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View Poll Results: Is my transmission running too cold?
Yes, get a thermostat! 1 50.00%
No, leave it alone! 1 50.00%
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Old 07-16-2019, 12:39 AM   #1 (permalink)
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Transmission running too cold?

Hello everyone, I have a somewhat unusual question. If you haven't figured out by the title, I am concerned that my 2005 Honda Civic's transmission might be running too cold. I installed a Hayden 679 transmission oil cooler (the biggest one I could find, rated at 30,000 pounds) when I installed my Mishimoto racing radiator 5 years ago. The transmission oil cooler seems to work very well, and I am concerned that my transmission may not be warming up enough.

I don't have a trans temp gauge (although I will probably install one eventually) and my trans has no temp sensor, so I rigged a thermometer as a test. I used a small hose clamp to attach my Klein multimeter's thermometer to the oil cooling pipe coming out of the transmission going to the cooler (the hot one) and I set up the multimeter inside the car to monitor trans temp while I drive.

I am actually surprised at how cool the transmission runs with my setup. Today was a cool day for Memphis (only about 80 degrees). The highest temperature I saw was around 135 degrees, and that was while I was pushing it up a pretty steep hill with the converter unlocked. The temp was around 115 degrees most of the time while cruising down the highway with the converter unlocked, and if I unlocked the converter the trans ran at around 120 to 125 degrees on the highway.

So I am wondering if this is too cold, or if this is a good temp to run at. I am pretty sure it is harmless to the transmission since my old trans lasted to 243K miles with good maintenance and this cooler, but I imagine MPG may suffer slightly from running so cold all the time, especially in the winter.

I can get various thermostats to control the trans temp. Most seem to open at around 180 degrees, although some are higher or lower. However, I would rather not install a thermostat unless I need to because I don't want to add another point of failure. If the thermostat were to stick closed and I didn't know it, the trans could quickly be destroyed.

So what do you think about my low trans fluid temperature? Is it harmful to the transmission, and how much would you expect for this to affect gas mileage? Thank you very much in advance!

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Old 07-16-2019, 01:00 AM   #2 (permalink)
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Is the pipe metal or rubber? Rubber is a good insulator, which would give you a false low.

I would expect the temperature to be closer to what the engine operating temperature is, because the engine is heating up the coolant.

135 sounds kind of low. I'm not sure what the norm is though; the AW4 transmission that my Jeep has is known for running hot.

Edit: I misread what you said. I did not know it is an oil cooler. 135 seems low still.
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Old 07-16-2019, 07:32 AM   #3 (permalink)
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Why would a hypermiler need a trans cooler that's bigger than stock? Even those of us in the hottest climates struggle for temp.
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Old 07-16-2019, 11:04 AM   #4 (permalink)
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good reason to stick with the cooler line running through the radiator...well, assuming the radiator is ever up to temp, which it never is...

Ok, good reason to run the oil cooling lines through the hot side of the cooling system.
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Old 07-16-2019, 11:23 AM   #5 (permalink)
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Quote:
Originally Posted by Taylor95 View Post
Is the pipe metal or rubber? Rubber is a good insulator, which would give you a false low.

I would expect the temperature to be closer to what the engine operating temperature is, because the engine is heating up the coolant.

135 sounds kind of low. I'm not sure what the norm is though; the AW4 transmission that my Jeep has is known for running hot.

Edit: I misread what you said. I did not know it is an oil cooler. 135 seems low still.
Great info, thank you. I clamped the thermometer to the metal part of the pipe, so I think I should be getting a fairly accurate reading.
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Old 07-16-2019, 11:32 AM   #6 (permalink)
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Quote:
Originally Posted by oldtamiyaphile View Post
Why would a hypermiler need a trans cooler that's bigger than stock? Even those of us in the hottest climates struggle for temp.
My main focus wasn't gas mileage, it was performance and longevity. The racing radiator that I installed had no built in trans oil cooler. I used to live in Asheville, so climbing long, steep mountains in hot weather was a daily occurrence, I didn't go easy on it while climbing, I added probably 30-40 HP over what this car had stock, I used to occasionally pull a "small" trailer (although sometimes I overloaded it pretty bad, the trailer was about 2500 pounds when I moved to Memphis. Oops.) And sometimes I push it hard, like flooring it from a stop.
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Old 07-16-2019, 11:37 AM   #7 (permalink)
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Quote:
Originally Posted by Stubby79 View Post
good reason to stick with the cooler line running through the radiator...well, assuming the radiator is ever up to temp, which it never is...

Ok, good reason to run the oil cooling lines through the hot side of the cooling system.
My racing radiator has no trans oil cooler. And even if it did I don't know how much heat it would actually add to the fluid being on the cold side of the rad. Probably not much, it may even cool the fluid in the winter when the thermostat isn't open much or at all.
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Old 07-16-2019, 01:55 PM   #8 (permalink)
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I would say yes just because you want the oil to thin out to spec. If its too cool, it is just causing extra friction and a slight loss in mpg.
I assume this car is an automatic?
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Old 07-16-2019, 05:17 PM   #9 (permalink)
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I would say yes just because you want the oil to thin out to spec. If its too cool, it is just causing extra friction and a slight loss in mpg.
I assume this car is an automatic?
Thank you for your help. Yes, it's an auto. How many MPG do you suspect I may be giving up by running the trans this cold? I would guess no more than 1 MPG, but I'm not sure
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Old 07-16-2019, 07:08 PM   #10 (permalink)
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For what it's worth, GM trucks let you see trans temp from the cluster, the '12 2500 at work (6.0 iron block and 6speed) runs around 160-170F. Our Yukon hybrid (6.0 aluminum block, very similar to LS2, with 2ML70 4 speed) also runs about 160-170F.

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Last edited by ksa8907; 07-20-2019 at 01:14 PM..
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