Why does my TH-400 not shift into 3rd until 32 mph.
Thread Starter
Racer
Joined: Oct 2001
Posts: 274
Likes: 2
From: Pampa Tx
Why does my TH-400 not shift into 3rd until 32 mph.
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
Phlash 72 BB Roadster :seeya
Instructor
Joined: May 2002
Posts: 121
Likes: 0
From: Outside Houston Texas
Re: Why does my TH-400 not shift into 3rd until 32 mph. (Phlash)
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:
Le Mans Master
Joined: Sep 2000
Posts: 5,635
Likes: 4
From: Orlando Floriduh
Re: Why does my TH-400 not shift into 3rd until 32 mph. (Phlash)
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.
Thread Starter
Racer
Joined: Oct 2001
Posts: 274
Likes: 2
From: Pampa Tx
Re: Why does my TH-400 not shift into 3rd until 32 mph. (silvervetteman)
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
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
