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I noticed my transmission temperature getting quite high when driving on trails or towing through traffic so I decided to add a secondary Derale 13900 transmission cooler in addition to the stock front mounted cooler. My truck is a 2020 Custom TB with the 5.3 and 6 speed 6l80e. This kit came with everything I needed except for a couple fittings, and the fan is wired to an ignition power source so it won't drain the battery. You can see that it is mounted to a winch mount that I had made, but without the winch there that would be a great spot to fit the cooler. Essentially I followed the return cooler line from the transmission forward until the hard lines split off into two rubber lines with the return going to the driver side of the truck. NOTE: The hard lines are 3/8" but the rubber line is 1/2" (learned that the hard way). I first held the cooler in place and measured what angle it needed to sit at and tacked together some mounts, then after a couple of iterations got it to where it fit and the skid plate did not interfere. I then finish welded, drilled, tapped, and painted the mounts. On the cooler itself, I mounted the 180 deg thermostat to the lower connection. With the cooler location determined, I then measured and cut the 3/8" rubber hose in the kit. I then clamped the return rubber line on the truck, cut the rubber hose, and connected the 3/8" cooler hoses to the pre-existing 1/2" hose on the truck. I used one 3/8-1/2 straight barbed connector and one 3/8-1/2 90 deg barbed connector. Some zip ties to tidy up the lines too. Wiring was simple as well, easiest way to get a for sure ignition power source was to get a fuse tap from autozone for a micro2 fuse and then you have a fused ignition source. Ran the power to the thermostat and then positive from motor to the other side of thermostat and the ground to the frame. The hole in the skid plate has an aluminum mesh behind it to keep small rocks out, and it's pretty high up so shouldn't see much abuse. Haven't given it a full test for cooling capability yet but the fan is nice and strong so I have no doubt it'll help. For about $200 total I'd say its a great upgrade for anyone who tows in the city or hits the trails on any steeper terrain.

Cooler Final.jpg

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  • 10 months later...

Wanted to post an update to this thread, have had the thermostat out and secondary cooler installed for a whole year and here are some observations: transmission temp on the highway sits around 135 in winter (not super cold here only gets into high 20’s) and 150 in summer (even up to 100°+ outside). City driving gets to around 165°. Have done some off-roading on long, steep service roads and got the secondary cooler to kick on. I’d say I was averaging about 5mph on some really steep grades and the cooler kept the temps down below 200° even with about 1000 lbs of people and equipment in the truck. Likewise with towing I have a 8.5x24 foot car hauler and have had no issues with transmission temps even in traffic. 
A note for installation: the Derale kit comes with a thermostat that has a tiny thread that is not tapered. You NEED to use a liberal amount of LIQUID TEFLON to get it to seal, or else you will have a drip. Also, injection fuel line style hose clamps are the way to go for these lines as they aren’t under much pressure but these give you peace of mind to prevent a line blowing off from a cracked clamp. DO NOT use the clamps that come with the Derale kit (ask me how I know…). 
Overall I would highly recommend the upgrade if you do any kind of off-roading or towing as it was cost effective and works great!

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