Hey guys in my 2001 Sierra 1500 I installed a Tru cool 40k and bypassed the radiator. I did install the thermal bypass. It's 51 degrees here in Florida this morning and here's results of what I did.

Idled truck for 15 min. Temp got to approx 100*.
Went for a quick 10 min ride and temps got up to about 130 in a little stop and go. As soon as you start driving (55 mph) the temps pretty quickly fall to below 100* and kept falling the more I drove.

The bypass isn't supposed to be open until 180. But nothing will even get to those temps.

After the drive idling in the driveway temps got up to 140.

Pan temp was about 125*.

Line from bottom of trans to the bypass was 125*.

Line from bypass back to trans was 129*

The 2 lines that go through the cooler after the bypass were 25-30* cooler than before the bypass.

I assume that some fluid (but not all) is going through the cooler which is why it's 25-30* cooler. But I thought the bypass was supposed to be fully closed until 180*?

The biggest problem is that nothing is getting up to 180*. Even after the drive sitting there idling for 10 min the trans pan would not go over 128*.

I can only assume that the fix for this would be to run in through the radiator, then to cooler?

I really wanted to bypass the radiator to get rid of that possible failure point. But I don't see a way around this. This truck is stock but when I had my turbo truck I was able to bypass the radiator, most likely because of the stall/turbo.

 

When it’s cold just use a piece of wax coated cardboard painted black to obstruct the air flow. That’s what I do.

 

That thought did cross my mind also. I would really like to bypass the radiator if possible..thanks for the tip

 

I was in the same boat and I needed a quick fix. Works so well it stayed that way, I’m making a new one today bc it’s actually cold at the beach this week. Yeah for me.

 

Alaskans have been putting cardboard in front of radiator since Anti-freeze was invented.
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I also have a 40K Tru-Cool on 4L80 install, trans would never even get warm.. ( Over-Cooling ).

I RE-Routed my cooler lines, so that Tru-Cool gets fluid feed , and then ATF goes thru OEM Radiator " cooling " loop, which- in Alaska WARMS UP the fluid, which then returns to the transmission at correct operating temperature.

 

The 40k definitely works to efficiently in cold weather for average vehicles. I put in a Derale bypass which is supposed to fully open it's thermostat at around 160 degrees. My trans will keep at least 130-140 degrees even highway driving down to like 30 degrees ambient temps. I also have mine running through the radiator, so it gains some heat from the engine coolant in winter, and I suppose it actually cools the engine coolant in the summer because trans always runs cooler than engine.

When it starts getting into 20's or colder I need to block off the Trucool because transmission may only get up to 100F during highway driving.

 

It's tough to have a 4l80e too cool
If you cruise the highway all day long at 90 degrees F...the only thing it's gonna hurt is 1% fuel economy
Cool oil is great for the trans

If you told me I had to choose between 80 or 180...both are ok but I'd pick 80

As for the temp sensor...what does the temp sensor in the pan say?
Factory has a sensor there and it's best to measure pan temp to get the best idea of average operating temp