kna4977

I have an 05 Yukon with the 4L60E that has recently started having issues in the 3-4 shift. All other shifts seem to be normal for now. When it shifts from 3-4 it will engage, disengage, re-engage. I assume this is the TCC going in and out of lock-up??? Don't know for sure though. Anyway, it does this cycle fairly quickly at least twice before it settles into overdrive/high gear. This usually occurs around 45-50 mph under light to moderate throttle. Usually can be mimicked by starting onto an interstate where it has to go through all the gears under normal/light acceleration. Can see it do this by watching the RPM gauge fluctuate up and down as well as hear and feel it. Can usually get it to stop if I press further down on the accelerator but then it seems to lug in the high gear unless I press further then it will upshift and if I let off it may or may not do the same thing again depending on the speed. Hope that makes sense. Another strange thing it will do is around 45 mph when it can't fully decide whether it wants to be in 3rd or 4th it will shift into third then klunk into lug in fourth. Usually occurs on back roads where the speeds are slower. Any ideas as to what is going on and how it can be fixed would be great. Unit has 63K miles on it fluid looks and smells normal and the temperature is within normal limits. There are no service engine lights on and no codes that I'm aware of. I've taken it to the GM dealer and they told me it was the normal function of the electronic throttle, but that sounds like B.S., plus it didn't do this when we got it 2 years ago up until a few months ago. Is there anything I can do to get things going/point the dealer in the right direction to get it fixed? Last time they drove it they said they couldn't duplicate the problem and the electronic throttle B.S. It's under warranty until June and would like to get it taken care of before that runs out. Thanks for your help!