Life inside a vacuum

Ah, the good old days - vacuum-operated wipers, choke controls and heat-riser valves. Then again, maybe those days weren't so good.


Ah, the good old days — vacuum-operated wipers, choke controls and heat-riser valves. Then again, maybe those days weren't so good when you consider some of the now-antiquated technology that used to be controlled by engine vacuum. Clearly, electrical and electronic systems provide superior control when compared to vacuum technology.

Yet, vacuum-operated devices are not gone altogether. They still hold a strong position in key areas of modern vehicles. Here's how to use a hand-held vacuum pump and vacuum gauge to troubleshoot vacuum systems.

Vacuum motors

There are probably more vacuum motors than any other vacuum-operated devices on cars. These motors may contain one or two diaphragms to push or pull. Vacuum motors are used in heating and air-conditioning systems and thermostatically controlled air cleaners, just to name a few.

Checking a vacuum motor is easy. Connect the pump to the fitting on the motor and apply at least 10" Hg. of vacuum. The motor's actuator should move. Once the actuator stops moving, watch the gauge on the pump to check for diaphragm leaks. It the diaphragm is OK, the vacuum should hold for one minute. Replace the motor if the actuator fails to move or the diaphragm leaks.

Vacuum switches

Like the switches used to turn electrical current on and off, carmakers also use switches to control vacuum. Some switches are temperature-sensitive such as those for the EGR circuit; others operate mechanically such as the vacuum switches for hidden-headlight systems or heater control valves.
It's important that you follow the carmaker's procedure when testing these switches. Frequently, you must apply vacuum to one switch port and watch for vacuum, using another vacuum gauge connected to another port.

Temperature-sensitive switches must be diagnosed based on the temperature at which they switch on and off. For example, you would apply vacuum to the switch, start and warm up the engine, and record the temperature at which the switch opens.

Vacuum valves

Many systems use various types of valves to tailor vacuum to the needs of the circuit. Check valves are often used to hold vacuum in a reservoir, or allow vacuum to flow only one way. Delay valves slow the passage of vacuum, slowing the application of vacuum to certain devices.

To test a check valve, connect your vacuum pump to the light-colored side of the valve and apply about 5" Hg. of vacuum. The vacuum should hold within 1" Hg. after 30 seconds. Then, connect your vacuum pump to the dark-colored side of the valve. Try to apply a vacuum. It should not build unless you cover the other end of the valve with your finger.

If the vacuum level drops off too quickly when connected to the light-colored side of the valve, or you can build a vacuum with the pump connected to the dark-colored side, replace the valve.

Vacuum solenoids

Nowadays, vacuum circuits are often controlled with solenoids. These solenoids may block, pass, or vent vacuum on command from the Powertrain Control Module. Usually, these solenoids are used in pairs, which gives them the joint capability of applying, releasing or holding vacuum to a certain device.
There are two basic kinds of vacuum solenoids. A normally open solenoid passes vacuum when de-energized and blocks it when energized. A normally-closed solenoid blocks vacuum when de-energized and passes vacuum when energized.

Checking the solenoid's operation will quickly tell you if the solenoid is OK, and what kind of solenoid it is. Turn the ignition switch on and connect the clip end of your test light to a good ground. Remove the hoses from the solenoid and connect your vacuum pump to the fitting that receives the vacuum supply. Apply vacuum to the fitting of about 15" Hg. Depending on the solenoid, the pump may or may not build a vacuum. Then, momentarily touch the tip of the test light to the ground terminal of the solenoid.

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