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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: coltonbily on April 29, 2018, 05:56:44 pm

Title: TH400 shifting problem
Post by: coltonbily on April 29, 2018, 05:56:44 pm

i have recently started driving my 1975 c20 with a 454 and th400. if i put the trans in Drive it starts in 3rd and goes really slow from a stop. i can shift it manually through all the gears but it will not shift automatically, the kickdown does not work and and the vacuum modulator does not suck ATF. i checked the governor and the gear was not stripped and the weights move, all seems to be OK except that it wont shift automatically. ATF levels are fine  ANY SUGGESTIONS?

Title: Re: TH400 shifting problem
Post by: Rapid Roy on April 29, 2018, 06:22:23 pm

Welcome.

Title: Re: TH400 shifting problem
Post by: VileZambonie on April 30, 2018, 09:32:04 am

Possible someone messed with the valve body? I would suggest a transgo shift kit and clean the valve body, make sure everything operates smoothly. Does the governor valve move freely?