2006 4L60E Transmission Issues

[bwood]

I just purchased my neighbors 2006 Silverado 1500, LWB w/ 4.8L and 4L60E. It's been sitting for about a year due to a damaged driveshaft (threw a u-joint). I know it had transmission issues prior to purchasing. I was able to weld and machine the damage on the driveshaft to get it moving again.

I can shift manually 1 to 2 with no issue, but once I put it in 3rd, it does seem to shift to 3rd (at least it changes gears), but once it does, it just starts slipping/flaring. If I put it in D, it appears to take off in 2nd and shifts the starts the slipping. RPMs rev out when it does depending on how much throttle I'm giving it. Fluid is full and does look like it needs a change, but I'm guessing that's not going to resolve anything. My old scanner only pulls codes from the PCM so I don't know if there are any transmission codes. The only engine code is P0449 for Evap System.

I'm trying to decide whether or not to pull the engine and harness for my son's 84 which was in the back of my mind, but I'm thinking about getting the transmission squared away for a beater to have. Engine runs smooth and so far only has the normal oil leaks.

What I'm looking for are items to check with the transmission and a ODB II scanner that can read transmission codes.