Hi Dave,
Sorry forgot to answer second part of your post, so here it is.
The manifold absolute pressure (MAP) sensor is a key sensor because it senses engine load. The sensor generates a signal that is proportional to the amount of vacuum in the intake manifold. The engine computer then uses this information to adjust ignition timing and fuel enrichment.
When the engine is working hard, intake vacuum drops as the throttle opens wide. The engine sucks in more air, which requires more fuel to keep the air/fuel ratio in balance. In fact, when the computer reads a heavy load signal from the MAP sensor, it usually makes the fuel mixture go slightly richer than normal so the engine can produce more power. At the same time, the computer will retard (back off) ignition timing slightly to prevent detonation (spark knock) that can damage the engine and hurt performance.
When conditions change and the vehicle is cruising along under light load, coasting or decelerating, less power is needed from the engine. The throttle is not open very wide or may be closed causing intake vacuum to increase. The MAP sensor senses this and the computer responds by leaning out the fuel mixture to reduce fuel consumption and advances ignition timing to squeeze a little more fuel economy out of the engine.
How a MAP sensor works:
MAP sensors are called manifold absolute pressure sensors rather than intake vacuum sensors because they measure the difference in pressure between the outside atmosphere and the vacuum level inside the intake manifold.
Ambient air pressure typically varies from 28 to 31 inches of Mercury (Hg) depending on your location and climate conditions. Higher elevations have lower air pressure than areas next to the ocean or someplace like Death Valley, California, which is actually below sea level. In pounds per square inch, the atmosphere exerts 14.7 PSI at sea level on average.
The vacuum inside an engine's intake manifold, by comparison, can range from zero up to 22 inches Hg or more depending on operating conditions. Vacuum at idle is always high and typically ranges from 16 to 20 inches Hg in most vehicles. The highest level of vacuum occurs when decelerating with the throttle closed. The pistons are trying to suck in air but the closed throttle chokes off the air supply creating a high vacuum inside the intake manifold (typically four to five inches Hg higher than at idle). When the throttle is suddenly opened, as when accelerating hard, the engine sucks in a big gulp of air and vacuum plummets to zero. Vacuum then slowly climbs back up as the throttle closes.
The reason why MAP sensors measure pressure differential rather than vacuum alone is because atmospheric pressure changes with the weather and elevation. Since this affects the balance of the air/fuel mixture, the computer needs a way to detect the changes so it can compensate. Some vehicles use a "baro" sensor to measure barometric pressure (that's meteorologist lingo for atmospheric air pressure) and a vacuum sensor connected to the intake manifold to measure intake vacuum. The computer compares the readings, calculates the difference and makes the necessary fuel mixture and timing adjustments. But it is easier to let the MAP sensor measure the difference. On some vehicles, the MAP sensor is also used to check barometric pressure when the ignition is first switched on. This is done as a sort of baseline calibration check.
On turbocharged and supercharged engines, the situation is a little more complicated because under boost there may actually be positive pressure in the intake manifold. But the MAP sensor does not care because it just monitors the difference in pressure.
On engines with a "speed-density" electronic fuel injection system, airflow is estimated rather than measured directly with an airflow sensor. The computer looks at the MAP sensor signal along with engine rpm, throttle position, coolant temperature and ambient air temperature to estimate how much air is entering the engine. The computer may also take into account the oxygen sensor rich/lean signal and the position of the EGR valve, too, before making the required air/fuel mixture corrections to keep everything in balance. This approach to fuel management is not as precise as systems that use a vane or mass airflow sensor to measure actual airflow, but it is not as complex or as costly either.
Another advantage of speed-density EFI systems is that they are less sensitive to vacuum leaks. Any air that leaks into an engine on the back side an airflow sensor is "un-metered" air and really messes up the fine balance that is needed to maintain an accurate air/fuel mixture. In a speed-density system, the MAP sensor will detect the slight drop in vacuum caused by the air leak and the computer will compensate by adding more fuel.
On many GM engines that have a mass airflow sensor (MAF), a MAP sensor is also used as a backup in case the airflow signal is lost, and to monitor the operation of the EGR valve. No change in the MAP sensor signal when the EGR valve is commanded to open would indicate a problem with the EGR system and set a fault code.
Analog map sensors:
The MAP sensor consists of two chambers separated by a flexible diaphragm. One chamber is the "reference air" (which may be sealed or vented to the outside air), and the other is the vacuum chamber which is connected to the intake manifold on the engine by a rubber hose or direct connection. The MAP sensor may be mounted on the firewall, inner fender or intake manifold.
A pressure sensitive electronic circuit inside the MAP sensor monitors the movement of the diaphragm and generates a voltage signal that changes in proportion to pressure. This produces an analog voltage signal that typically ranges from one to five volts.
Analog MAP sensors have a three-wire connector: ground, a five volt reference signal from the computer and the return signal. The output voltage usually increases when the throttle is opened and vacuum drops. A MAP sensor that reads one or two volts at idle may read 4.5 volts to five volts at wide open throttle. Output generally changes about 0.7 to 1.0 volts for every five inches Hg of change in vacuum.
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Saturday, July 24th, 2010 AT 6:48 AM
