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C-6 Shifting Problem?

415 Views 1 Reply 2 Participants Last post by  09Frank65
My 72 Mach 1 Q Code has a C-6 tranny. The problem is that when it's in Drive and I'm starting from a stop, it will shift into 2nd then 3rd before I reach 10 mph. I had it in the shop and they said that it was the modulator valve so I had them replace it but no improvement.

I'm running 3.70 gears and am wondering if that can have anything to do with it. Doesn't the tranny rely on engine vacuum to decide when to shift? Could it be that because I'm at higher RPMs quicker then it would shift quicker? It works fine otherwise.

I shift it manually now because of this. It's no big deal, but there are times I just want to put it in drive and go.


My 72 Mach 1 Q Code
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I had the same problem when I changed to a bigger cam. The vacuum dropped and the tranny thought I was going faster than I was.

I turned the modulator adjustment screw (inside the vacuum hose connection) in about 4 turns (mine was all the way out) and now it winds out nice and long.

My guess is the modulator diaphragm spring just needs to be torqued down a little bit.
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