# 67 GTO 400 Vacuum Modulator Seal



## rickmpontiac (Jul 31, 2015)

I just replaced the vacuum modulator attempting to eliminate a hard shift at slow speeds and have a question about the o'ring seal around the tube that slides into the side of the transmission case. Does the o'ring just fit between the lip on the tube of the modulator and against the perimeter of the hole in the transmission case, or is it supposed to go into a recess in the side of the transmission case. I couldn't see anything like that, but it just seems like a poor design to have an o'ring between two flat surfaces. Thanks for any help or suggestions as always.


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## Goat Roper (Oct 28, 2014)

O ring seals against the flat inside the tranny and the flat on the modulator.
Put the fork that holds it into a vise and bend the two tines in a little to put more pressure on the ring.
The fork is made of some tough spring steel so just bend them in a little with a Crescent wrench.


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## chuckha62 (Apr 5, 2010)

Did you replace the modulator with an adjustable one? Typically, a low or no vacuum signal to the modulator would result in a harsh 1-2 upshift, but it would be delayed, was yours? With a good vacuum signal and an adjustable modulator you should be able to fine tune your shift.


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## rickmpontiac (Jul 31, 2015)

Thanks for the advice to bend the retainer forks a little to put more pressure on the valve body.

Yes, I replaced it with an adjustable vacuum modulator and have adjusted it a few times. Just can't seem to get a good setting that shifts fairly smoothly and at not so high rpm.


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## chuckha62 (Apr 5, 2010)

"... not so high RPM", is a clue that you have a weak vacuum signal to the modulator. Have you checked for a vacuum leak?


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## rickmpontiac (Jul 31, 2015)

chuckha62 said:


> "... not so high RPM", is a clue that you have a weak vacuum signal to the modulator. Have you checked for a vacuum leak?




No I haven't, but that sounds like a great next step in my investigation. Thanks for the suggestion.


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