14 GEARS May 2002 I n the last issue of GEARS, we looked at how easily you can con- nect a scope or meter to typical sensors without having a schematic or menu to guide you. In this issue, we’ll look at a few sensors that aren’t quite so obvious. Mass Airflow (MAF) Sensors Mass airflow sensors provide the computer with an indication of engine load. By measuring the amount of air entering the engine, the computer can determine engine load with much greater accuracy than with a typical MAF sensor (figure 1). NOTE: Some GM vehicles have both a mass airflow sensor and a MAP sensor. On those vehicles, the MAP provides a backup for the mass airflow sensor, in case the MAF fails. The problem with testing mass air- flow sensors is there are a number of different ones in use, with different wiring configurations. In addition, some of those sensors have an air charge temperature sensor built in, which adds a couple more wires to the connector. But in general, there are only two different types of mass airflow sensors: frequency sensors and analog sensors. Frequency sensors provide a digi- tal signal that increases in frequency as the flow of air through the engine increases. Analog sensors provide a variable voltage signal, much like a TPS does. There are two types of analog sen- sor: the hot-wire sensor, commonly used on GM vehicles, and the “flapper” design, which is more common on imports. But in each case, the sensor by Vince Virgilio, EAST Training, Inc. and Steve Bodofsky, Steve Bodofsky Productions Figure 1 Common Mass Airflow (MAF) sensors. Left to right: 1990 Mazda B2200 and 1988 Toyota Camry. Figure 2 A waveform from a digital Mass Airflow (MAF) sensor. The frequen- cy of the pulses will increase as engine RPM increases.
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