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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Joe.cummings on December 11, 2018, 09:07:04 am

Title: Th400 shifting issue
Post by: Joe.cummings on December 11, 2018, 09:07:04 am

Issue: th400 not shifting correctly. Feels like when the truck shifts into 2nd that first gear does not disengage.

Additional information: I never used this transmission before so I do not know the history of the trans. Also, I did do a complete rebuild including installing a 4l80 direct drum and 32 element sprag. It does need to be said that this is my first trans rebuild and I easily could have messed something up.

I did replace the vacuum modulator and ensured the vacuum line is coming from manifold vacuum. Any help is greatly appreciated.

Title: Re: Th400 shifting issue
Post by: Rapid Roy on December 11, 2018, 09:08:14 am

Welcome

Title: Re: Th400 shifting issue
Post by: Ozark on December 11, 2018, 10:29:33 am

Maybe worth replacing the check valve at the brake booster?

Title: Re: Th400 shifting issue
Post by: Joe.cummings on December 11, 2018, 02:52:51 pm

Why would this cause an issue?

Title: Re: Th400 shifting issue
Post by: VileZambonie on December 12, 2018, 04:26:05 am

Disconnect the detent solenoid to see if any change.

Recheck the manual valve before you pull it apart.