4l80e

I have a '94 CrewCab 1 ton dually 4WD. I had 4L80E Tranny rebuilt about year and a half ago and what it has been doing the past 6-9 months is when towing or with a load in box of say 2000 lbs. or more, it'll run fine for about 15-20 miles then gets to a point where it will not shift into OD from Drive, and even on occasion, it won't let me out of second gear, until I shut it off and let it sit for about 20 minutes or so. It does have tranny cooler, and I don't see this problem when truck is empty. When starting vehicle from complete stop under load, I shift it down to first and work my way manually up thru the gears (transmission place told me to do this - doubles the bands used?). I've checked tranny fluid and all is well there. Any ideas?

Thanks in advance.

Sounds to me like your tranny is getting hot.

NEVER use OD when towing. You should always keep it in D, unless you are towing something really really small.

My words exactly, place transmission in drive never overdrive when towing or carrying heavy loads, I am sure if you read the owners maual (if you have one) it will tell you this. I would say your transmission is getting hot. You might try cleaning the outside of the tranny cooler. If the outside is dirty it can slow down the heat transfer from the tansmission fluid to the air passing through the cooler. If you have a fin type tranny cooler you could try upgrading to a plate type cooler, I would only try this if you know the fluid is getting too hot and the above doesn't seem to work.