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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Jason87 on October 17, 2015, 08:14:34 am

Title: TH 400 help needed
Post by: Jason87 on October 17, 2015, 08:14:34 am

Not sure if I should post here or start a new one, but I'm having problems with the th400 2-3 shift. It worked fine before the swap and now won't go into 3rd. I've replaced the vacuum module and adjusted it out and in. I've got 18lbs of vacuum at idle all new clear lines. Pulled the governer cleaned and replaced the gear. Flushed the tranny twice now. The vacuum line does have a tee in it to the heater controls with a one way valve and it's all working as it should. No problems with reverse. My last thought is the detent solenoid o ring is leaking pressure back into the pan and causing a kick down?? Any thoughts??

Title: Re: TH 400 help needed
Post by: Chisholmcody on October 17, 2015, 01:51:46 pm

Did the flushed fluid smell burnt at all?


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Title: Re: TH 400 help needed
Post by: Jason87 on October 17, 2015, 04:18:56 pm

Bright red and no burnt smell.

Title: Re: TH 400 help needed
Post by: bd on October 17, 2015, 04:49:10 pm

...18" Hg of vacuum, not 18 lbs...

Refer to the 1987 Service Manual, Section 7A, Page 22, Figure 22 (http://www.73-87chevytrucks.com/techinfo/7387CKMans//Service/ST_330_87_1987_Chevrolet_Light_Duty_Truck_Service_Manual.pdf#page=971) and the 1987 Unit Repair Manual, Section 7, Page 400-1 (http://www.73-87chevytrucks.com/techinfo/7387CKMans//Unit_Repair/ST_333_87_1987_Chevrolet_Light_Duty_Truck_Unit_Repair_Manual.pdf#page=663).

Title: Re: TH 400 help needed
Post by: VileZambonie on October 17, 2015, 06:08:09 pm

So you have vacuum at the modulator, you've checked the governor, pop the wire off of the detent solenoid and make sure you don't have B+ there with the key on.

Title: Re: TH 400 help needed
Post by: Jason87 on October 17, 2015, 07:09:03 pm

Unhooked the detent and no change. Can the module be getting to Michigan vacuum at high RPMS and and causing it to not 2-3 shift? I'm dropping the pan on Monday to check the filter and the valve body. I'm grasping at straws but think I need to check the filter o ring and the detent o ring.

Title: Re: TH 400 help needed
Post by: VileZambonie on October 17, 2015, 07:57:27 pm

getting to Michigan? Did you adjust the modulator screw?

Title: Re: TH 400 help needed
Post by: Engineer on October 18, 2015, 09:40:48 am

If you unplugged the kick down solenoid and nothing changed I would be focused on a stuck 2-3 shift valve.

Definitely sounds like it is in the valve body.

Title: Re: TH 400 help needed
Post by: Jason87 on October 20, 2015, 06:02:33 pm

Pulled the pan off and found the detent solenoid broken. Also broke the dipstick so while I wait for a replacement I'll check the 2-3 valve