Pictured above is Oscium’s iMSO-104 handheld oscilloscope measuring the waveform of a Mass Air Flow (MAF) sensor for General Motors Vortec V8 engine. In modern post 1990’s vehicles, MAF sensors monitor the flow of air through the engine’s intake and provide real time information to the vehicle’s computer for adjusting fuel/air mixture.
Air Flow Represented as an Analog Waveform
There are two commonly used MAF sensor topologies in the automobiles we drive. The first and probably most common topology, called the “Hot Wire” sensor, uses a calibrated, temperature controlled heating element to convert the flow of air into an analog voltage.
The second MAF sensor topology, called the “Cold Wire” sensor, is based on the fundamental operating theory of LC tank circuit oscillators. The fine mesh in the sensor changes inductance as air moves across it. The resonant frequency for the circuit changes with the changing inductance. The vehicle’s computer monitors the waveform at the output of the MAF sensor. In the figure below, the waveform at idle is compared to the waveform produced when the engine speed is increased slightly. As the engine speed increases, the pulse width decreases and the frequency increases. It’s possible that the vehicle’s computer uses an edge-counting algorithm to measure the frequency, and thereby determine the flow of air through the sensor.
Connecting to the Sensor for Making Measurements
Connecting to the MAF sensor to measure the output waveform is relatively easy. In this case, any point of the vehicle chassis that is tied electrically to ground will suffice for the voltage reference. The trickiest part of making this kind of measurement is getting at the signal conductor without disabling the sensor, or without permanently damaging the vehicle’s wiring.
In the figure below you can see where the portable oscilloscope probe is clipped to a small exposed segment of the signal wire (yellow), and the reference is taken from a nearby stud welded to the vehicle’s frame.
Test Sensors Before Replacing Them
Anyone that’s ever taken an older car or truck through vehicle emissions tests know about the sensors that can frequently cause test failures and the check engine light to light up the dash. Oscium’s handheld oscilloscope provides the mechanically inclined do-it-yourselfer a way to test a sensor’s function before shelling out the big bucks to the neighborhood mechanic. Check out the video and see an iPad be transformed into an automotive oscilloscope.MAF_autoshop101.pdf