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AW4 Transmission cooler inlet & return lines

J

Jayzbird

NAXJA Forum User
Installing a B&M cooler ... I've read and looked at the FSM. Though there have been some dissenters, everyone usually says that:
-The line from the bottom of the radiator is the return line; and
-This runs to the back of the transmission

Well, I have triple-checked the routing, and I am sure that on my 99 AW4, the line from the TOP, not the bottom, of the radiator runs to the back of the transmission. Completely contradictory to the FSM and what I've read here. Can anyone help sort this out? Which is the return line?
 
Ok, just verified it by pulling the hose that exits from the top of the radiator and starting it up. Fluid gushing out of that hose. So, on a 1999 AW4 at least, the return line to the trans comes out the TOP of the radiator, not the bottom and then runs to the back of the tranny.
 
Go drive it around and feel both lines with your hand, the cooler of the two is the return line.

-Mike
 
Jayzbird said:
Ok, just verified it by pulling the hose that exits from the top of the radiator and starting it up. Fluid gushing out of that hose. So, on a 1999 AW4 at least, the return line to the trans comes out the TOP of the radiator, not the bottom and then runs to the back of the tranny.
Click to expand...
If you pulled the hose on top and fluid is pressurized coming out of the hose once started that would tell me it is pumping fluid into the top of the radiator not going back to the trans
If the fluid is pressurized coming out of the radiator on the top then you are correct
but someone correct me if I am wrong on my 96 the return line is at the bottom there should be no difference if it is a AW4
I have my cooler inline before it goes to the radiator cooling tank to first let the external trans cooler do the brunt of the cooling(take the load off the radiator and let it cool the water) and next to bring the trans fluid back to nominal operating temp before going back to the trans this is critical... too cool and you have problems. That trans is designed to run at specific temps not to hot not to cold
 
Last edited:
I haven't got around to putting a cooler on mine yet, but have seen this topic several times. Everyone seems to be concerned about whether top or bottom is return line and how to determine ..

Probably a good reason it hasn't been put forward and I am a dumbass for asking but ... wouldn't it be easier to definitively describe the locations of the ports on the transmission (which presumably are always the same) and then simply tell everyone "The return line is the one that goes in the X side port of the transmission Y inches from the Z" and then everyone can look on their own jeep, visually trace that lines to the radiator, and know for sure which is which.

What's the obvious point that I am missing? :D
 
Jayzbird said:
Installing a B&M cooler ... I've read and looked at the FSM. Though there have been some dissenters, everyone usually says that:
-The line from the bottom of the radiator is the return line; and
-This runs to the back of the transmission

Well, I have triple-checked the routing, and I am sure that on my 99 AW4, the line from the TOP, not the bottom, of the radiator runs to the back of the transmission. Completely contradictory to the FSM and what I've read here. Can anyone help sort this out? Which is the return line?
Click to expand...


Both the 1995 and 2000 FSM have this drawing.
trans_cooler.jpg


If you look carefully, one picture shows the top being the inlet and the other shows the bottom being the inlet. Perhaps it's different if you have the aux cooler option? In all cases, the oil comes out of the front port on the AW4 and returns to the rear port.

I think having the top rad port be the return line makes sense as going the other way makes it much harder to purge air from the system.
 
lawsoncl said:
If you look carefully, one picture shows the top being the inlet and the other shows the bottom being the inlet. Perhaps it's different if you have the aux cooler option?
Click to expand...
Bingo.

The difference is the aux cooler.

Either way, the pressure line on the transmission is the forward port, the return is the rear.
 
I agree that a lot of the confusion comes from the way the inlet and outlets are labeled for the auxiliary cooler in the FSM. However, no matter what reference is chosen, every source says that the return line is the one going to the back of the transmission. If you follow the line to the back of the transmission, you will have found your return line. If you have any doubt, do as I did: pull one of the lines and start your engine. You will have your definitive answer in seconds. The pressurized side of the circuit will begin gushing fluid in short order.

Having "correctly" chosen the return line for my cooler, I'm now wondering if it was really such a good idea after all. I installed a temperature guage at the same time, and in normal driving at mild NorCal late summer ambient temperatures (mid 70s) I'm barely getting over 125 degrees. I'm starting to think all the people who accidently installed their cooler on the feed line rather than the return may have unintentionally installed it more optimally.
 
Jayzbird said:
I agree that a lot of the confusion comes from the way the inlet and outlets are labeled for the auxiliary cooler in the FSM. However, no matter what reference is chosen, every source says that the return line is the one going to the back of the transmission. If you follow the line to the back of the transmission, you will have found your return line. If you have any doubt, do as I did: pull one of the lines and start your engine. You will have your definitive answer in seconds. The pressurized side of the circuit will begin gushing fluid in short order.

Having "correctly" chosen the return line for my cooler, I'm now wondering if it was really such a good idea after all. I installed a temperature guage at the same time, and in normal driving at mild NorCal late summer ambient temperatures (mid 70s) I'm barely getting over 125 degrees. I'm starting to think all the people who accidently installed their cooler on the feed line rather than the return may have unintentionally installed it more optimally.
Click to expand...

Yes if you want the most effective trans cooling install your cooler on the hot line coming from the trans first then route to the radiator then back to trans. The return line to the trans comes from the bottom of the radiator
This will also take some of the heat load of the radiator effectively helping with engine operating temps
 
MrMom said:
....install your cooler on the hot line coming from the trans .....this will also take some of the heat load of the radiator effectively helping with engine operating temps
Click to expand...

This is what I did last week and this weekend I drove my rig up a very steep and long (couple miles) hill climb doing 55 with a bunch of camping crap in the back and even more on top in the rack and my engine temp was just above the middle between the mid point and 3/4's....normally it would be just past the 3/4 mark with the electric fan running. Transmission temperature (just feeling the shifter and the tunnel because I have no temp guage) felt much MUCH cooler than last time.
 
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