I have a 2000 Burb K1500 with 225,000 miles. It's prob been 100,000 since I did the last trans service. I know! Did the service and filled it up, run it through the gears, checked fluid, perfect level. Put it in reverse, backed out of driveway dropped it in gear and wouldn't pull up in driveway hardly like something was holding it back. RPMS didn't skyrocket but just went up like I was pulling out. Manually shift to 1st gear or 2nd gear and all seems fine. Won't pull in 3rd gear or Drive hardly. If you accelerate hard in 1st or 2nd gear it takes off like a scared rabbit. If you just go from reverse to D and accelerate hard the rpms jump to about 1500 and it feels like you're power braking it. It doesn't feel like it's slipping. This vehicle drove perfectly fine until i did the trans service today. I think I must have knocked an electrical connection somewhere loose because it feels like it's trying to pull out in what gear I've selected instead of 1st gear. So when I put it in D, it's trying to pull out in D instead of 1st gear. What controls the tranny to tell it to pulll out in 1st gear when you just put it in drive like you normally would. I hope it's an electrical connection outside the tranny and not inside. I don't feel like pulling the pan off again. LOL!

 

It's either the harness on the side of the transmission or the internal wiring harness.
they control the solenoids which control what gear you are in. Normally there are two A and B. Get a scanner and see if the solenoids are switching when you move the gear selector. Should be like A-on B-off. Or A-on B-on , etc through the gears. If one shows no change you likely popped a wire off inside. If the harness is bad they are about $15 and easy to change.

 

Yep , You are correct sir. i checked the outside electrical hookups and all was fine. Pulled the pan again and I guess when I dropped the pan the first time and slid it forward, the pan grabbed the plug on the shift solenoid and pulled it off. I was pissed. i've never done that before.