Hi All,
New to this site so bear with me. I recently had to repo a 2000 Blazer from my grandson and tackle needed repairs. It has the vortec V6 with auto trans. It shifts thru all gears just fine untill it goes into overdrive and at about 45 it just can't make up its mind and wants to drop back to 3rd. At this point it will continue to go back and forth between these gears. My mechanic said there are no codes to read so I took it to a trans shop to see what they think. They test drove it with me and said they think one of the sensors are feeding bad info to the computer. I wouldn't know where to start looking so wondered if someone else has had this problem and would share some help so I don't have to fork out big $$$$$$$ to get the trans shop to fix it.

 

It likely is not dropping back and forth between 3rd and 4th, but going back and forth from locked torque converter to unlocked torque converter. This can indicate a failing torque converter clutch, a leak in the TCC apply path, a weak solenoid, or a problem in the electrical connection to the TCC solenoid.

 

Thanks so much for the quick response, but sounds like a problem I will have to submit to the trans shop since they have the equipment and I don't. I don't know what a TCC is or for that matter where it is located, but if the "leak" is a reference to a vacuum line I can inspect or test along with the electrical connections then I might have some luck fixing the problem.