Manifold Pressure/Vacuum Sensor: Testing and Inspection

Manifold Absolute Pressure Output Check:

Wiring Diagram For Code 33 - MAP Sensor Circuit (Signal Voltage High - Low Vacuum):

Circuit Description:
The Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure which result from engine load (intake manifold vacuum) and rpm changes; and converts these into a voltage output. The ECM sends a 5 volt reference voltage to the MAP sensor. As the manifold pressure changes, the output voltage of the sensor also changes. By monitoring the sensor output voltage, the ECM knows the manifold pressure. At lower pressure output voltage will be about 1 to 2 volts at idle. While at higher pressure or Wide Open Throttle (WOT) the output voltage will be about 4 to 4.8 volts. The MAP sensor is also used, under certain conditions, to measure barometric pressure, allowing the ECM to make adjustments for different altitudes. The ECM uses the MAP sensor to control fuel delivery and ignition timing.

Test Description:
Numbers below refer to circled numbers on the diagnostic chart.

Important
^ Be sure to use the same diagnostic test equipment for all measurements.

1. Checks MAP sensor output voltage to the ECM. This voltage, without engine running, represents a barometer reading to the ECM.

^ When comparing Tech 1 "Scan" readings to a known good vehicle, it is important to compare vehicles that use a MAP sensor having the same color insert or having the same "Hot Stamped" number.

2. Applying 34 kPa (10" Hg) vacuum to the MAP sensor should cause the voltage to change. Subtract second reading from the first. Voltage value should be greater than 1.5 volts. Upon applying vacuum to the sensor, the change in voltage should be instantaneous. A slow voltage change indicated a faulty sensor.

3. Check vacuum hose to sensor for leaking or restriction. Be sure that no other vacuum devices are connected to the MAP hose.

NOTE: Make sure electrical connector remains securely fastened.

4. Disconnect sensor from bracket and twist sensor by hand (only) to check for intermittent connection. Output changes greater than .1 volt indicates a bad connector or connection. If OK replace sensor.