasy running over 240°F, although brief spikes above this will not instantly kill your trans.
Cooler is better (for durability). Life goes down as the temp goes higher, and the relationship is not linear, but exponential, so the hotter it gets, the faster your life drops. But let's suppose (just to pull numbers out of the air, for an example) that your trans life at 220°F sump is one-half what it is at 190°F. If you run at 220°F sump for 10 minutes, then you have done the same damage as driving it for 20 minutes at 190°F. So while higher temps are not desirable, they will not instantly fail the trans. You simply use up its life a little faster. Overall then, keep an eye on your temps, but you don't need to panic or obsess if the temp spikes up higher for a few minutes.
Two tips to help keep your temps under control: (1) When climbing a steep grade towing a trailer at low speeds (like mountain switchbacks), drop the trans down to the LOWEST gear you can get ("1" at speeds below about 22 mph). This will give you higher engine speed, but it will greatly reduce the slip within the torque converter, which will greatly reduce the amount of heat you are dumping into the trans. (2) When stopped in traffic, shift the trans to Neutral. This will also reduce the heat being generated inside the converter.