Halderman ch075 lecture

Objectives The student should be able to: Prepare for ASE Engine Performance (A8) certification test content area “E” (Computerized Engine Controls Diagnosis and Repair). Discuss how MAF sensors work. List the methods that can be used to test MAF sensors.

Objectives The student should be able to: Describe the symptoms of a failed MAF sensor. List how the operation of the MAF sensor affects vehicle operation. Discuss MAF sensor rationality tests.

Airflow Sensors Electronic fuel injection systems that do not use the speed intensity system for fuel calculation measure the airflow volume delivered to engine

Airflow Sensors Older systems use movable vane in intake stream called vane airflow (VAF) sensor Vane is deflected by intake airflow

Figure 75-1 A vane air flow (VAF) sensor.

Airflow Sensors Vane airflow sensor is movable vane connected to laser-calibrated potentiometer Vane is mounted on pivot pin and is deflected by intake airflow proportionate to air velocity

Airflow Sensors As vane moves, it moves the potentiometer This causes change in signal voltage reported to computer Potentiometer provides information computer needs to vary injector pulse width proportionate to airflow

Figure 75-2 A typical air vane sensor with the cover removed. The movable arm contacts a carbon resistance path as the vane opens. Many air vane sensors also have contacts that close to supply voltage to the electric fuel pump as the air vane starts to open when the engine is being cranked and air is being drawn into the engine.

Airflow Sensors A special dampening chamber is built into VAF to smooth out vane pulsations Many vane airflow sensors include a switch to energize the electric fuel pump

MASS Airflow Sensor Types Most newer fuel injection systems use a Mass Air Flow (MAF) sensor Calculates amount of air volume delivered to engine Several types of MAF sensors

MASS Airflow Sensor Types Hot Film Sensor Uses temperature sensing resistor (thermistor) to measure temperature of incoming air Through electronics in sensor, conductive film is kept at 70°C above temperature of incoming air

Figure 75-3 This five-wire mass air flow sensor consists of a metal foil sensing unit, an intake air temperature (IAT) sensor, and the electronic module.

MASS Airflow Sensor Types Amount and density of air contribute to cooling effect as air passes through sensor MASS airflow sensor measures the mass, not the volume, of air entering engine Output of this type sensor is a frequency based on amount of air entering sensor

MASS Airflow Sensor Types The more air that enters the sensor, the more the hot film is cooled Electronics in sensor increase the current flow through the hot film to maintain the 70°C temperature differential between air temperature and temperature of hot film

MASS Airflow Sensor Types The change in current flow is converted to frequency output that computer uses to measure airflow Most of these types of sensors are referred to as mass airflow (MAF) sensors MAF sensor takes into account relative humidity, altitude, and air temperature

MASS Airflow Sensor Types Hot Wire Sensor Hot wire sensor is similar to hot film type Hot wire sensor uses hot wire to sense mass airflow Hot wire sensor uses temperature-sensing resistor to measure temperature of air entering sensor

Figure 75-4 The sensing wire in a typical hot wire mass air flow sensor.

Mass Airflow Sensor Types Electronic circuitry in sensor keeps temperature of wire 70°C above temperature of incoming air Hot wire and hot film sensors operate much the same

Mass Airflow Sensor Types Resistor wire or screen in path of intake airflow is heated to constant temperature Air flowing past the screen or wire cools it Degree of cooling varies with air velocity, temperature, density, and humidity

Mass Airflow Sensor Types These factors indicate the mass of air entering engine As screen or wire cools, more current is required to maintain temperature

Mass Airflow Sensor Types As screen or wire heats up, less current is required More intake air volume = cooler sensor, more current Less intake air volume = warmer sensor, less current

Mass Airflow Sensor Types Some hot wire-type MAF sensors use burn-off circuit to keep sensing wire clean High current is passed through wire Wire glows due to heat

Mass Airflow Sensor Types Some hot wire-type MAF sensors use burn-off circuit to keep sensing wire clean Burn-off is turned on when ignition switch is turned off after engine has been operating at normal operating temperature ?

