1994 Pontiac Firebird 33 map sensor

Checking The Manifold Absolute Pressure (MAP) Sensor

Purpose:
The Manifold Absolute Pressure (MAP) sensor is used to monitor intake manifold pressure (engine load). It sends voltage signals to the Powertrain Control Module (PCM) that represent the engines varying load conditions.

Theory/Operation:
PCM supplies 5 volt sensor reference voltage. The sensor, connected to manifold vacuum at throttle body, converts intake manifold pressure into voltage.

A silicon crystal in the MAP sensor senses changes in manifold absolute pressure. This crystal changes the resistance of the sensor depending upon the manifold absolute pressure acting upon it, and the change in resistance affects the amount of voltage that the sensor allows to flow back to the PCM.

Manifold absolute pressure and voltage to PCM are directly proportional (manifold absolute pressure increases, low vacuum, voltage to PCM increases and vice versa).

Sensor resistance and manifold absolute pressure are inversely proportional (as manifold absolute pressure increases, (low vacuum), sensor resistance decreases and vice versa).

MAP Sensor (Typical)

Typical Readings:
Sensor output voltage range is 0.5 to 4.5 volts.

Output voltages between 0.5 and 1.5 volts indicate a high vacuum (low pressure) situation, such as idle or deceleration.

Output voltages between 1.5 and 3.0 volts indicate a medium level of vacuum (pressure) such as a cruise or slight acceleration condition.

Output voltages between 3.0 and 4.5 volts indicate a low vacuum (high pressure) situation such as hard acceleration or a mechanical failure.

Any reading of 0 volts or over 4.5 volts indicates a problem.

NOTE: The following procedure tests the MAP sensor only.

Inspect the rubber nipple (fitting) from the MAP sensor to the throttle body. Repair as necessary. CAUTION: When testing the MAP sensor, be sure that the harness wires are not damaged by the test meter probes.
Test the MAP sensor output voltage at the MAP sensor connector terminal B. With the ignition switch ON, and the engine OFF. Output voltage should be 4 to 5 volts.
Test the MAP sensor output voltage at the MAP sensor connector terminal B at a hot, neutral idle speed condition. The voltage should drop to 1.5 to 2.1 volts.
Test MAP sensor supply voltage at sensor connector terminal C with the ignition ON. The voltage should be approximately 5 volts (±O.5 V).