Same Transmission Problem, Same Question
 

I have a 2000 Chevy S-10 (250,000+) - it's been a wonderful vehicle, purchased new, regular transmission maintenance. The mechanic is saying it needs a whole new transmission! Another mechanic is saying it's a sensor. Is it possible that this transmission problem could be just a sensor? Do the additives really work? I have to try the cleaning idea first. The engine in this truck is still humming along perfectly.
Anyone had any experience with the additives? Is 250,000 miles about all one can expect from a Chevy transmission.
The symptoms are erratic shifting - sometimes in 1st, sometimes in 4th - more in 4th than 1st. But lots of times - no trouble at all! I've also noticed, that if just drive the thing like nothing is wrong (don't baby it on take-off, etc.) the shifting is smoother! If I try to take off really easy, let up off the gas when RPM's are just rigth...it slams into gear - especially in first. This just started to occur about a month ago, and I've been using another vehicle until I figure out what to do. Last service was about 20,000 miles ago. (18 mos) Hopefully, I have not made matters worse, but I do still drive it. Check engine light has gone on and off 3 times in the last month. The "code" is signaling timing. It's not timing - this truck runs as smooth as silk. This whole thing started when the mechanic changed the serpentine belt - the first code hit that day. The mechanic insists the serpentine belt is not causing it. I've used these mechanics for 5 years - they are as honest and fair as they can be - and I've never had them steer me the wrong way. I believe them...but I also know my truck. Something is not quite right. Any ideas?

*Moved to a new thread so as not to hijack the other one*

What was the specific code that was being returned? Which solenoid/sensor was the one mechanic claiming was the cause of your symptoms? Erratic shifting could be caused by a number of different things...