I have a 99 S-10 Blazer. When I bought it, it had a transmission issue where at low speeds, it would shift to high gear / torque convertor lock up. My wife noticed that when the 4wd lights were on, it was less likely to do this. We also had a problem where we had to either disconnect the battery, or disconnect the atc module everytime we wanted to change from 4wd to 2wd, regardless of what mode we were in. When in 4hi or 4lo, everything seems to be fine. But when in auto4 or 2wd, that is when the shifting problem starts. It will also turn on the cel when it does this a few times. I unplugged the atc module with the truck in 2wd and haven't had a problem with it as far as the transmission is concerned. My question is, and this has baffled GM technicians as well, is what does the atc module have to do with the shifting characteristics of the transmission. The transmission is controlled by the pcm, not the atc. Keep in mind, I am a former GM technician myself, all my training coming from GMSTC. I have a few friends who specialize in transmissions inChevrolet dealerships, as is all of my experience is in Chevrolet dealerships. Like I said, I have discussed this with them, and they seem just as lost as I am. Can anyone help? Does anyone have any ideas? Thank you.

 

Bear in mind that everything I know about automatic transmissions is written on the inside of my left eyelid........

With that in mind, I would hazard a guess that when the truck is in 4WD, the PCM pays attention to this, and alters shift points accordingly. (hold gears a little longer on accel, downshift a little sooner? taking into account the additional power required while in 4wd.) Perhaps swapping out the PCM, or, at the very least, reprogramming it, even if you put the stock program back in there, erasing the EPROM, and re-writing it may fix this issue. Then again, it may not do a damn thing.

 

well with my limited knowledge I'll give it a shot. [8D]

Yes, the puter read teh ATC as one of the paremeters it monitors to control engine/tranny perofrmance. When in 4wdr it changes shift points, when in 4lo it changes them more. So yes the ATC has a bearing on tranny shift points and engine performance.

As to your problem, look for a short or loose wiring harness at either the ATC or puter. I have heard of a faulty ground short causeing problems, and believe there is a service bulletin out on it.

Also check for water in the harnesses. It will cuase some goofy things to happen as well.