Been playing with an '01 K2500 6L 4L80E Silverado. Quick question though, after reading some of joecar's excellent technical info, I am still wondering just what AppEn is and how to control it. From reading, I would guess that this is simply the TCC "slightly" turned on (i.e. dragging). As to what controls it, it appears that this happens anytime the transmission is in 2nd - 4th gear (never seen it kick in in 1st gear) and certain TCC Slip (RPM) values are not exceeded.
The reason I am trying to understand this is for two reasons. The first is that my dad's truck (99' same configuration except with 3.73's not 4.10's like the '01) seems to hunt a lot between Off and AppEn during 2nd - 4th gear. I would really like to either turn it on, or turn it off so it doesn't keep hunting. The second reason is that for this '01, we are kicking around swapping out the 4.10's for either 3.73's (could be the same problem that my dad's truck has) or even 3.55's (he doesn't tow much so MPG is more important). I have a gut feeling that if we go 3.55's that the '01 will hunt even more.
So far, I have logged data and figured out that if the TCC Slip (RPM) gets too high, then it kicks out. Didn't know if anyone else had played with or experience dealing with this condition.