2004 GMC 1500 transmission swap problems

[cgerritsen]

Customer of mine lost 3rd and 4th in his transmission and wanted me to swap a used one in for him. I told him I couldn't warranty anything because I have no idea if it's good or not so that's his gamble. Anyways I swap in the "good used" transmission and it won't kick down at WOT and also will not go into overdrive. It takes a second to go into gear when you go from park to reverse and even longer when you go from park to drive. Any idea what the problem might be? Just a bad transmission or? Thanks in advance!

[cgerritsen]

Truck is a 2004 GMC 1500 5.3 4x4 excab 4l60e. At first I thought it might be the linkage not putting it in "D" and only putting it in "3" but it falls into the park and reverse positions just fine. When you are going down the road in "D" and let off the gas it drops right down to idle. If you move the shifter to "3" it doesn't shift but it holds rpm when you let off the gas. Thanks!

[cgerritsen]

Also! The transmission he brought me to put in had a 2plug connector for the shift rod thing and the one originally from his truck just had one large connector. I swapped his old one onto the transmission that was going in. Could that have something I do with it? There is an adjustment on that piece.

[cgerritsen]

Code P0753 came up. 1-2 shift circuit A fault.