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Transmission Trouble


larry n

I have a 59 powerglide. I am just now installing it in my rebuilt baby. It was rebuilt about 5 years ago. It will shift great when its cold. From first to second and then downshift. When the engine temp gets to about 150 it will not shift out of first. I have tried a new modulator valve and it helped, but now it will not upshift after the temp gets to be about 180. I have talked to several transmission people. Some say its the transmission some say its not due to it shifting when cold. I don't think I am getting a lot of vacuum but I am think I am getting enought to make it shift.
Any suggestions???????
Jan 1, 2002
Washington, Michigan
'67 Marina Blue Convertible
Two things come to mind - solid vacuum supply, and correct kickdown linkage adjustment. You should have a steel vacuum line from the manifold vacuum source all the way down to the modulator; I've seen many with just a rubber hose, and they can deteriorate and crack over time, causing two issues - an engine vacuum leak, and inadequate vacuum signal to the modulator. The other is kickdown linkage adjustment, which is covered in the assembly manual and in the ST-12 Shop Manual; this needs to be done correctly in order to get proper shift points and wide-open throttle downshifts.

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