The manifold absolute pressure (MAP) sensor is used by the powertrain control module (PCM) to monitor intake manifold pressure. Since pressure is inversely proportional to vacuum, the PCM can also determine engine vacuum and load from the MAP sensor signal. In some cases, the MAP sensor is also used to determine barometric pressure. Depending on the application, the PCM will use this information to help calculate one or more of the following: fuel control, ignition timing, and EGR operation.
How a MAP Sensor Works
Typically, there are three wires connected to the MAP sensor: reference voltage, return signal, and ground. The PCM sends the MAP sensor a 5-volt reference signal. The MAP sensor then varies the voltage according to manifold pressure and sends a return signal to the PCM.
To sense manifold pressure, the MAP sensor is connected to the engine via a vacuum hose or tube. Inside the MAP sensor are two chambers, one that contains ambient air and another that is connected to engine vacuum. The chambers are separated by a diaphragm. A circuit inside the sensor measures diaphragm movement to determine manifold pressure.
When the engine is off, manifold pressure is the same as atmospheric pressure. When the engine is running, a vacuum is created in the intake manifold by the pistons moving up and down and the restriction of the throttle plate. At idle, manifold vacuum is high (although it’s the highest when decelerating with the throttle closed). As engine load increases, the throttle opening also increases and manifold vacuum drops.
There are two basic types of MAP sensors: analog and digital. With an analog sensor, return signal voltage increases as the throttle opens and vacuum drops. In other words, voltage and pressure are proportional. Sensor voltage typically varies from 1 volt at idle to 5 volts during wide open throttle. An analog sensor produces a traditional DC waveform pattern.
On the other hand, digital sensors produce an on/off signal that can be viewed as a square waveform pattern. The frequency of the signal decreases as the throttle opens and vacuum drops. At idle, the MAP sensor output may be only 100 Hertz, whereas, at wide open throttle, it may be as high as 150 Hertz.
Symptoms of a faulty MAP sensor
Symptoms of a faulty MAP sensor can include an illuminated check engine light, poor engine performance, and reduced fuel economy. These symptoms can also result from anything that prevents the MAP sensor from performing correctly. This can include wiring problems, intake manifold vacuum leaks, a leaking MAP sensor vacuum hos,e or even a faulty PCM.
MAP sensor testing
The MAP sensor is usually found in the engine compartment mounted to the intake manifold or firewall. The analog MAP sensor can be tested using a digital multimeter (DMM). To do this, turn the ignition on and set the meter to the DC volts setting. Connect the positive meter lead to the MAP sensor return signal terminal using a back-probe test lead. Connect the other meter lead to ground. With the ignition on, the MAP sensor output should be around 5 volts. Start the engine and check the reading; at idle, the MAP sensor should read 1 to 2 volts. This indicates the MAP sensor is responding to changes in vacuum.
Digital MAP sensors are best tested with an oscilloscope. However, they can also be tested with a tachometer, which is a type of frequency counter. As was stated earlier, a properly operating digital MAP sensor should produce a signal that increases in frequency as manifold pressure goes up and vacuum goes down. Most MAP sensors can also be tested by viewing the voltage output on a hand-held scan tool.
MAP sensor replacement
In most cases, MAP sensor replacement is straightforward. Simply disconnect the vacuum hose from the MAP sensor, then remove the retaining screws. Reinstall the new sensor in the reverse order of removal.
Now you know a little more about MAP sensors and their fundamental role in the electronic engine control system. Hopefully, you’ll be better prepared should your vehicle experience MAP issues.