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  •  » C4 transmission vacuum modulator

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    9/03/2022 12:32 PM  #1

    nrackison@yahoo.com
    Active Member

    Offline

    Registered: 8/16/2022
    Posts: 9

    C4 transmission vacuum modulator

    Hello,
    I had my C4 transmission rebuilt but the guy didn't change the vacuum modulator which is at least 20 years old even though I gave him a new one. My question is about adjusting the new vacuum modulator. the instructions say that they are peadjusted to factory specs but does there have to be a certain amount of vacuum from the line that connects to the modulator. I've heard about 15 inches but perhaps some uses know what works best for the C$

    Thanks,

    Norm
     

     

    9/05/2022 1:43 PM  #2

    Ron68
    Senior 24-7 FYI Addict

    Offline

    From: Mukilteo, Washington
    Registered: 4/01/2013
    Posts: 1,575

    Re: C4 transmission vacuum modulator

    Back about 20 years ago when I rebuilt my C4, I had to adjust the modulator as it was shifting too early. I did the adjustment of turning in the screw a couple of turns and it eventually was shifting as I thought it should. I do remember that when I was adjusting the idle mixture on the Edelbrock 1406 I installed that the engine vacuum at the best and smoothest idle was in the range of 15 inches, and that is what I had when I adjusted the modulator.  The C4 served me well for a good ten years until I removed it and installed a 4R70W. I had been running an 8" 3.25 rear end that I swapped out for a 9" 3.25 rear that I was running for a couple of years on the  C4, so the gear ratios were the same on the C4 for a few years and I didn't have to readjust the modulator after the 9" swap.
    Check out this video for some good info on adjusting the modulator: https://www.youtube.com/watch?v=lu5gnSVL76w

    Last edited by Ron68 (9/05/2022 1:44 PM)

    68 coupe - 351W, 4R70W, 9" 3.25 -- 65 convertible - 289 4v, C4, 8" 3.00
     

     

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