Why would your 1994 Pontiac Grand Prix downshift while being driven and what can be done to fix the problem?

Answer 1

While I may not have the answer, I can sure offer a possible. I suffered the same question with my 1987 Pontiac 6000 STE. I took the car to a number of qualified mechanics and Pontiac service centers. As you can imagine, every one of them were more than willing to install a new transmission. I even took it to an old family friend who told me it's time to "put her down". After all the car did have 390K miles on the drive train. Anyway, after months of agonizing over this issue, it came to me like a light. Many of the functions of these modern cars, are inter-related. If one function characteristic changes, others compensate for that change. In my case, I was able to establish a pattern. By watching the temperature gauge.Once it dropped below 160 degrees the transmission would down shift. When the temp rose back to 180+ it would upshift. I deduced my thermostat was stuck open and everytime the engine temp dropped too low to keep the engine fucntioning effeciently the transmisssion responded. I changed the thermostat and solved my problems. I drove that car for another 80K miles without another downshift issue. That $6.00 purchase saved me $2800 on a new transmission! Give it a shot.