Check The Rubber Lines First.
D.D. - I have a 1970 SS396 Chevelle with a TH400. My problem occurs when the transmission shifts. Taking off from a standing start under normal acceleration, the transmission shifts into second at a very high rpm. The same is true for second to third, but the rpm does not seem to be as high. Also, while driving along, the transmission wants to down shift very easily.
G.M. - Check the vacuum line that goes from the intake manifold, behind the carburetor, to the modulator valve on the passenger side of the transmission. Particularly check the rubber lines at the ends. If they are in good condition, then chances are the modulator valve is bad. Also, make sure that the other line which hooks up to the same vacuum port behind the carburetor is not leaking. This may be a line that is only used on air conditioned cars.
Archivist: Tom Wilson
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