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Suppliment to AUT 126

Suppliment to AUT 126. MAP/BARO Sensors. Barometric manifold absolute pressure (BMAP) sensor Barometric pressure (BARO) sensor Manifold absolute pressure (MAP) sensor. Piezoresistivity Pressure differential Speed density Vacuum. KEY TERMS. AIR PRESSURE—HIGH AND LOW.

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Suppliment to AUT 126

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  1. Suppliment to AUT 126 MAP/BARO Sensors

  2. Barometric manifold absolute pressure (BMAP) sensor Barometric pressure (BARO) sensor Manifold absolute pressure (MAP) sensor Piezoresistivity Pressure differential Speed density Vacuum KEY TERMS

  3. AIR PRESSURE—HIGH AND LOW • Think of an internal combustion engine as a big air pump. • As the pistons move up and down in the cylinders, they pump in air and fuel for combustion and pump out exhaust gases. • They do this by creating a difference in air pressure. • The air outside an engine has weight and exerts pressure, as does the air inside an engine.

  4. AIR PRESSURE—HIGH AND LOW • As a piston moves down on an intake stroke with the intake valve open, it creates a larger area inside the cylinder for the air to fill. • This lowers the air pressure within the engine. • Because the pressure inside the engine is lower than the pressure outside, air flows into the engine to fill the low-pressure area and equalize the pressure. • The low pressure within the engine is called vacuum. • Vacuum causes the higher-pressure air on the outside to flow into the low-pressure area inside the cylinder. • The difference in pressure between the two areas is called a pressure differential.

  5. AIR PRESSURE—HIGH AND LOW • As an engine is accelerated under a load, the engine vacuum drops. • This drop in vacuum is actually an increase in absolute pressure in the intake manifold. • A MAP sensor senses all pressures greater than that of a perfect vacuum. • The relationship between absolute pressure, vacuum and gauge pressure.

  6. PRINCIPLES OF PRESSURE SENSORS • Intake manifold pressure changes with changing throttle positions. • At wide-open throttle, manifold pressure is almost the same as atmospheric pressure. • On deceleration or at idle, manifold pressure is below atmospheric pressure, thus creating a vacuum. • In cases where turbo- or supercharging is used, under part- or full-load condition, intake manifold pressure rises above atmospheric pressure. • Also, oxygen content and barometric pressure change with differences in altitude, and the computer must be able to compensate by making changes in the flow of fuel entering the engine.

  7. PRINCIPLES OF PRESSURE SENSORS • A plastic MAP sensor used for training purposes showing the electronic circuit board and electrical connections.

  8. CONSTRUCTION OF MANIFOLD ABSOLUTE PRESSURE (MAP) SENSORS • The manifold absolute pressure (MAP) sensor is used by the engine computer to sense engine load. • The typical MAP sensor consists of a ceramic or silicon wafer sealed on one side with a perfect vacuum and exposed to intake manifold vacuum on the other side. • As the engine vacuum changes, the pressure difference on the wafer changes the output voltage or frequency of the MAP sensor.

  9. CONSTRUCTION OF MANIFOLD ABSOLUTE PRESSURE (MAP) SENSORS • SILICON-DIAPHRAGM STRAIN GAUGE MAP SENSOR • CAPACITOR-CAPSULE MAP SENSOR • CERAMIC DISC MAP SENSOR

  10. MAP sensors use three wires: • 5-volt reference from the PCM • Sensor signal (output signal) • Ground. • A DMM set to test a MAP sensor. • Connect the red meter lead to the V meter terminal and the black meter lead to the COM meter terminal. • Select DC volts. • Connect the test leads to the sensor signal wire and the ground wire. • Select hertz (Hz) if testing a MAP sensor whose output is a varying frequency; otherwise keep it on DC volts. • Read the change of voltage (frequency) as the vacuum is applied to the sensor. Compare the vacuum reading and the frequency (or voltage) reading to the specifications. CONSTRUCTION OF MANIFOLD ABSOLUTE PRESSURE (MAP) SENSORS

  11. CONSTRUCTION OF MANIFOLD ABSOLUTE PRESSURE (MAP) SENSORS • A waveform of a typical digital MAP sensor.

  12. CONSTRUCTION OF MANIFOLD ABSOLUTE PRESSURE (MAP) SENSORS • Shown is the electronic circuit inside a ceramic disc MAP sensor used on many Chrysler engines. The black areas are carbon resistors that are applied to the ceramic, and lasers are used to cut lines into these resistors during testing to achieve the proper operating calibration.

