[MCD]

Codes may not set in ECM/TCM, yet shifting is not happening on the 4L60E. Really, shifting might, just the actions are not being followed by the electronic signals. The sun-shell can have the teeth ripped out of it, the aluminum forward clutches housing can be cracked, the input shaft oil circuit holes can be plugged, the servo can leak (takes 2nd oil to apply and continue other shifting circuits), the band can be extremely glazed, lose ya yet? The guy who said he'd take it down is, in my honest opinion, giving you the best option as a rebuild to the "original" equipment. I have rebuilt hundreds of units over the past 30 years in business. Never has any two been identical in exact troubles. I have taken several down with less than 10k miles and found structure cracks, porosity, and even incomplete boring from the manufacturer. The best "other" fix is go to the dealer and buy a SRTA(Service Replacement Transmission Assembly). That way ALL of the most current updates and performance fixes are installed according to GM technology. Downside: the unit may have more miles than your original tranny has or less for that matter? The torque convertor may have blown the seal internal and you lose power to accelerate, yet the trans shifts, ... best guess from here. I would recommend the SRTA from what I understand of your trans knowledge. It comes with a national warranty at any GM dealership. The local guy may not be able to compete with that offer. No, I don't want it, ... LOL

MCD

[C-10]

This thread is a little old, so I doubt this wordy fella is paying attention.

I bet it was the 'sprag' (maybe not)

He seems to like that 'word'...

sprag, sprag, spraggin the spraggity sprag!

[buff52]

I have the same problem on my 2004 K1500 Sierra. I don't have a fix yet but I might. I have a scanner and a shop/service manual. Just as you stated, I thought that the driveline fell off or something. Also when I scanned the transmission parked and driving it set no DTC codes. But the manual has a list of standard parameters for all the circuit components. All control valve solenoids in park with the engine running should be in the ON position. Turns out that the 3-2 downshift solenoid was OFF and it should have been ON. If that hydraulic circuit in the valve body causes some sort of fluid bypass, that could be the problem. I'm going to swap mine out and I'll let you know. It also could be a hardwire problem to that switch, but I can do a circuit continuity check.