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EcoCivic · 07-16-2019, 01:39 AM

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!

07-16-2019, 01:39 AM	#1 (permalink)
EcoCivic Master EcoModder Join Date: Jul 2017 Location: Memphis, Tn Posts: 463 Thanks: 320 Thanked 107 Times in 81 Posts	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!