Like so many others, I had lots of problems with the "Hot Oil" idiot light on my 2007 Rubicon Unlimited. I installed the popular B&M transmission cooler and haven't had a problem since.

I recently added a transmission temperature gauge and noticed that the transmission is running very cool. The highest temperature I've seen so far is about 105 degrees Fahrenheit (street and highway driving).

Does anyone have a good idea of what operating temperature range is ideal for these transmissions?

Thanks!

 

105 is good. Mine gets as hot as 200 in the city, which is why I have a Mopar aux cooler waiting for me to install. A lot of guys report their trans temp in the 130-160 range.

 

Where is the temp reading taken from?

Normal operating temp for the 42RLE is anything over 80°F.
The computer prevents torque converter lock-up until that temp is reached. Preventing it from locking up shortens the time it takes to reach 80°F.
Mine, with an aux cooler, and an engine driven fan, will run in the 90°F to 130°F range, no matter what the outside air temp is. Even when the temp is -10°F, it will get there in less than 3 miles.

Some people will chime in saying it's too cool, but that is not the case. The transmission and its ATF+4 love being that cool.

 

I agree 100 %. The main killer of an auto trans is heat.. the cooler she can run the better and longer it will run.
Heat breakes down the additives in atf and once that happens clutches start to burn and u get a slipping trans.
I would say your temp is excellent.

 

just thought id add a lil here... a tranny cooler is nice but for even bigger gains in thermal reduction, a good 1" body lift does a lot to reduce temps too

 

The temperature reading is from the computer via an OBD-II adapter so the reading is from wherever Chrysler chose to put the sensor. I know that cooler is better but there are lower limits. I've had other vehicles with transmission coolers and gauges and I'm seeing numbers that are easily 30-50 degrees cooler than I saw on those, which is why I was curious.

The transmission operates properly and the fluid looks great so if the temp is in the operating range that I have nothing to worry about.

Thanks for the feedback!

 

Well, I have a 1.5" body lift so I'm doubly good!

 

ahhh well that explains your nice n cool tranny temps

 

Then you are seeing sump temperature, which is fine.

Most other vehicles run the transmission fluid through a coil in the engine radiator, so it generally stays at engine temperature.
The JK doesn't do this. The stock cooler is in front of the lower portion of the radiator.
There is a general feeling the normal transmission temp should be around 180°F. That was true back when engines had 180°F thermostats, and the fluid ran through the radiator.

The 3.8 in the JK runs as high as 215°F, or even a little higher; too hot for the transmission fluid. That may be why it comes with an external cooler, although it's too small and not in the best of locations.

I have not seen a lower temperature limit for the 42RLE. The closest thing to a lower limit I have seen is that normal shifting commences at 80°F. The computer takes steps to get it there as quickly as possible.
That suggests the normal temperature is at least 80°F.