Map Sensor
The MAP serves as a PCM input, using a silicon based sensing unit, to provide data on manifold vacuum that draws air/fuel mixture into combustion chamber. The PCM requires this information to determine injector pulse width and spark advance. When MAP equals Barometric pressure, pulse width will be at maximum.
Also like cam and crank sensors, a 5 volt reference is supplied from PCM and returns a voltage signal to PCM that reflects manifold pressure. The zero pressure reading is 0.5V and full scale is 4.5V. For a pressure swing of 0-15 psi voltage changes 4.0V. The sensor is supplied a regulated 4.8-5.1 volts to operate sensor. Like cam and crank sensors ground is provided through sensor return circuit.
The MAP sensor input is number one contributor to pulse width. The most important function of MAP sensor is to determine barometric pressure. The PCM needs to know if vehicle is at sea level or is it in Denver at 5000 feet above sea level, because air density changes with altitude. It will also help to correct for varying weather conditions. If a hurricane was coming through pressure would be very, very low or there could be a real fair weather, high pressure area. This is important because as air pressure changes barometric pressure changes. Barometric pressure and altitude have a direct inverse correlation, as altitude goes up barometric goes down. The first thing that happens as key is rolled on, before reaching crank position, PCM powers up, comes around and looks at MAP voltage, and based upon voltage it sees, it knows current barometric pressure relative to altitude. Once engine starts, PCM looks at voltage again, continuously every 12 milliseconds, and compares current voltage to what it was at key on. The difference between current and what it was at key on is manifold vacuum.
During key On (engine not running) sensor reads (updates) barometric pressure. A normal range can be obtained by monitoring known good sensor in you work area.
As altitude increases air becomes thinner (less oxygen). If a vehicle is started and driven to a very different altitude than where it was at key On barometric pressure needs to be updated. Any time PCM sees Wide Open throttle, based upon TPS angle and RPM it will update barometric pressure in MAP memory cell. With periodic updates, PCM can make its calculations more effectively.
The PCM uses MAP sensor to aid in calculating the following:
- Barometric Pressure
- Engine Load
- Manifold Pressure
- Injector Pulse-Width
- Spark-Advance Programs
- Shift-Point Strategies (F4AC1 Transmissions Only, Via The CCD Bus)
- Idle Speed
- Decel Fuel Shutoff
The MAP sensor signal is provided from a single piezoresistive element located in center of a diaphragm. The element and diaphragm are both made of silicone. As pressures changes diaphragm moves causing element to deflect which stresses silicone. When silicone is exposed to stress its resistance changes. As manifold vacuum increases, MAP sensor input voltage decreases proportionally. The sensor also contains electronics that condition signal and provide temperature compensation.
The PCM recognizes a decrease in manifold pressure by monitoring a decrease in voltage from reading stored in barometric pressure memory cell. The MAP sensor is a linear sensor; as pressure changes, voltage changes proportionately. The range of voltage output from sensor is usually between 4.6 volts at sea level to as low as .3 volts at 26" of Hg. Barometric pressure is pressure exerted by atmosphere upon an object. At sea level on a standard day, no storm, barometric pressure is 29.92 in Hg. For every 100 feet of altitude barometric pressure drops .10" Hg. If a storm goes through it can either add, high pressure, or decrease, O2 SENSOR low pressure, from what should be present for that altitude. You should make a habit of knowing what average pressure and corresponding barometric pressure is for your area. Always use Diagnostic Test Procedures Manual for MAP sensor testing.