Auto Service Professional

APR 2016

Magazine for the auto service professional

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18 | April 2016 before the throttle and supplied with a constant voltage. The sensor's electronics control the current fowing through the wire to heat it to a specifc temperature. Air fowing past the wire carries away heat, so the current must be increased to maintain its temperature. The amount of current fowing through the wire has a direct cor- relation to the mass of air fowing past the wire: More current equals higher airfow. Depending on the sensor design, the signal sent to the powertrain control module (PCM) is either analog or digital, so the sensor is usually called analog or digital. A typical analog sensor has two elements that measure temperature, a "cold" wire that measures the intake air temperature, and a "hot" wire mounted next to it that is maintained at a specifc temperature above the cold wire. Electronics inside the sensor reads both temperatures, controls the level of current fowing through the hot wire and produces a 0 – 5-volt output signal that's proportional to the current applied to the hot wire. The output signal is typically about 0.7 volts at idle and about 4.5 volts at wide-open throttle. Digital sensors work basically the same way, but instead of reporting the current fow as a voltage signal, the sensor reports the duty cycle of the voltage fowing through the hot wire (that's how current is controlled). On MAF sensors from the 1980s and early 1990s, the duty cycle generally tops out at about 160 Hz, but on more modern sensors the duty cycle is in the kilo-hertz range, generally topping out at about 8,500 kHz. On many models, the MAF sensor also reports intake air temperature (IAT). On some the PCM simply uses the signal from the "cold" wire for IAT. Other MAF sensors include a separate (non-replaceable) IAT sensor as part of the same assembly. That IAT sensor has its own reference voltage and signal return wires, so these MAF sen- sors have at least fve wires. Regardless of whether the MAF signal is digital or analog, a scan tool will report its reading as "grams per second" (g/s) because Powerplant Why is it called 'mass' airfow sensor? It's called a "mass" airfow sensor because the PCM needs to know the density of the air fowing into the engine, not just the volume. A liter of air at 90 degrees is less dense (the molecules are farther apart) than it would be at 40 degrees, so there is less oxygen in that liter of hot air. This hot-wire measuring technique automati- cally accounts for variations in temperature and humidity that affect the air's density. How much does that variation matter? On racing engines that use carburetors, the crew chief chooses the main jets after hearing the race-day weather forecast. A cool, moist day means the air is "fat" with oxygen, so bigger jets can be used. Ever notice how a fuel-injected engine seems to make a bit more power and uses a little more gas in cold weather? This MAF sensor is mounted in the top of the air flter housing. Notice the IAT sensor on the right; it's part of the MAF assembly but it is not used for measuring airfow.

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