Hello Glenn and Wallace,
Actually the "venturi switch" has been an active topic on this forum. It is one of the essential ingrediants to having proper linkage adjustment. It is critical to the proper shifting of the automatic transmissions. Improperly adusted linkages can result in an improperly functioning switch which may cause the transmission shifts to be harsh and late. The last downshift before a stop may be very abrupt.
There is not much detailed documentation on the system but it is fairly straight forward. This "venturi swich" along with the "kick down switch" (passing gear) activate a three position solenoid on the automatic transmission. This three position solenoid moves a linkage rod which raises or lowers modulator pressure in the transmission. The change in pressure influences when the tranmission shifts, and shift quality (firmness, harshness, smoothness etc,) by raising and lowering the pressure.
The venturi switch should be adjusted to activate the three position solenoid on the tranmsission to position "1" (lower position)at idle thus lowering the pressure in the transmission. As soon as the accelerator is depressed the solenoid moves to position "2" (middle position), raising the transmission pressure. The solenoid will move to position "3" (uper position) if the accelerator pedal activates the "kick down switch" thus raising the pressure even more. With the ignition off the three position solenoid is at rest in position "2".
Another problem to watch out for are frozen linkages or solenoid at the transmission whcih can render the solenoid useless.
Read the "linkage tour" for more information on setting up your venturi swich.
On the later cars with standard tranmissions, the venturi swich was used fot the fuel shut off solenoid on the injection pump, in place of the earlier fire wall "micro switch".
The link Bob has posted gives an even more in depth description of parts of the system.