Karman Vortex Sensors This type of sensor sends sound wave through the turbulence created by incoming air Air mass is calculated based on time required for sound waves to cross turbulent air Two designs for Karman Vortex air flow sensors

Karman Vortex Sensors Ultrasonic Uses ultrasonic waves to detect vortexes Sensor produces digital signal where frequency is proportional to amount of air passing through sensor

Figure 75-5 A Karman Vortex air flow sensor uses a triangle-shaped rod to create vortexes as the air flows through the sensor. The electronics in the sensor itself converts these vortexes to a digital square wave signal.

Karman Vortex Sensors Pressure-Type Uses pressure sensor to detect vortexes As airflow through sensor increases, so do number of pressure variations

Karman Vortex Sensors Pressure-Type Electronics in sensor convert these variations to square wave signal where frequency is proportional to airflow

PCM Uses for Airflow Sensors PCM uses information from airflow sensor for these purposes: Airflow sensor used to determine amount of fuel need and base pulse-width numbers

PCM Uses for Airflow Sensors PCM uses information from airflow sensor for these purposes: Airflow sensors back up the TP sensor in the event of a loss of signal or an inaccurate throttle position sensor signal

Testing Mass Airflow Sensors Visual Inspection Look at all hoses that direct and send air between MAF sensor and throttle body

Testing Mass Airflow Sensors Visual Inspection Also check electrical connectors for Corrosion Terminals that are bent or pushed out of the plastic connector Frayed wiring

Testing Mass Airflow Sensors MAF Sensor Output Test MAF sensors calculate air mass by weight in a given amount of time A digital multimeter, set to read DC volts on the signal wire, can be used to check the MAF sensor

Testing Mass Airflow Sensors MAF Sensor Output Test See the chart for voltage output compared to grams per second of airflow Normal airflow is 3 to 7 grams per second ? ?

Testing MASS Airflow Sensors Tap Test With engine idling, gently tap MAF sensor with fingers If engine stumbles or stalls, MAF sensor is defective

Testing MASS Airflow Sensors Digital Meter Test of a MAF Sensor Use digital multimeter to measure frequency (Hz) output of sensor and compare with specifications

Testing MASS Airflow Sensors Digital Meter Test of a MAF Sensor Frequency output and engine speed in RPM can also be plotted on graph to check proportion of frequency to RPM Graph should show straight line

MAF Sensor Contamination Dirt, oil, silicon, and other debris can coat sensing wire Debris insulates wire At engine idle, contaminated sensor can overestimate amount of air entering engine

MAF Sensor Contamination At higher speeds near WOT, contamination can cause sensor to underestimate the amount of air entering engine To check for contamination, check fuel trim numbers

MAF Sensor Contamination Other tests for contaminated MAF sensor: At WOT, grams per second on scan tool should exceed 100 At WOT, voltage on digital voltmeter should exceed 4 volts for analog sensor

MAF Sensor Contamination Other tests for contaminated MAF sensor: At WOT, frequency on meter or scan tool should exceed 7 kHz for digital sensor

MAF-RELATED DIAGNOSTIC TROUBLE CODES

MAF-Related Diagnostic Trouble Codes

FREQUENTLY ASKED QUESTION What Is the Difference Between an Analog and a Digital MAF Sensor? Some MAF sensors produce a digital DC voltage signal whose frequency changes with the amount of airflow through the sensor. ? BACK TO PRESENTATION The frequency range also varies with the make of sensor and can range from 0- to 300-Hz for older General Motors MAF sensors to 1,000- to 9,000-Hz for most newer designs. Some MAF sensors, such as those used by Ford and others, produce a changing DC voltage, rather than frequency, and range from 0- to 5-volts DC.

