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So I've read on FB KL pages that shifting into 4 low cools your transmission when on the trail. Last week while in Colorado on a simple up and down trail while in auto my TH transmission over temp warning came up so I hit the 4 low button and kept going. The trans temp dropped. My friend's Compass TH had a similar issue where he stated power got pulled if his trans temp went over 180 - 200. He hit the 4 low button and his trans temp also lowered and driving became normal. As far as I know there aren't any gears that physically change when in 4 low. Does the pump speed up and circulate the fluid better or what causes the cooling??
 

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Simple - higher pump speed moving more fluid through the radiator.

My JK was notorious for having high transmission temperatures despite having an auxiliary cooler. I quickly discovered that 4lo cools down the transmission in short order. I also discovered that turning the A/C ON also had a quick cooling effect. In the JK the A/C forces the radiator fan on high. Simple solutions! Of course, this only works at trail speeds.

I know the KL transmission cooling responds well to 4lo. I didn't notice the fan automatically coming on high with the A/C, though it might. I tried it a couple of times but ended up having to go to 4lo for "trail reasons" before I could determine if the A/C would have made a difference.
 

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Only the computer programmers know why.
Since you are not going down the highway fast to move air through the radiator you have to engage 4 low to get the radiator fans running.
When I am off road in 4 low the radiator fans are really working hard.
I run the AC whether I need it or not, that really gets the fans going.
 

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The trans will heat up faster in offroad type driving when not in 4x4 low because it has to work harder to get up inclines ect. The rad fans are not controlled by 4x4 position its still all based on temperature sensors. In 4 low the engine does heat up a bit (higher rpms) which in turn activates the cooling fans.
If you have AlfaOBD of another ODB reader you can read the position of your PWM fan by % to see how fast the fans are going.
 

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Let me try from a different angle.

When in 4lo, the engine tends to run faster (transmission turning faster). The torque converter tends to lock up sooner (less slippage, less heat). The transmission oil pump is gear driven, so spinning the transmission faster pumps more oil through the radiator (more oil exposed to the cooling area). The fan, if needed, makes use of the higher airflow to extract heat from the cooling fins in the radiator (faster cooling). So: Faster transmission speeds + less torque converter slippage + higher oil flow + more oil exposed to cooling fins + (if needed) higher fan airflow to extract the heat faster = lower transmission temps.

There's one more variable for non-Trailhawk owners (and maybe others) - Active Grille Shutters. I assume they open up in 4lo?
 
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So, what can those of us who don't have 4LOW do to avoid excessive transmission temps if doing a little light off-roading?

Turn the AC on?
 
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