So last month my transmission went out and i bought a used 4l60e. After installing i found i had no reverse and only 1 forward gear. After about 50 miles it started to slip out of gear. After it would slip out it would not go into another gear until i shut off the truck and let it sit a few minutes. After starting it back up or would go into gear and roll for about 5 seconds, then slip back out. You buy used you get used. So i stayed looking for another. Well last week i bought another tranny that i knew for a fact worked. Well it's doing the same exact thing the last one did, minus the gear problems. I unplugged the wiring and it still does it so it is not an electrical issue, as it should have gone into limp mode. The only thing i can think of is maybe my cooler is stopped up. Would a blockage cause the transmission to slip out of and not go back into gear?

 

yes. the cooler lines have to be flushed anytime you replace the tranny. When you pump that old contaminated fluid through your replacement tranny you can end up right back where you started.