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Old 03-31-2018, 10:46 PM   #1
'65 chevy lover
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TH400 help

I have a TH 400 that I just got in my truck and, it's not shifting right. Could it have to do with this (see pic below) is this supposed to be hooked up to a vacuum line?
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Old 03-31-2018, 10:50 PM   #2
B. W.
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Re: TH400 help

Yes, vacuum modulator. Hook it to vacuum & see if it shifts better.
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Old 03-31-2018, 11:05 PM   #3
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Re: TH400 help

Just to clarify, it needs to be connected to manifold vacuum.
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Old 04-01-2018, 12:03 AM   #4
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Re: TH400 help

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Originally Posted by B. W. View Post
Yes, vacuum modulator. Hook it to vacuum & see if it shifts better.
Thanks, will do.
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Old 04-01-2018, 12:05 AM   #5
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Re: TH400 help

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Just to clarify, it needs to be connected to manifold vacuum.
Thanks, Captain, I'll check it out.
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Old 04-01-2018, 10:45 PM   #6
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Re: TH400 help

Just some additional tidbits.......From the pic, I believe the modulator maybe adjustable as well....so you can take a small screw driver and adjust the set screw in the modulator....turn the screw clockwise will firm up the shift (later) and counter clockwise with soften the shifts (earlier) ....
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Old 04-02-2018, 12:23 AM   #7
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Re: TH400 help

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Originally Posted by YoungPup1977 View Post
Just some additional tidbits.......From the pic, I believe the modulator maybe adjustable as well....so you can take a small screw driver and adjust the set screw in the modulator....turn the screw clockwise will firm up the shift (later) and counter clockwise with soften the shifts (earlier) ....
Thanks, I'll check it out.
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Old 04-05-2018, 11:39 PM   #8
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Re: TH400 help

Hooking up to vacuum fixed the problem. It shifts good now. I felt and looked around on the valve body, but didn't see or feel a screw to adjust how it shifts. Thanks to all for the help!
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Old 04-06-2018, 12:37 AM   #9
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Re: TH400 help

The adjustment screw on a vacuum modulator is inside and accessed thru the port for the vacuum hose.
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Old 04-06-2018, 10:22 AM   #10
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Re: TH400 help

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The adjustment screw on a vacuum modulator is inside and accessed thru the port for the vacuum hose.
This. And on a Turbo 400 the modulator is the sole control of shift points and firmness, except for the electric switch for WOT downshifts which most, but not all have.
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Old 04-06-2018, 03:24 PM   #11
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Re: TH400 help

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Originally Posted by Captainfab View Post
The adjustment screw on a vacuum modulator is inside and accessed thru the port for the vacuum hose.
Ah, I'll check it out. It shifts fairly quick from 1st to 2nd, but from 2nd to 3rd takes a bit longer. I'll have to drive it more to see if I need to adjust it. Thanks for the info.
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Old 04-06-2018, 03:33 PM   #12
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Re: TH400 help

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Ah, I'll check it out. It shifts fairly quick from 1st to 2nd, but from 2nd to 3rd takes a bit longer. I'll have to drive it more to see if I need to adjust it. Thanks for the info.
Glad you got to the root of it. I'll bet it revved way up and then slammed the 1-2 shift. I had that vacuum hose fail once because the PO put the exhaust too close to it and burned the hose until it was extremely brittle.
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Old 04-06-2018, 03:42 PM   #13
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Re: TH400 help

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The adjustment screw on a vacuum modulator is inside and accessed thru the port for the vacuum hose.
Be very careful and make changes 1/4 to 1/2 turn at a time. You can puncture the diaphragm in them if you crank it in too far. As me how I know...
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Old 04-06-2018, 08:48 PM   #14
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Re: TH400 help

Also there was a steel line that ran down there from the factory. Attached at the intake behind the carb and to the modulator with a 4" piece of rubber hose. A long rubber vacuum hose going all the way down there can weaken and colapse.

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Old 04-08-2018, 12:21 AM   #15
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Re: TH400 help

Thanks to everyone for the information, it is very appreciated!
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