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I had trouble getting a clear answer searching the net for the 42RLE return line while I was installing an auto trans and power steering fluid cooler. My 42RLE is on a 2005 LJ Rubicon. To help others, attached is a picture that shows the return line being pointed out by the red arrow. I started the motor for a few seconds to see the direction of the flow. I hope this helps others.
 

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So when you ran the engine, did fluid come out of the line indicated by the red arrow or the white arrow?
 

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So when you ran the engine, did fluid come out of the line indicated by the red arrow or the white arrow?
yes, the trans should pump ATF+4 out of the line on the passenger side of the radiator. its a great way to change out the trans fluid as well
 

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So when you ran the engine, did fluid come out of the line indicated by the red arrow or the white arrow?

The red arrow. In the picture you can see the fluid still coming out of the line. It's the passenger side.
 

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I had these questions when I saw this post originally as well.



So, lets see if I have it - the red arrow points to the "return line" and it carries trans fluid to the radiator?


What is the line pointed to in white - it must carry trans fluid from the rad to the trans?







Straighten me out...
 

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Red arrow on the passenger side is the supply from the transmission to the radiator. The white arrow on the driver side is the return line from the radiator to the transmission. You want the tap into the return (driver side) side. This will allow the ATF to be cooled even more after going through the radiator and then through the additional new cooler and back to the transmission. We just installed an addition cooler in front of the radiator on our grandson TJ. We cut the metal tube in the middle made a slit flare on the two ends so the hose will not slip off. NO leaks.
 

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I had these questions when I saw this post originally as well.



So, lets see if I have it - the red arrow points to the "return line" and it carries trans fluid to the radiator?


What is the line pointed to in white - it must carry trans fluid from the rad to the trans?


Straighten me out...

The red arrow points to what is referred to as the "return line." This is on the passenger side of the Jeep. That's the hot fluid that is run through the radiator and cooled down.


The white arrow points to the "supply line." That line supplies the transmission with the cooled down fluid. This is on the driver side of the Jeep.


The reason I created this post is when I searched the web for 42RLE return line, I saw conflicting information in posts. Look closely at the return line and you'll see the fluid dripping out which happened after I ran the engine for a couple seconds. If you have a 42RLE and are installing an aftermarket product, like a cooler, the instructions may want you to tap into the return line of the system so this picture should provide clarification. IHTH.
 

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just an FYI, the radiator acts as a trans line heater as well as a cooler depending on climate. it helps warm up the trans in the colder areas.

due to this its generally a good idea to install a trans cooler after the radiator and some have even gone as far as installing a transmission oil thermostat to bypass the trans cooler until ATF the ATF fluid reaches 180 deg F
 

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The red arrow points to what is referred to as the "return line." This is on the passenger side of the Jeep. That's the hot fluid that is run through the radiator and cooled down.


The white arrow points to the "supply line." That line supplies the transmission with the cooled down fluid. This is on the driver side of the Jeep.


The reason I created this post is when I searched the web for 42RLE return line, I saw conflicting information in posts. Look closely at the return line and you'll see the fluid dripping out which happened after I ran the engine for a couple seconds. If you have a 42RLE and are installing an aftermarket product, like a cooler, the instructions may want you to tap into the return line of the system so this picture should provide clarification. IHTH.

Thanks for the clarification and the photo. I've always been confused as to which line is the "return" and which is the "supply". I guess it depends on point of view - whether from/to the radiator or from/to the transmission. What you've described makes sense. I've got a temp sensor on the tubing at my transmission, but have been curious as to whether it was located before or after the cooler.
 

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just an FYI, the radiator acts as a trans line heater as well as a cooler depending on climate. it helps warm up the trans in the colder areas.

due to this its generally a good idea to install a trans cooler after the radiator and some have even gone as far as installing a transmission oil thermostat to bypass the trans cooler until ATF the ATF fluid reaches 180 deg F



Agree. I was installing a cooler and the instructions specifically referred to the "return line" so I had to find it somehow. PSC has a dual cooler setup using 2 Derale coolers sitting on a bracket in front of the radiator. On that note, I was concerned if those 2 coolers in front of the radiator would be blocking air flow or cause engine temp to rise, but nothing at all like that happened, and I'm wheeling in the hot SoCal summer.
 

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Thanks for the clarification and the photo. I've always been confused as to which line is the "return" and which is the "supply". I guess it depends on point of view - whether from/to the radiator or from/to the transmission. What you've described makes sense. I've got a temp sensor on the tubing at my transmission, but have been curious as to whether it was located before or after the cooler.
No it doesn't... the supply line supplies hot fluid from the trans and the return line returns cooler fluid back to the trans.... op is mixed up

Thr transmission is the source of the fluid, ie the pump, and the rad/cooler is the secondary device
 

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Agree. I was installing a cooler and the instructions specifically referred to the "return line" so I had to find it somehow. PSC has a dual cooler setup using 2 Derale coolers sitting on a bracket in front of the radiator. On that note, I was concerned if those 2 coolers in front of the radiator would be blocking air flow or cause engine temp to rise, but nothing at all like that happened, and I'm wheeling in the hot SoCal summer.
i've got the same cooler with PS going to the second cooler plus a rather large winch (warn 9.5ti) and also experienced zero difference in engine cooling
 

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No it doesn't... the supply line supplies hot fluid from the trans and the return line returns cooler fluid back to the trans.... op is mixed up

Thr transmission is the source of the fluid, ie the pump, and the rad/cooler is the secondary device
You are absolutely wrong. The original intent of this post was to clear up the conflicting information on the net. Go read the installation instructions of any cooler and they will refer to the "return line." I am supplying a photo of where the "return line" is. Readers who are interested in the topic can verify by calling a tech support line from a cooler vendor if they choose as the year of the vehicle may be a factor. I am specifically referring to a 2005 LJ Rubi with a 42RLE.

I'm not going to get into a back and forth with you on this so I won't reply further on whether you think I am wrong.
 

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You are confused and aren't clearing anything...
The return line is the one that returns the transmission fluid to the transmission...

The fact that fluid came out of the passenger side line MEANS it is the SUPPLY line NOT the return line....
 
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