Regardless of whether the MAF signal is digital or analog, a scan tool will report its reading as “grams per second” (g/s) because that’s the data used for fuel control. That IAT sensor has its own reference voltage and signal return wires, so these MAF sensors have at least five wires. Other MAF sensors include a separate (non-replaceable) IAT sensor as part of the same assembly. On some the PCM simply uses the signal from the “cold” wire for IAT. On many models, the MAF sensor also reports intake air temperature (IAT). 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. The output signal is typically about 0.7 volts at idle and about 4.5 volts at wide-open throttle.ĭigital sensors work basically the same way, but instead of reporting the current flow as a voltage signal, the sensor reports the duty cycle of the voltage flowing through the hot wire (that’s how current is controlled). Electronics inside the sensor reads both temperatures, controls the level of current flowing through the hot wire and produces a 0 – 5-volt output signal that’s proportional to the current applied to the hot wire. The amount of current flowing through the wire has a direct correlation to the mass of air flowing past the wire: More current equals higher airflow.ĭepending 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.Ī 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 specific temperature above the cold wire. Air flowing past the wire carries away heat, so the current must be increased to maintain its temperature.
The sensor’s electronics control the current flowing through the wire to heat it to a specific temperature. A resistance-calibrated wire is mounted in the engine’s intake airflow before the throttle and supplied with a constant voltage. So the question is, if this really is a bad MAF sensor, how can it have such a dramatic affect on fuel delivery and engine performance without setting a code?īefore exploring that question, let’s review how a MAF sensor works.Īlthough there are some differences, electronic MAF sensors all work on the same principle. That could be caused by low fuel pressure, but the engine continues running smoothly even at WOT. The clue is in the fuel trim numbers and the lack of power at WOT.įuel trim is negative at idle, the opposite of what you would see with a vacuum leak, and it changes to positive at higher speed/load but the oxygen sensor readings still indicate it’s running lean.
It’s tempting to think these symptoms indicate a vacuum leak or low fuel pressure, but this is an article about mass air flow sensors (MAF), so at this point you already know that’s the problem to be considered. Both fuel trims are now positive, but the O2 sensor shows a lean air/fuel ratio. After a few minutes at steady cruise, the engine seems smoother but throttle response is still lacking and power is definitely down, especially at wide-open throttle (WOT), almost like the engine has a governor. There’s a hesitation and not much power off the line, so it certainly feels like it’s not getting enough fuel. The idle is a bit rough long-term fuel trim is way negative but short-term is making up for it and the oxygen sensor trace looks normal.
OK, there’s nothing obvious, so now it’s time for the scan tool and a test drive. Once you get snookered by something as simple as a split vacuum hose or a loose ground wire, you’ll start every diagnosis with a quick visual/touch inspection. Most techs will grab a scan tool right away, but some of us older guys are more likely to start by looking under the hood for something obvious, like a broken wire or severe neglect or (aha!) signs of a recent repair. It’s tough enough when the trouble codes indicate something vague like “random misfire” or “system too lean,” but it can be even worse when there are no codes at all, with only a driver’s complaint about stalling or sluggish performance. Sometimes the hardest part of chasing driveability problems is knowing where to start. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.
#2012 nissan altima mass air flow sensor problems manuals
His began his writing career writing service manuals at Chilton Book Co. Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer.
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