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transmission fluid is heated by coolant?

modeler

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I start to monitor Tranny temp when tow in the summer now.
Looks like Tranny temp even increases when it is idle, so my suspicion is transmission fluid is heated by coolant, is that true?

Generally, I found my Tran temp doesn't change a lot, stay under 90c, my question is if the fluid temp is maintained by coolant, then what's the point to add a secondary cooler for towing? Is that because coolant heat exchange is still not enough to cool transmission fluid. Then what about in winter, by adding secondary cooler is that possible the fluid will be too cold to properly work?
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Trans fluid is cooled by a heat exchanger in the radiator. The trans generates heat in the fluid in the converter when it is slipping at a stop light ect. with the car in gear. Once the converter is locked up or the converter reaches stall speed this heat source is reduced. Trans fluid is designed to operate at engine temperature. An external cooler can be added when towing extra heavy loads.. its primary function is to remove excess heat. Over heated trans is the kiss of death mechanically.
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I start to monitor Tranny temp when tow in the summer now.
Looks like Tranny temp even increases when it is idle, so my suspicion is transmission fluid is heated by coolant, is that true?
Yes

Generally, I found my Tran temp doesn't change a lot, stay under 90c, my question is if the fluid temp is maintained by coolant, then what's the point to add a secondary cooler for towing? Is that because coolant heat exchange is still not enough to cool transmission fluid.
Yes
A secondary cooler is typically mounted on the line returning to the transmission and provides further cooling before the fluid is returned.

Then what about in winter, by adding secondary cooler is that possible the fluid will be too cold to properly work?
Transmissions work reasonably well with cold fluid but to alleviate harsh shifts and reduce friction some warming is desirable. A couple methods are often used to help with this:

1) The transmission cooler lines can enter the cold side of the radiator and exit the hot side. This helps limit the maximum transmission temperature while still providing some warming in the winter during minimum levels of coolant flow (thermostat closed).

2) Some transmissions disable the torque converter lock-up function (which increases fluid heating) until the transmission reaches a minimum operating temperature.
 

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the tranny has a separate cool that sits in front of the of the engine radiator, there may be a cooling loop in the main radiator in addition, assuming this based on rockauto part descriptions. i have a manual so idk what the tranny cooling loop looks like

but it is expect that there should be some heat increase while sitting at idle, the torque converter is spinning on the engine side and stopped on the tranny side so that is generating heat there, and when you are sitting at idle there is no air flow going across the radiators so they are just soaking in theer own heat, the electric fan only kicks on when the temps get to high
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