48RE Remote Transmission Cooler Install





Installing a Derale 15870 Remote Cooler in a 2007 Dodge 5.9 4WD Mega Cab Truck with a 48RE Transmission


We noticed the transmission temperature on our 2007 5.9 4WD Mega Cab 48RE transmission was climbing to 225 degrees when towing our 10K# toyhauler. We talked to our transmission builder, Jim Galatioto, at ATO Performance Transmissions in Rancho Cordova, California, who suggested replacing the stock front transmission cooler with a Mishimoto replacement transmission cooler, and eliminating the stock engine coolant to transmission oil heat exchanger.

Mishimoto model # MMTC-RAM-03SL


Click on pic for Mishimoto replacement transmission cooler source


I decided to take part of Jim's advice, to eliminate the stock heat exchanger, but not to replace the stock front transmission cooler, because we live in California where keeping a stock under hood appearance is important because of CARB compliance issues.

Jim wanted the heat exchanger removed from the transmission cooling system because the heat exchanger has been known to crack, letting engine coolant into the transmission and transmission oil into the engine coolant.

Derale model # 15870

Click on pic for Derale replacement transmission cooler source


I purchased a Derale p/n 15870 remote cooler and mounted it on the left frame rail just ahead of the fuel tank. There is already an auxiliary fuel filter and water separator mounted on the frame rail, but there was still ample room to mount the cooler.

We like the Derale remote cooler because of its high cooling capacity, finished appearance, and "suck fan" design (pulling air through the cooler from above and exhausting the air out to the road).

There are 3 major steps to the cooler install:
1. Selecting a mounting location, fabricating a bracket, and mounting the bracket to the frame.
2. Installing the fan relay with the wiring harness, and running wires to the thermostat and fan motor.
3. Mounting the remote cooler and plumbing the transmission cooling system (installing the fittings and routing the hoses).

Remote auxiliary cooler mounting bracket

Click on pic for Derale bracket fabrication pictures


I fabricated the mounting bracket from 1 1/2" x 3/16" flat steel strap & 1" x 1/8" flat steel strap.

The cooler mounting bracket was made in 3 pieces:

1. I welded a rectangular frame to hold the cooler at a 45 degree angle next to the frame rail with ears to bolt the bracket to the inside bottom of the frame rail.

2. I formed 2 pieces to go over the top of the frame rail and bolt to the outside of the frame rail, and attach to the middle of the rectangular frame on the inside of the frame rail. This arrangement allowed bolting the bracket in place, and enabled me to do lots of trial fitting and adjusting before drilling the holes in the bracket to join the pieces together.

3.(4) 1/4" holes were drilled in the mounting flange of the cooler, and corresponding holes were drilled in the rectangular frame to bolt them together. Larger holes were drilled in the cooler mounting flange to provide clearance around the bolt heads sticking up through the rectangular frame.

Installing the relay & thermostat

We used the Derale relay & wiring harness p/n 16763, 12 gauge silicone jacket wire with 1/2 inch nylon braided wire shielding, a Derale in-line thermostat switch P/N 15721 (not recommended, see Notes below), and a 30 amp in-line fuse. The fan on the Derale 15870 draws approximately 8.7 amps. We placed the relay in the left rear corner of the engine compartment with the fuse accessible, so it can be removed easily. We used a 12v switched hot, so the cooler fan only runs when the ignition is on. Good instructions come with the relay & wiring harness. Only 1 wire needs to be run from the relay to the thermostat switch. The other side of the thermostat switch is connected to the fan hot lead. The other wire on the fan is connected to ground.

Plumbing the system

We removed the thermostat from the front stock transmission cooler and replaced it with a transmission cooler thermostat delete kit from Dynamic Transmissions ($29.95 with free shipping).

Click on pic for Delete Kit source


A good instruction sheet comes with the kit. As advertised, the delete kit reduces the transmission temperature about 25 degrees.

We had removed the check ball from the stock transmission cooler line several years ago when Jim built our transmission. The check ball keeps transmission fluid from draining back into the pan when the truck is parked.

I didn't want to cut the stock transmission cooling system hard lines, so we removed the line from the 48RE coolant output to the heat exchanger and capped the heat exchanger fitting with an -8AN cap.

We disconnected the heat exchanger line that connects the heat exchanger to the front transmission cooler, and capped that heat exchanger fitting with another -8AN cap. We adapted the hard line to a -8AN push-lock hose to complete the fluid path from the transmission to the front cooler.

We connected the output of the stock front transmission cooler to the input of the Derale remote cooler with a -8AN push lock hose, and the output of the remote cooler to the input at the rear of the 48RE transmission with another -8AN push lock hose, thus completing the fluid path.

Stock 48RE transmission cooling system

I was not able to find a good diagram or illustration of the 48RE transmission cooling system in the 2007 Dodge Truck Service Manual. From what I can determine, this is the complete fluid path for the 48RE transmission cooling system:




1. The transmission fluid exits the outlet port at the front of the transmission via a brass 1/4" NPT male -> double flared 1/2" female fitting.

2. A 1/2" female flare swivel connects the steel and rubber hard line to the 1/2" male flare fitting on the side of the heat exchanger mounted on the left side of the engine block.

3. An identical 1/2" male flare fitting on the outlet of the heat exchanger connects to a 1/2" female steel swivel flare fitting on the end of the 1/2" steel hard line that goes to the front transmission oil cooler located behind the intercooler, and ahead of the radiator at the front of the truck.

4. There is a thermostat at the left top corner of the transmission cooler which controls fluid flow through the cooler.

5. The outlet of the transmission cooler is a 1/2" hard line ended with a 1/2" steel male swivel that connects to the brass 1/2" female flare -> 1/4" NPT male fitting that screws into the inlet port on the rear of the transmission case on the left side of the 48RE transmission.

