Vacuum Assist Shifting Problems

[DKY]

I'm having a problem with my vacuum shift assist on my 39 chevy. With the car not running and the transmission in neutral, when I start the car the vacuum assist automatically shifts the transmission into reverse. I checked the vacuum and air inlet lines and they are correct. Any idea what could be causing this?

Any help would be appreciated.

[Guest De Soto Frank]

Sounds like the shifter mechanism needs to be cleaned and adjusted...

If you have a Chevrolet or MoToR's shop manual that covers the year of your car, there should be information on how to adjust the gearshift mechanism.

Steering Column gearshift linkages tend to get gunked-up from grease and road dirt; if such is the case with your car, the shift linkage may be preventing the valve from operating correctly; make sure everything is clean and working freely before attempting to make asjustments....

Good luck !

[DKY]

Thanks Frank, but I've spend the last year restoring the car and everything is clean. I also followed the manual on adjusting the cylinder, It just seems like there is too much vacuum.

[Guest De Soto Frank]

I have limited experience with the Chevy power-shift; most of the cars I drove that originally had them, the vacuum shift had long ago been disconnected and "abandoned in place".

I don't think your problem is one of too much vacuum...rather, I suspect the problem is in the valve that controls the power cylinder...either it is leaking or it needs further adjustment.

The engine vacuum is simply a power source (like the electric service to your house): it "just sits there" until called upon to do work, say by turning on a switch (like when you turn a light on).

There is a reaction valve that is part of the linkage to the power cylinder: it senses movment of the mechanical shifter linkage, which when moved far enough, "tells" the valve to apply vacuum to the appropriate side of the piston.

My 1935-'42 MoToR's manual has a fairly detailed service section on the Chevrolet power shift, and therein, they mention using an "adjusting bushing" to achieve proper adjustment of the linkage, with dimensions as to the size of said bushing... there is supposed to be a certain amount of "float" between the valve rods and the surrounding brackets...

Aside from the "phantom" shift into Reverse upon starting the engine, does the unit seem to function correctly?

Without trying to sound critical of the '39 Chevy, the transmission appears to be the same box as the previous year's floor shift ("Top-loader"), with a modified top cover to adapt it to the new column shift...the 1940 and later trannys had the shift mechanism through the side of the tranny.

You might be running into a "sensitivity issue" with the 1939 mechanism...the MoToR's manaul does stress making sure that the mechanical linkages are correctly adjusted before attempting to adjust the power shift...

I hope this is of some help...good luck!