BlueSpot 07932 - Testing Thermal Vacuum Switches; Testing Vacuum Operated Heater Taps

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1. Locate the solenoid to be tested.
2. Remove the line that goes to the component being tested.
3. Connect the vacuum tester to the solenoid port (figure 8).
4. Start the engine.
5. With the system turned off there should be a zero gauge reading.
6. Turn the system to the ‘on’ position.
7. A gauge reading equal to the manifold vacuum should be observed. If no reading exists
remove the vacuum supply line and test for manifold vacuum.
If the vacuum does exist this indicates that the solenoid is faulty or is not receiving a
‘switch on’ voltage. You can use a multimeter to test this.
If no vacuum exists trace the supply line back to the vacuum source checking for kinks and
breaks.
Testing Thermal Vacuum Switches
There are many vacuum controlled circuits that must only operate when the engine reaches
the normal operating temperature. To do this you use the thermal switches that remain in
an ‘off’ position until a given temperature is reached.
To test this type of switch
1. Remove the vacuum supply line coming from the manifold to the switch and test for
manifold vacuum.
2. If this vacuum is correct refit the supply line to the thermal switch and remove the
opposing from the switch.
3. Attach the vacuum tester to the port (figure 9).
4. Start the engine.
5. With a cold engine, no reading should be noted.
6. When the engine reaches normal operating temperature a manifold vacuum reading
should be noted.
Testing Vacuum Operated Heater Taps
On newer vehicles, climate control ventilation systems are becoming more popular. Most of
these systems use vacuum operated taps to control the heating modes. The system on the
majority of makes and models use vacuum to turn the heater tap ‘on’.
To test these
1. Remove the supply line from the tap vacuum module.
2. Connect the vacuum tester (figure 10).
3. With the engine at normal operating temperature locate and feel the heater return hose.
4. With the heater tap in the ‘off’ position, this hose should be cold.
5. Open the tap by operating the vacuum pump.
6. The gauge reading must hold.
If the return hose begins to heat the tap is in working order. If the tap is faulty the hose
will not begin to heat.