6. There is a check ball in the line between the front transmission cooler and the transmission inlet, so the fluid can't drain back to the transmission pan when the engine is not running.

Transmission cooling system after remote cooler install





The transmission fluid leaves the outlet port on the front of the 48RE transmission

1. 1/4" NPT male straight->-8AN female swivel
2. -8AN male 90 -> -8AN male
3. -8AM female swivel straight-> -8AN push lock

and flows through a -8AN hose adapted to a 1/2" steel hard line inlet to the front transmission cooler.

1. -8AN push lock-> -8n female swivel
2. -8AN male 90-> -8AN male
(attaches to 1/2" female flare end on hardline-37 degree to 45 degree interference fit)

The fluid leaves the front transmission cooler and flows through a long 1/2" steel hard line that ends with a 1/2" male flare fitting near the inlet port at the rear of the 48RE transmission.

1. Brass female 1/2" straight -> 1/4" NPT male
2. Steel 1/4" female 1/2" male (tapped fitting 1/4" NPT)
3. -8AN female swivel 90 -> -8AN push lock

This fitting is connected to a -8AN push lock hose routed over the transfer case and connected to the thermostat switch fitting at the inlet port of the remote auxiliary cooler.

1. -8AN push lock -> -8AN female swivel straight
2. -8AN male -> -8AN female swivel with 1/8" NPT port

The fluid flows through the thermostat housing fitting and into the remote auxiliary cooler.

1. -8AN male -> 7/8" adapter

The fluid leaves the outlet port of the auxiliary cooler

1. 7/8" adapter -> -8AN male
2. -8AN female swivel straight -> -8AN push lock

and flows through a -8AN hose routed over the transfer case connected to the input port at the rear of the 48RE transmission.

1. -8AN push lock -> -8AN female swivel straight
2. -8AN male -> -8AN female swivel 90
3. -8AN male -> 1/4" NPT male straight

Notes to make the cooler install easier

Don't use the Derale in-line fan thermostat switch, P/N 15721, because it leaks. We replaced the Derale in-line fan thermostat switch with an American Volt 1/8" NPT probe and a 170 degree on - 155 degree off dual pin thermostat switch SKU AV-T18P17 ($21.95 on Amazon). We screwed the probe into a -8AN male -> -8AN female swivel with a 1/8" NPT port. ($13.69 on Amazon). We used a little heat sink thermal conducting compound when we assembled the switch into the probe. Be careful not to screw the probe too far into the port, or it will restrict fluid flow into the remote auxiliary cooler.

Make sure you have all the -8AN push lock fittings, -8AN push lock hose, and other adapters before you start plumbing. We used about 10' of -8AN push lock hose. Insure the hose works with the fittings, since some fittings & hoses are proprietary. Using white lithium grease, and heating the hose with a heat gun makes the push lock fittings easier to assemble onto the hose. We used worm clamps over the push-lock connections, for extra insurance against leaks.

Finding adaptors to connect the 1/2" male and female flare fittings to the -8AN fittings is difficult. I ended up tapping a 1/2" male steel fuel line fitting with a 1/4" NPT tap.

Be careful not to over tighten the 1/4" NPT fittings into the 48RE transmission case.

We used (2) -8AN aluminum hose separator clamp brackets to locate and stabilize the push lock hose over the transfer case and away from the driveshaft.

Dodge truck auxiliary transmission cooler fittings list


(2) -8AN male-> 7/8" adapters (ships with the Derale 15870 Remote Cooler)

(2) -8AN hose separators

Install an -8AN hose separator 14" toward the front of the truck from the remote cooler, and another one 6" ahead of the rear transmission inlet

(2) -8AN female plug (fittings to cap off heat exchanger inlet & outlet)

1/2" steel male fuel line fitting (tap with 1/4" NPT tap)

Remove the brass 1/4" NPT -> 1/2" female fitting from the rear of the 48RE transmission case and use it in the front transmission cooler return line.

1/4" NPT male straight -> -8AN female swivel

(2) -8AN male 90 -> -8AN male

(5) -8AM female swivel straight -> -8AN push lock

-8AN female swivel 90 -> -8AN push lock

-8AN female swivel 90 -> -8AN male

-8AN male straight -> 1/4" NPT male

Shopping list


1. Derale p/n 15870 remote cooler ($344.99 from Dynamic Transmission)
2. 5' 1 1/2" x 3/16" flat steel strap (cooler mounting bracket)
3. 2' 1 x 1/8 flat steel strap (cooler mounting bracket)
4. (8) 1/4" x 20 x 1" bolts, nuts, and lock washers
5. (4) 1/4" x 3/4" self tapping screws
6. 10' -8AN push lock hose
7. Derale p/n 16763 relay & wiring harness
8. 10' 12 gauge silicone jacket wire
9. 10' 1/2" nylon braided wire shielding
10. 30 amp in-line fuse
11. Gen 3 cooler thermostat delete kit from Dynamic Transmissions ($29.95 with free shipping)
12. American Volt 1/8" NPT probe
13. 170 degree on - 155 degree off dual pin thermostat switch SKU AV-T18P17 ($21.95 on Amazon)
14. -8AN male -> -8AN female swivel with a 1/8" NPT port. ($13.69 on Amazon)
15. (16) -8AN fittings from list above (includes (2) -8AN hose separators)
16. (6) worm gear clamps for -8AN hose
17. 1/4" NPT tap

















Article written by Joe Leonard 11-01-2022

Joe Leonard 2022-11-01