1968 Thunderbird 429 c6 not shifting into third

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Skinnygerman
Posts: 1
Joined: Thu Mar 26, 2020 2:57 am

1968 Thunderbird 429 c6 not shifting into third

Post by Skinnygerman »

So I just installed a new power brake booster in my tbird to find my tranny doesnt shift into third and feels weird going into reverse. Aka doesnt feel like its engaging so no clunk. The old power brake booster had a check valve with two plugs one to cruise and one to the tranny the new one only has one. I hooked up the tranny one to the one available plug from the check valve. It shifted fine before changing the brake booster. Could it be because its getting to much or to little vaccume? I hooked it right up to the carb and no change. Changed the vaccume valve on tranny and only made the problem worse so I put the old one back on. Doesnt manually shift into third, fluid level is normal. Doesnt smell burt or anything. First and second work but sometimes it slips out of gear when I have it in drive. When I manually change gears it doesnt slip. I may have bumped into the kickdown lever. Should I just have it in the off position and see if it shifts into third. Sorry was rambling I'm just very confused as to what's going on.

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Alan H. Tast
Posts: 3077
Joined: Wed Aug 20, 2003 10:52 pm
Location: Omaha, NE

Re: 1968 Thunderbird 429 c6 not shifting into third

Post by Alan H. Tast »

Try readjusting the shifter and kickdown linkage first. If you don't have a copy of the '67 T-bird Shop Manual and '68 Supplement, I highly suggest you get the pair. NOW! And read up on how to make the adjustment. The vacuum connections to the booster should also be covered.
Alan H. Tast, AIA
Technical Director/Past President,
Vintage Thunderbird Club Int'l.
Author, "Thunderbird 1955-1966" & "Thunderbird 50 Years"
1963 Hardtop & 1963 Sports Roadster

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