Trans cooler issues 4L60, 4L65 & 4L70

Dave’s Tech Tip from Larry Cleeton,

My Hot Rod Buddy, Pierre was having issues with his brand new GM Performance 4L70e transmission and LS3 525hp crate engine this summer having shifting issues.  I had driven it locally when we went to our favorite joint for lunch, Jersey’s Mikes in Sammamish and seemed like it shifted great.  On Pierre’s return trip from Everett, he had problems once he got into stop and go traffic and at low speeds, it just wasn’t shifting right, Pierre called the local transmission guy, had them check it out and it still had problems.

Just by chance I was talking to Larry Cleeton and mentioned it and Larry not only had the same issue, but had solved the problem.  It turns out that these electronically controlled transmissions have a built in safety that locks up the torque converter when it gets too hot.  GM figures that by reducing the slippage of the torque converter (which produces heat)  it will protect the transmission from failure and get you home in more or less “limp mode.”  I had experienced the same type of issue on my LT4 engine which had a oil cooler attached to the engine sump, but was cooled by radiator fluid.  Larry said that the cooling in the front mounted engine radiator was insufficient to cool down the transmission enough to prevent lockup.

Most of the aftermarket swap radiators have an option for transmission cooling in the bottom or side of the newer style aluminum radiators.  This is a separate coil that is cooled by the radiator fluid.  As is the case with the supercharged LT4, why would I want to try and cool down hot oil or hot transmission fluid with a radiator that was at 200 degrees or more?  Most of the Gen IV and Gen V engines run at or around 200-208d before the fans kick on.  I thought that was a poor idea and plumbed in an external oil cooler in the front of the vehicle.  My thinking was why not take a clue from the off-road buggy guys that run the Baja 1000.

I’ve been looking at these compact Derale fan cooled radiators for sometime now thinking that I might want to upgrade and try them out, they also come in a single fan configuration.  I suggested this to Pierre and he jumped on it and solved the issue right away.  Here’s a few pictures for your review: