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.
Typical MAP sensor outputs for an older GM application.
HOW A MAP SENSOR WORKS
MAP sensors are called manifold absolute pressure sensors rather than intake vacuum sensors because they measure the pressure (or lack thereof) inside the intake manifold. When the engine is not running, the pressure inside the intake manifold is the same as the outside barometric pressure. When the engine starts, vacuum is created inside the manifold by the pumping action of the pistons and the restriction created by the throttle plates. At full open throttle with the engine running, intake vacuum drops to almost zero and pressure inside the intake manifold once again nearly equals the outside barometric pressure.
Barometric 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.
When the ignition key is first turned on, the powertrain control module (PCM) looks at the MAP sensor reading before the engine starts to determine the atmospheric (barometric) pressure. So in effect, the MAP sensor can serve double duty as a BARO sensor. The PCM then uses this information to adjust the air/fuel mixture to compensate for changes in air pressure due to elevation and/or weather. Some vehicles use a separate "baro" sensor for this purpose, while others use a combination sensor that measures both called a BMAP sensor.
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 doesn't care because it just monitors the absolute pressure inside the intake manifold.
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 isn't 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's 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 1 to 5 volts.
Analog MAP sensors have a three-wire connector: ground, a 5-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 1 or 2 volts at idle may read 4.5 volts to 5 volts at wide open throttle. Output generally changes about 0.7 to 1.0 volts for every 5 inches Hg of change in vacuum.
FORD DIGITAL MAP SENSORS
Ford BP/MAP sensors (barometric pressure/manifold absolute pressure) also measure load but produce a digital frequency signal rather than an analog voltage signal. This type of sensor has additional circuitry that creates a 5 volt "square wave" (on-off) voltage signal. The signal increases in frequency as vacuum drops.
At idle or when decelerating, vacuum is high and the BP/MAP sensor output may drop to 100 Hz (Hertz, or cycles per second) or less. At wide open throttle when there is almost no vacuum in the intake manifold, the sensor's output may jump to 150 Hz or higher. At zero vacuum (atmospheric pressure), a Ford BP/MAP sensor should read 159 Hz.
MAP SENSOR DRIVABILITY SYMPTOMS
Anything that interferes with the MAP sensor's ability to monitor the pressure differential may upset the fuel mixture and ignition timing. This includes a problem with the MAP sensor itself, grounds or opens in the sensor wiring circuit, and/or vacuum leaks in the intake manifold (airflow sensor systems) or hose that connects the sensor to the engine.
Typical driveability symptoms that may be MAP related include:
* Rough idle.
* A rich fuel condition, which may cause spark plug fouling.
* Detonation due to too much spark advance and a lean fuel ratio.
* Loss of power and/or fuel economy due to retarded timing and an excessively rich fuel ratio.
A vacuum leak will reduce intake vacuum and cause the MAP sensor to indicate a higher than normal load on the engine. The computer will try to compensate by richening the fuel mixture and retarding timing -- which hurts fuel economy, performance and emissions.
MAP SENSOR CHECKS
First, make sure engine manifold vacuum is within specifications at idle. If vacuum is unusually low due to a vacuum leak, retarded ignition timing, an exhaust restriction (clogged converter), or an EGR leak (EGR valve not closing at idle).
A low intake vacuum reading or excessive backpressure in the exhaust system can trick the MAP sensor into indicating there is a load on the engine. This may result in a rich fuel condition.
A restriction in the air intake (such as a plugged air filter), on the other hand, may produce higher than normal vacuum readings. This would result in a load low indication from the MAP sensor and possibly a lean fuel condition.
A good MAP sensor should read barometric air pressure when the key is turned on before the engine starts. This value can be read on a scan tool and should be compared to the actual barometric pressure reading to see if they match. Your local weather channel or website should be able to tell you the current barometric pressure reading.
Check the sensor's vacuum hose for kinks or leaks. Then use a hand-held vacuum pump to check the sensor itself for leaks. The sensor should hold vacuum. Any leakage calls for replacement.
An outright failure of the MAP sensor, loss of the sensor signal due to a wiring problem, or a sensor signal that is outside the normal voltage or frequency range will usually set a diagnostic trouble code (DTC) and turn on the Check Engine light.
MAP SENSOR SCAN TOOL CHECKS
On 1995 and newer vehicles with OBD II self-diagnostics, a DTC code P0105 to P0109 would indicate a fault in the MAP sensor circuit.
P0105....Manifold Absolute Pressure/Barometric Pressure Circuit
On older pre-OBD II vehicles, the MAP codes are:
* General Motors: Codes 34, 33, 31
* Ford: Codes 22, 72
* Chrysler: Codes 13, 14
On vehicles that provide data stream through a diagnostic connector and allow a scan tool to display sensor values, the MAP sensor's output voltage can be read and compared to specifications. Basically, you want to see a quick and dramatic change in the MAP sensor signal when the throttle on an idling engine is snapped open and shut. No change would indicate a sensor or wiring fault.
If the sensor is reading low or there is no reading at all, check for proper reference voltage to the sensor. It should be very close to 5 volts. Also check the ground connection. If the reference voltage is low, check the wiring harness and connector for looseness, damage or corrosion.
Scan tools that display OBD II data will also display a "calculated load value" that can be used to determine if the MAP sensor is working or not. The load value is computed using inputs from the MAP sensor, TPS sensor, airflow sensor and engine speed. The value should be low at idle, and high when the engine is under load. No change in the value, or a higher than normal reading at idle might indicate a problem with the MAP sensor, TPS sensor or airflow sensor.
If you display the MAP sensor output on a Digital Storage Oscilloscope (DSO), this is
what the waveform might look like as the throttle position, engine load and speed change.
MAP SENSOR TESTS
A MAP sensor can also be bench tested by applying vacuum to the vacuum port with a hand vacuum pump. With 5 volts to the reference wire, the output voltage of an analog MAP sensor should drop, and on a Ford digital MAP sensor the frequency should increase.
An analog MAP sensor's voltage can also be read directly using a voltmeter or oscilloscope. A digital MAP sensor's frequency signal can be read with a DVOM if it has a frequency function, or an oscilloscope. The leads would be connected to the signal wire and ground.
Warning: Do NOT use an ordinary voltmeter to check a Ford BP/MAP sensor because doing so can damage the electronics inside the sensor. This type of sensor can only be diagnosed with a DVOM that displays frequency, or a scope or scan tool.
Another way to check out a Ford digital MAP sensor circuit is to input a "simulated" MAP sensor signal with a tester that can generate an adjustable frequency signal. Changing the frequency of the simulated signal should trick the computer into changing the fuel mixture (look for a change in the injector pulse width signal).
No change would indicate a possible computer problem.
MAP SENSOR REPLACEMENT
If a MAP sensor needs to be replaced, make sure the replacement is the correct one for the application. Differences in calibration between model years and engines will affect the operation of the engine management system.
If a vehicle is more than five years old, the vacuum hose that connects the MAP sensor to the engine should also be replaced.
More Engine Sensor Articles:Making Sense of Engine Sensors
Air Temperature Sensors
Crankshaft Position CKP Sensors
Mass Airflow MAF Sensors
Vane Airflow VAF Sensors
Throttle Position Sensors
Wide Ratio Air Fuel (WRAF) Sensors
Understanding Engine Management Systems
Powertrain control modules (PCMs)
Flash Reprogramming PCMs
All About Onboard Diagnostics II (OBD II)
Zeroing in on OBD II Diagnostics
Controller Area Network (CAN) Diagnostics
OEM Automotive Service Information Websites & Access Fees
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