Like most General Motors cars of this era, this car (1986 Buick Century) has a vacuum-operated air conditioning system and cruise control
The air condiotioner depends on vacuum actuators to direct the air through the selected vent (defrost / floor / dash / recirc). With a loss of vacuum to the air controls, the A/C system defaults to "defrost."
The Cruise system uses a vacuum powered actuator to operate the throttle based on the signals to its electric over air solenoid valves. Without vacuum, it will not open the throttle.
The vacuum to operate all this comes from the engine. At less than full-throttle intake manifold vacuum is adequate for these accessories. The factory setup includes a vacuum reservoir hidden in the left-front fenderwell, and a check valve to hold vacuum in this reservoir when engine vacuum is not adequate. This system works well enough as long as there are plenty of times when the engine is developing strong vacuum, and the vacuum-operated accessories are not leaking.
On a turbo engine, there are times when the manifold contains pressure instead of vacuum. First problem was the the factory check valve blew apart. Then every time I accellerated, the A/C came out the defrost. I installed an industrial check valve rated at 150 PSI and it has held up fine. But still under proloned operation at low vacuum, the reservoir is depleted and the cruise starts to loose speed and the A/C malfunctions. Pulling a long hill in overdrive (which will produce 3 or 4 PSI boost) would do it. The cruise would loose speed on the Interstate at prolonged high-speeds.
The solution is an auxiliary vacuum pump. The pump I have used was originally a cruise-control pump on an Audi. I got it at the junkyard complete with a vibration isolating bracket. It's pretty well-made with a rollerbearing eccentric shaft and ball-bearing motor. The relay-like device you see inside the pump is the dump valve for the Audi's cruise control. I took this out and replaced it wioth a Hobbs vacuum switch which will turn ON when vacuum is lost to 10"Hg.
The pump only runs when vacuum is low in the vacuum reservoir AND the engine is running. So basically only under prolonged high-load operation. This is accomplished by using the ECM's fuelpump drive signal for the relay coil power source. It prevents the auxiliary vacuum pump from running constantly if the key is turned on but the engine is not started.
Electrical and air schematic below.