  13. If It’s Green, It’s a Signal Wire • Ford-built vehicles usually use a green wire as the signal wire back to the computer from the sensors. It may not be a solid green, but if there is green somewhere on the wire, then it is the signal wire. The other wires are the power and ground wires to the sensor.

  14. PCM USES OF THE MAP SENSOR • The PCM uses the MAP sensor to determine the following: • The load on the engine • Altitude, fuel, and spark control calculations • EGR system operation • Detect deceleration (vacuum increases) • Monitor engine condition • Load detection for returnless-type fuel injection • Altitude and MAP sensor values

  15. Altitude affects the MAP sensor voltage. PCM USES OF THE MAP SENSOR

  16. BAROMETRIC PRESSURE SENSOR • A barometric pressure (BARO) sensor is similar in design, but senses more subtle changes in barometric absolute pressure (atmospheric air pressure). • It is vented directly to the atmosphere. • The barometric manifold absolute pressure (BMAP) sensor is actually a combination of a BARO and MAP sensor in the same housing. • The BMAP sensor has individual circuits to measure barometric and manifold pressure. • This input not only allows the computer to adjust for changes in atmospheric pressure due to weather, but also is the primary sensor used to determine altitude.

  17. The Cavalier Convertible Story • The owner of a Cavalier convertible stated to a service technician that the “check engine” (MIL) was on. The technician found a diagnostic trouble code (DTC) for a MAP sensor. • The technician removed the hose at the MAP sensor and discovered that gasoline had accumulated in the sensor and dripped out of the hose as it was being removed. • The technician replaced the MAP sensor and test drove the vehicle to confirm the repair. Almost at once the check engine light came on with the same MAP sensor code. • After several hours of troubleshooting without success in determining the cause, the technician decided to start over again.

  18. The Cavalier Convertible Story • Almost at once, the technician discovered that no vacuum was getting to the MAP sensor where a vacuum gauge was connected with a T-fitting in the vacuum line to the MAP sensor. • The vacuum port in the base of the throttle body was clogged with carbon. • After a thorough cleaning, and clearing the DTC, the Cavalier again performed properly and the check engine light did not come on again. • The technician had assumed that if gasoline was able to reach the sensor through the vacuum hose, surely vacuum could reach the sensor. The technician learned to stop assuming when diagnosing a vehicle and concentrate more on testing the simple things first.

  19. TESTING THE MAP SENSOR • Most pressure sensors operate on 5 volts from the computer and return a signal (voltage or frequency) based on the pressure (vacuum) applied to the sensor. • If a MAP sensor is being tested, make certain that the vacuum hose and hose fittings are sound and making a good, tight connection to a manifold vacuum source on the engine.

  20. TESTING THE MAP SENSOR • Four different types of test instruments can be used to test a pressure sensor: • 1. A digital voltmeter with three test leads connected in series between the sensor and the wiring harness connector or back-probe the terminals. • 2. A scope connected to the sensor output, power, and ground. • 3. A scan tool or a specific tool recommended by the vehicle manufacturer. • 4. A breakout box connected in series between the computer and the wiring harness connection(s).

  21. TESTING THE MAP SENSORTESTING THE MAP SENSOR USING A DMM OR SCOPE • Use jumper wires, T-pins to back-probe the connector, or a breakout box to gain electrical access to the wiring to the pressure sensor. • Most pressure sensors use three wires: • 1. A 5-volt wire from the computer • 2. A variable-signal wire back to the computer • 3. A ground or reference low wire • The procedure for testing the sensor is as follows: • 1. Turn the ignition on (engine off) • 2. Measure the voltage (or frequency) of the sensor output • 3. Using a hand-operated vacuum pump (or other variable vacuum source), apply vacuum to the sensor

  22. Visual Check of the MAP Sensor • A defective vacuum hose to a MAP sensor can cause a variety of driveability problems including poor fuel economy, hesitation, stalling, and rough idle. • A small air leak (vacuum leak) around the hose can cause these symptoms and often set a trouble code in the vehicle computer. When working on a vehicle that uses a MAP sensor, make certain that the vacuum hose travels consistently downward on its route from the sensor to the source of manifold vacuum. Inspect the hose, especially if another technician has previously replaced the factory-original hose. It should not be so long that it sags down at any point. Condensed fuel and/or moisture can become trapped in this low spot in the hose and cause all types of driveability problems and MAP sensor codes.

  23. Visual Check of the MAP Sensor • When checking the MAP sensor, if anything comes out of the sensor itself, it should be replaced. This includes water, gasoline, or any other substance.

