4L80E plumbing

PLascell · June 1, 2005

My 4L80E ATF pan temp gets up to 250 degrees pulling steep grades with 5th wheel. Trying to cool it down more than what the factory aux coller is doing. Engine water temp doesn't get above 195 degrees.

I want to add additional trany cooling to 2000 3500 CC DRW 4WD 7.4L that came with aux trany cooler. Dealer can't tell me which way the ATF flows in the top and bottom radiator tubing. Which is from the torque converter (hotest)?

Also, what kind of fittings (thread sizes) are the metal tubing connections to the radiator transmission coil? Dealer parts man says 5/8 -18 UNF but I can't find fittings like this on the web. Is this a typo?

PLascell · June 5, 2005

Worked on the truck today and confirm the front ATF connection does go the the top radiator connection and it is hotter than the rest which confirms the TC connection. The fittings at the radiator are 3/8 Inverted flare and generally know by this at the auto parts store. The thread pitch is 18 TPI. The outside dia. of the male threads is 0.618. The wrench required is a 5/8 open end.

I ended up adding a second aux cooler (7500 BTU) between the output of the 4L80E and the input to the radiator. The factory tow package place a larger aux cooler on the outout of the radiator in the return line to the transmission.

It will be a few weeks before I try pulling the 5th wheel up the 12% grade to see what effect it will have.

GM-Tech · June 2, 2005

The cooler lines on the trans are arranged one in front of the other. The forward most connection is the output to the top of the cooler in the radiator.

I don't know offhand what the thread size is, but no way it's 5/8ths. 5/8ths is the size of a heater hose! I'd say more like 3/8ths, but maybe 5/16ths. Never really paid much attention to the size.