How bad is transmission fluid temperature that is too cold?

I installed an auxiliary transmission cooler on my 1996 Nissan Maxima, because I sometimes use the car to do some towing. I bought the second smallest Haynes transmission cooler. According to the manufacturer, I would have needed the smallest one for occasional towing of upto 1,000-2,000 pounds, but I decided to go one size larger, just to be on the safe side.

A few days ago, the temperature went down to about 40 degrees in the morning, and I happened to get on the freeway only after about 5 minutes of city driving with no traffic and stop light. For a few minutes on the freeway at 60 MPH, the torque converter would not lock up (I know this because it should be little more than 2000 rpm at 60MPH, but it was running about 2300 rpm.) A few minutes on the freeway, the torque converter finally locked up and the engine fell to about 2000 rpm.

My Question:

How bad is it on the transmission to run it cold for a prolonged time? I am planning to move to the east coast where the winter lows get down below 0 degree.

Should I just block the transmission cooler with a piece of cardboard when it gets too cold?

Was your engine temp up to normal operating temperature? Maybe you have a thermostat issue? I’m no expert, but I think the computer doesn’t let the torque converter lock up until the engine is warm enough, not based on tranny fluid temp. It gets below zero here and tranny fluid temp isn’t an issue.

So why not try the cardboard thing on a day with similar temps with a cold vehicle and see how it works? Our '95 Civic is an automatic and I can see the same thing in wintertime . . . waiting for the rpms to drop takes longer . . . transmission never shifts to top gear until a little later . . . until the car/engine/tranny warms up. It DOES eventually warm up and I change the fluid on a regular basis, so I don’t worry about it. Rocketman

I cannot speak for other makes or models. But, my 2002 Sienna, since new, does not want to shift up beyond second, until the motor warms up. I found somewhere that this is design, part of the smog stuff, to make it warm up quick or something.

It will shift up if I floor it hard, but I prefer not to do that. And, this is in rather warm temps, even 70’s and up.

I am not an expert like some here, but I do not think you should bother covering the cooler. Just my opinion. If there is a problem, get it fixed.

Converter clutch lockup is generally locked out (Temperature-wise) until the ATF reaches a certain temp, usually 65-70 degrees F’. External coolers will cause a delay in lockup in cold climates. But they WILL heat up enough to activate lockup. You have a transmission running with no lockup which in itself will heat the fluid and you have the engine coolant heating up which will also heat the ATF. Like goldwing said, I would make sure the thermostat in the engine is operating properly and the engine is heating up properly.

Now with Irlandes, your issue is most likely within the clutch seals hardening and being too stiff when cold causing internal leaking. You can pour some Berrymans in it to soften up the seals a bit but otherwise your problem will just be getting worse.


I guess my question was whether running the transmission too cold is bad.
Engine warmed up more or less normally on that day, and the thermostat is O.K.

No, it is not bad for it to run cooler.