Phlash

My 72 TH-400 won't shift until 32 mph. It will downshift into second once my speed drops to 26 mph without romping on it. It will downshift to 2nd until about 60 mph when I stand on it. What is going on and how do I adjust this. I haven't found this in the tech tips or in the archives. Is it a bad modulator?

Phlash 72 BB Roadster :seeya

StingRayPonyGirl

Sounds similar to the problem I had, it turned out to be a clogged vacuum line between the modulator and the intake. We found it accidently while replacing the modulator. Hope it turns out to be something as simple as mine. :cheers:

Larry B.

The modulator controls normal upshift/downshift firmness and the point at which it shifts (based on engine vacuum). If you car has a vacuum leaks or the engine is in poor tune producing lower than normal vacuum, the trans will shift late and harsh. Check ALL of the vacuum lines... (Yes it is a pain ). If they are ok and the engine is running good the replace the modulator. If even ONE drop of fluid is visible in the line the modulator is bad. Most replacement modulators have an adjusting screw in the tube the vacuum lines goes over. You can tailor the shift point within reason. Now the next point is wide open throttle downshift. There is a switch either on the carb or under the gas pedal depending on year. This forces a 3-2 downshift at higher speeds and a 2-1 or 3-1 downshift at speeds up to 35. That will never happen without the electric solenoid system functioning.

Phlash

Sivervetteman,

Thanks, I will be checking it out tomorrow. My vette has good vacuum and idles well. The 1-2 shift is good and smooth, but once I hit 32 or higher, the shift into 3rd is stiff, but not harsh. Slight pedal movement will shift it back to 2nd, but not into 1st. Full pedal still doesn't shift to 1st, so I may have multiple problems.

Phlash 72 BB Roadster :seeya