Transmission cooler ....er heater?

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Y33TJPN
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Looking for a little clarification in the purpose of the AT fluid cycling through the bottom of the radiator. I’ve read conflicting Information that it is ran through the bottom channel of the rad in an effort to heat it to optimal temps during cold weather.

That being said it is referred to most commonly as a trans “cooler”. FSM advises 176 degrees F is considered hot for the trans, but if that is optimal coolant temps for the engine why would running the trans fluid through this area be beneficial?

Thank you in advance!


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VStar650CL
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2004 Nissan Altima 2.5 S

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For an engine to run at, say, 190F at the water outlet, the bottom tank on the radiator will be much cooler than that, generally 140F or lower. The engine is the heat source and the temp at the upper hose is the highest in the system, the coolant having just made a trip through the block and heads. The radiator is the sink, so after a trip through the rad tubes, the water at the lower tank and hose is the coolest point in the system, generally at least 50F cooler than the inlet. So the answer to your question is both. When the tranny has no load, it warms up much slower than the engine, so the "cooler loop" actually serves to help it warm up. Once it's warm, the loop serves to keep it stable at a safe temp.

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Q451990
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The consensus for the 1st gen has been that if you were in a warmer climate, it was better to bypass the factory cooler completely, and go with an external cooler. The B&M stacked plate design supposedly bypasses itself when the fluid is thicker and cold at startup - so it was a favorite. In colder climates we've been running the factory cooler in series with the external cooler. I think the 2nd generation transmissions have been shown to be more reliable, and the bypass is probably not necessary.

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VStar650CL
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Q451990 wrote:
Mon Apr 26, 2021 9:09 am
The consensus for the 1st gen has been that if you were in a warmer climate, it was better to bypass the factory cooler completely, and go with an external cooler. The B&M stacked plate design supposedly bypasses itself when the fluid is thicker and cold at startup - so it was a favorite. In colder climates we've been running the factory cooler in series with the external cooler. I think the 2nd generation transmissions have been shown to be more reliable, and the bypass is probably not necessary.
My feeling is, unless you're in Mojave, a series cooler always beats an independent one. Trannies wear faster at dead cold just like engines do, so getting to an optimal temperature quickly is arguably as important as keeping it optimal. A series cooler benefits from quick warmup via the radiator but adds heat sinking to the loop under hot engine conditions, the best of both.


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