  24. TESTING THE MAP SENSORTESTING THE MAP SENSOR USING A SCAN TOOL • A scan tool can be used to test a MAP sensor by monitoring the injector pulse width (in milliseconds) when vacuum is being applied to the MAP sensor using a hand-operated vacuum pump. • STEP 1 Apply about 20 in. Hg of vacuum to the MAP sensor and start the engine. • STEP 2 Observe the injector pulse width. On a warm engine, the injector pulse width will normally be 1.5 to 3.5 ms. • STEP 3 Slowly reduce the vacuum to the MAP sensor and observe the pulse width. A lower vacuum to the MAP sensor indicates a heavier load on the engine and the injector pulse width should increase.

  25. FIGURE 22–7 A typical hand-operated vacuum pump. TESTING THE MAP SENSORTESTING THE MAP SENSOR USING A SCAN TOOL

  26. FUEL-RAIL PRESSURE SENSOR • A fuel-rail pressure (FRP) sensor is used on some vehicles such as Fords that are equipped with electronic returnless fuel injection. • This sensor provides fuel pressure information to the PCM for fuel injection pulse width calculations.

  27. MAP/BARO DIAGNOSTIC TROUBLE CODES • The diagnostic trouble codes (DTCs) associated with the MAP and BARO sensors include:

  28. SUMMARY • Pressure below atmospheric pressure is called vacuum and is measured in inches of mercury. • A manifold absolute pressure sensor uses a perfect vacuum (zero absolute pressure) in the sensor to determine the pressure. • Three types of MAP sensors include: • Silicon-diaphragm strain gauge • Capacitor-capsule design • Ceramic disc design

  29. SUMMARY • A heavy engine load results in low intake manifold vacuum and a high MAP sensor signal voltage. • A light engine load results in high intake manifold vacuum and a low MAP sensor signal voltage. • A MAP sensor is used to detect changes in altitude, as well as check other sensors and engine systems. • A MAP sensor can be tested by visual inspection, testing the output using a digital meter or scan tool.

  30. REVIEW QUESTIONS • What is the relationship among atmospheric pressure, vacuum, and boost pressure in PSI? • What are two types (construction) of MAP sensors? • What is the MAP sensor signal voltage or frequency at idle on a typical General Motors, Chrysler, and Ford engine? • What are three uses of a MAP sensor by the PCM?

  31. CHAPTER QUIZ 1. As the load on an engine increases, the manifold vacuum decreases and the manifold absolute pressure ________. • Increases • Decreases • Changes with barometric pressure only (altitude or weather) • Remains constant (absolute)

  32. CHAPTER QUIZ 2. A typical MAP sensor compares the vacuum in the intake manifold to ________. • Atmospheric pressure • A perfect vacuum • Barometric pressure • The value of the IAT sensor

  33. CHAPTER QUIZ 3. Which statement is false? • Absolute pressure is equal to barometric pressure plus intake manifold vacuum. • A decrease in manifold vacuum means an increase in manifold pressure. • The MAP sensor compares manifold vacuum to a perfect vacuum. • Barometric pressure minus the MAP sensor reading equals intake manifold vacuum.

  34. CHAPTER QUIZ 4. Which design of MAP sensor produces a frequency (digital) output signal? • Silicon-diaphragm strain gauge • Piezoresistivity design • Capacitor-capsule • Ceramic disc

  35. CHAPTER QUIZ 5. The frequency output of a digital MAP sensor is reading 114 Hz. What is the approximate engine vacuum? • Zero • 5 in. Hg • 10 in. Hg • 15 in. Hg

  36. CHAPTER QUIZ 6. Which is not a purpose or function of the MAP sensor? • Measures the load on the engine • Measures engine speed • Calculates fuel delivery based on altitude • Helps diagnose the EGR system

  37. CHAPTER QUIZ 7. When measuring the output signal of a MAP sensor on a General Motors vehicle, the digital multimeter should be set to read ________. • DC V • AC V • Hz • DC A

  38. CHAPTER QUIZ 8. Two technicians are discussing testing MAP sensors. Technician A says that the MAP sensor voltage on a General Motors vehicle at idle should be about 1.0 volt. Technician B says that the MAP sensor frequency on a Ford vehicle at idle should be about 105–108 Hz. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

  39. CHAPTER QUIZ 9. Technician A says that MAP sensors use a 5-volt reference voltage from the PCM. Technician B says that the MAP sensor voltage will be higher at idle at high altitudes compared to when the engine is operating at near sea level. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

  40. CHAPTER QUIZ 10. A P0107 DTC is being discussed. Technician A says that a defective MAP sensor could be the cause. Technician B says that a MAP sensor signal wire shorted-to-ground could be the cause. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

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