Accessory Vacuum check valve? on 1996 Chevrolet Lumina

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What is the function of the Accessory Vacuum check valve? For the heck of it I replaced it and all of a sudden my vents in the cabin for air/heat/AC are now WORKING?
(1) Answer
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On some vehicles, engine vacuum is used to operate the cruise control and heater/AC mode and temperature function. If vacuum is lost these systems will not function correctly. The check valve will is a one way valve that allows vacuum to be supplied and also traps vacuum for the accessory system during times of low engine vacuum - under heavy engine load, when the throttle is open.