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can a transmission be too cool?


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My 2014 Ram 2500 Cummins Diesel with 68rfe auto trans overheats in traffic especially while towing heavy. There is an aftermarket transmission cooler thermostat delete part which makes the transmission much slower to warm up and run about 30 to 50 degrees cooler than normal (170*F). Will this harm the tranny or is it there to save a little mpg? I see that most transmissions are designed to run at 170*F. Thanks for any input.

2 Answers

Yes a transmission can be too cool. The transmission has an optimal temperature range, just like an engine does.


In fact, some functions such as torque converter clutch lockup, and overdrive are disabled until the transmission fluid reaches a temperature threshold.


Do not modify the factory cooler, which helps warm up the fluid. You can also purchase stacked-plate coolers which have a bypass built in, which also help the transmission warm up, but plumb it inline with the factory one.

Also, how do you know that the tranny overheats?

Trans temp in stop and go gets up to 220! Normally its 170. On the Cummins Forum people without a tranny thermostat run 30* cooler at all speeds.

I asked HOW you know. Are you using a factory sensor and scan tool? Did you install your own temp sender? infrared thermometer aimed at the pan? Different parts of the fluid circuit are at very different temperatures. The fluid coming out of the torque converter can be very hot.


"thermostat delete part"

All the thermostate deletes I heard of delay the transmission warm-up = prolong the process of its getting to its working temperature, but do not (and have no means to) influence this working temperature in itself. Since the thermostate is not a cooling or heating device - it is (just) a flow limiter. Once it is fully open, which happens soon after you started driving (and which equals a thermostate delete) - that is it: from this point forward, it has zero influence on the temperature. So you may want to double-check if it is not a marketing lie that your delete makes the transmission run about 30 to 50 degrees cooler than normal. Maybe it does make this difference for a couple of minutes right after startup - but not always. Or does it also do taxes?

BTW running on too cold ATF is not good for the transmission - that is why they actually install thermostates to have this fluid reach the correct temperature ASAP.

Could it be that your thermostate is just out of order and needs to be replaced, and that is it? A malfunctioning thermostate can indeed cause overheating.

If it is OK, what you need is not a thermostate delete, but a larger ATF cooler, or smth to blow more air through the existent one as the ATF gets too hot.

Thank you for the response. I'll have to look into an aux fan for the radiator since the trans cooler is inside it. Be well, Mani

Check your existent ATF thermostate first.

An additional cooling fan installed on the main radiator will probably not do the desired job in your case, cause - the way most main radiators are - this main radiator does not cool the ATF directly, BUT via the main engine coolant some small atf radiator (which is an integral part of the main one) is submersed into. You can not see this small one cause it is totally inside the main one. Such a multilevel system is designed this way on purpose in order to have vour ATF heated to its operational temperature ASAP via the main engine coolant which in its turn gets heated by the engine (much sooner, than your ATF would have gotten heated by internal friction happening in your transmission). So in order to super-cool your atf you would virtually have to super-cool your whole main radiator, meaning your whole main engine coolant, meaning also your whole engine, which is not an easy job and which is not good for the engine either - the engine does not enjoy too low coolant temperatures just like the transmission does not enjoy too low atf temperatures. So your proposed solution is a) hard to implement and therefore probably will not work as expected, and b) if it would, it would be bad for your car overall.

I am sorry that this whole story may sound overly complicated, but it is the way things are, unfortunately. Even an additional atf cooler added in sequence with the one which is part of the main eradiator will not work well enough - cause in many cases this additional atf cooler, and the atf cooler hidden inside your main radiator, will be "battling" against each other, one atempting to cool the atf, while the other one is trying to warm it up, etc... Not a good solution.

What I would do, is:
1) make sure your atf thermostate is OK - this is the main point; if yes, and if your atf still overheats -
2) I would install a separate large ATF radiator in front of the main one (with or without additional fans), and route the whole atf flow via this new atf radiator, simply skipping and forgetting the atf-cooling main radiator circuitry.
Imo this is technically the most correct solution - it will do the required atf cooling, avoiding excessive atf cooling, and your existent atf thermostate will make sure that it works optimally. The only thing this solution will not do - it will not have that accelerated atf heating via the heat of the main coolant, but this is the price you will have to pay for having an atf system which does not overheat even under heavy loads.

Good luck!