Chart C-1E Vacuum Sensor Check

- Chart C-1E Differential Pressure (VAC) Sensor Check.:

- Wiring Diagram for Chart C-1E Vacuum Sensor Check.:

CHART DESCRIPTION
1. This checks the output of the sensor. The VAC sensor is a differential pressure sensor that measures the difference in pressure between atmosphere and manifold. The voltage output is opposite the MAP sensor in that it supplies high voltage at high vacuum. Normal reading with engine stopped, key "ON" is less than 1 volt, while at idle should be above 3 volts. High voltage increases spark advance, while low voltage reduces spark advance.
2. This checks the rate of change of the output with vacuum. Again, the normal reading would be about in the middle of the range indicated. Also, the voltage should change as soon as the vacuum changes. If it doesn't, it could result in detonation or a sag on acceleration. Rapid voltage changes can only be measured with a DVM since a "Scan" tool has a time delay. Detonation or a sag could also be caused by poor fuel or a restriction in the hose to the sensor.