C4 Swap Help

Scrappy91

Member
Jul 31, 2016
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so i recently swapped the aod in my 91 mustang for a c4. its a stock c4 but has a shift kit. the problem im having is the car doesn't seem to shift very hard under heavy throttle like it should with the shift kit and also it wont shift into 3rd gear until about 45mph. i have the vacuum modulator hooked up to the vacuum tree im not sure if that is part of the problem. im not sure if i try to adjust the modulator to get the 2-3 shift at a lower speed if that will cause the transmission to shift really soft which is the opposite of what i want. if anyone has run into a similar problem or knows about c4's all help is appreciated, thanks.
 
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Rcdgl

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Feb 19, 2018
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A couple of possible causes, but it sounds like it may be a worn transmission or a dirty valvebody.
Could also be worn ring seals on the tail shaft, or a dirty or stuck governor. Also suggest that the kickdown lever be installed and properly adjusted.
I tried a shift kit in a used C4 once, and got no improvement in shifting. Gave up on that, rebuilt one the cars originals and installed a complete TCI valve body, bolted it in and that fixed it.
 

Scrappy91

Member
Jul 31, 2016
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A couple of possible causes, but it sounds like it may be a worn transmission or a dirty valvebody.
Could also be worn ring seals on the tail shaft, or a dirty or stuck governor. Also suggest that the kickdown lever be installed and properly adjusted.
I tried a shift kit in a used C4 once, and got no improvement in shifting. Gave up on that, rebuilt one the cars originals and installed a complete TCI valve body, bolted it in and that fixed it.
I cleaned the valve body when I put the shift kit in and I’m not putting a kick down on because had other cars and heard plenty of people say they don’t run kickdowns and they work fine I’m wondering if Its Related to the vacuum going to the modulator since I just have t hooked to the vacuum tree on my 91
 

85ragtop

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Mod Dude
Nov 29, 1999
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That could be, the vacuum that is. But I had a similar problem and ended up going with a manual valve body. I never could get one to shift like I thought it should. I however never used stock AODs or C4s so that's outside of my realm.