REAL WORLD FIX The Dirty MAF Sensor Story The owner of a Buick Park Avenue equipped with a 3800 V-6 engine complained that the engine would hesitate during acceleration, showed lack of power, and seemed to surge or miss at times. A visual inspection found everything to be like new, including a new air filter. There were no stored diagnostic trouble codes (DTCs). BACK TO PRESENTATION A look at the scan data showed airflow to be within the recommended 3 to 7 grams per second. A check of the frequency output showed the problem. Idle frequency = 2.177 kHz (2,177 Hz) Normal frequency at idle speed should be 2.37 to 2.52 kHz. Cleaning the hot wire of the MAF sensor restored proper operation. The sensor wire was covered with what looked like fine fibers, possibly from the replacement air filter. NOTE: Older GM MAF sensors operated at a lower frequency of 32 to 150 Hz, with 32 Hz being the average reading at idle and 150 Hz for wide-open throttle.

FREQUENTLY ASKED QUESTION What Is Meant By a “High-Authority Sensor”? A high-authority sensor is a sensor that has a major influence over the amount of fuel being delivered to the engine. For example, at engine start-up, the engine coolant temperature (ECT) sensor is a high-authority sensor and the oxygen sensor (O2S) is a low-authority sensor. ? BACK TO PRESENTATION However, as the engine reaches operating temperature, the oxygen sensor becomes a high-authority sensor and can greatly affect the amount of fuel being supplied to the engine. See the chart.

FREQUENTLY ASKED QUESTION What Is False Air? Airflow sensors and mass airflow (MAF) sensors are designed to measure all the air entering the engine. If an air hose between the MAF sensor and throttle body was loose or had a hole, extra air could enter the engine without being measured. This extra air is often called false air. ? Because this extra air is unmeasured, the computer does not provide enough fuel delivery and the engine operates too lean, especially at idle. A small hole in the air inlet hose would represent a fairly large percentage of false air at idle, but would represent a very small percentage of extra air at highway speeds. To diagnose for false air, look at long-term fuel trim numbers at idle and at 3000 RPM. NOTE: If the engine runs well in reverse, yet runs terrible in any forward gear, carefully look at the inlet hose for air leaks that would open when the engine torque moves the engine slightly on its mounts. BACK TO PRESENTATION Figure 75-6 Carefully check the hose between the MAF sensor and the throttle body assembly for cracks or splits that could create extra (false) air into the engine that is not measured by the MAF sensor.

TECH TIP The Unplug It Test If a sensor is defective yet still produces a signal to the computer, the computer will often accept the reading and make the required changes in fuel delivery and spark advance. If, however, the sensor is not reading correctly, the computer will process this wrong information and perform an action assuming that information being supplied is accurate. BACK TO PRESENTATION For example, if a mass airflow (MAF) sensor is telling the computer that 12 grams of air per second is going into the engine, the computer will then pulse the injector for 6.4 ms or whatever figure it is programmed to provide. However, if the air going into the engine is actually 14 grams per second, the amount of fuel supplied by the injectors will not be enough to provide proper engine operation. If the MAF sensor is unplugged, the computer knows that the sensor is not capable of supplying airflow information, so it defaults to a fixed amount of fuel based on the values of other sensors such as the TP and MAP sensors. “If in doubt, take it out.” If the engine operates better with a sensor unplugged, then suspect that the sensor is defective. A sensor that is not supplying the correct information is said to be skewed. The computer will not set a diagnostic trouble code for this condition because the computer can often not detect that the sensor is supplying wrong information.

REAL WORLD FIX The Rich Running Toyota A Toyota failed an enhanced emission test for excessive carbon monoxide, which is caused by a rich (too much fuel) air–fuel ratio problem. After checking all of the basics and not finding any fault in the fuel system, the technician checked the archives of the International Automotive Technicians Network ( www.iatn.net ) and discovered that a broken spring inside the air flow sensor was a possible cause. The sensor was checked and a broken vane return spring was discovered. Replacing the air flow sensor restored the engine to proper operating conditions and it passed the emission test. BACK TO PRESENTATION

Halderman ch075 lecture

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