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TH400 vacuum modulator adjustment

This is a discussion on TH400 vacuum modulator adjustment within the Technical / Maintenance forums, part of the General Discussion category; hey guys how's it going? a buddy of mine told me about an adjustment screw inside the vacuum modulator on ...

  1. #1
    Livin' truck by truck Muddin_Maniac89's Avatar
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    TH400 vacuum modulator adjustment


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    hey guys how's it going? a buddy of mine told me about an adjustment screw inside the vacuum modulator on the side of my TH400 Trans. apparently depending on which way you turn it you can make the gears wind out longer or shift faster. anyone know which way does what?
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  2. #2
    Registered User ToyHauler's Avatar
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    Re: TH400 vacuum modulator adjustment

    interesting info i found on a different site;


    "At full throttle my turbo 400 upshifts at 4,500 rpm. I want it to upshift at at least 5,000, maybe even 5,500. I know that the modulator controls upshifts, but only up to WOT I think. After that, the springs in the Governor decide when to upshift right?

    So should I get different springs for the governor or adjust the modulator? If I adjust the modulator won't all upshifts be higher up in the rpm range? I mean, If say I adjust it to 5,500 WOT upshift, won't my partial/cruising throttle upshifts also be up much higher?



    Your understanding of the modulator and governor operations is incorrect.

    The governor weights and springs work together

    The springs in the governor control shift points primarily at light throttle. Changes to the springs affect shift points more up to 35 mph area depending on gear ratio - tires ,etc.

    The weights affect higher speed shift points more than the springs do.

    Later WOT shift = higher RPM will happen by lightening the weights ( inner) on the governor.

    The vacuum modulator affects mostly th e light throttle shift points. At WOT the vacuum will be near zero and the modulator will be ineffective. This will have main line pressure raised.

    I would suggest you purchase a kit with different weights and springs to modify your governor. Or simply start grinding on the inner weights for later WOT shift points.

    The lighter weights will affect light throttle shifts slightly on the t-400 trans , but mostly the higher speed shift points"
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