Auxiliary Transmission Cooler Install

[et_pb_section admin_label=”section”][et_pb_row admin_label=”row”][et_pb_column type=”4_4″][et_pb_text admin_label=”Text” background_layout=”light” text_orientation=”left” use_border_color=”off” border_color=”#ffffff” border_style=”solid”]

Driving an American truck to South America where parts are not available could be a risky business.  I am pretty confident 5.9 Cummins Turbo Diesel is as reliable as any engine out there. One thing I am nervous about however, is Ram 3500’s 48RE transmission.  My truck’s rebuilt 48RE has about 35K miles on it.  I will probably be fine for a while but I wanted to make sure it operates smoothly and stays healthy and does not leave us stranded in the middle of nowhere.

The automatic transmission produces a great deal of heat.  Ideal operating temperature is between 150 to 200  Fahrenheit.  As temperature rises, the automatic transmission fluid begins to break down and damage occurs. Excessive heat also causes expansion and changes tolerances on internal components.

I have been paying extra attention to my ScanGauge II to monitor the transmission fluid temperature.   With the added weight of the camper, it has been operating around 185 F.  When it reached 205 F in a stop-and-go traffic, I decided to install an auxiliary transmission cooler.

I picked up Hayden Automotive 405 Ultra-Cool Tube and Fin Transmission Cooler for $35.

Auxiliary Transmission

Model 405 is the biggest Ultra-Cool trans-cooler with the dimensions of 2.1 x 16.7 x 11.1 inches.


Unit comes with 3/8″ x 3′ hose and is not long enough to complete the project. I purchased additional Gates 27057 Transmission Oil Cooler Hose.

Cooler Hose

I also needed Lisle 39400 Angled Disconnect Tool Set and half dozen of  Hose Clamps.

Disconnect Tool

Here are step by step instructions:

1.  To access the transmission fluid lines, loosen the clamps and remove the intercooler hose.

InterCooler Hose

InterCooler Hose2

2.  Prep the cooler by attaching the hoses and sticky foams.

Back side

3.  Remove top two intercooler bolts and raise it up a bit to access behind.

interCooler disconnect

interCooler disconnect2

4. Position the aux cooler against intercooler and make sure when hood comes down it does not touch it. Insert the plastic ties at each corner that go through the foams. Insert the locking round washers behind the intercooler and secure the unit.

Behind InterCooler

5. Detach the cold transmission fluid line by inserting and simply pushing disconnect tool.

detaching cold transmission line

6. Hayden auxiliary transmission cooler model 405 uses 3/8″ connections. 2007 Dodge Ram 3500 however uses 1/2″ hoses. So to be able to connect those together, I needed two female 1/2″ and two male 3/8″ connectors.


7. I cut the cold transmission fluid line hose with sharp knife and prepped the hose by attaching connectors. I am using metal pipe joint compound and two clamps.


8. Here is the final picture of the connections. I am tapping into cold transmission fluid line coming from factory cooler and creating another loop.

Transmission Fluid Loop

Before I put everything back together, I turned on the truck and made sure there were no leaks. Once project was completed, I took a long test drive with the camper. Here is the temperature after 30 minutes of driving around. 158 F, not bad for $70, 4 hours project…

Transmission Fluid Temp

Project Difficulty Rating     Easy Medium Hard

Project Shopping List

Required Tools




  1. Thanks for posting this. I am using your write up to install an auxiliary cooler on my truck.
    I couldn’t get my cooler lines off so I ordered that Lisle tool and hopefully that will work.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.