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1968 -- Lyndon Johnson was still President, the Viet Nam War was in painful escalation, NASA was counting down to manned moon missions, ecology was becoming a big issue, and the first popular electronic fuel injection system appeared on the Volkswagen Type III fastback/squareback. That original Robert Bosch D-Jetronic EFI used a vacuum sensor (the "D" is for "Druck," which means "pressure" in German) to inform the electronics about the intake situation, but in '74 the air flow meter showed up on L-Jetronic (the L stands for "Luftmengenmessung" auf Deutsch, meaning "air flow management"), also known as AFC (Air Flow Controlled) fuel injection. At the time, we thought the concept was ingenious: A vane rotated against spring pressure according to how much of the atmosphere the engine was ingesting, and this movement turned the shaft of a variable resistor, thus changing the reference signal before it returns to the computer. Simple, and much more accurate than anything else ever conceived. The jump to direct mass measurement occurred in 1984 when LH-Jetronic with its hot-wire sensor was introduced (to continue our foreign language lesson, the "H" stands for "Heisz," German for "hot"). Then came the GM hot film MAF (Mass Air Flow) sensor and other configurations from such outfits as Hitachi and Mitsubishi that may be of the wire, film, or Karman-Vortex variety. *Air sales We've used a retail analogy before to help illustrate this evolution, but it bears repeating. Suppose you were in the business of selling air. You could calculate the amount delivered by using a complicated formula that takes pressure, temperature, and the diameter of the valve into account, but that would be slow, unwieldy, and probably not very accurate. You'd improve the efficiency of your operation considerably if you had a way of directly measuring the volume flowing into your customers' air canisters in cubic feet per minute. That would sure speed things up, but it wouldn't be perfect, either. On cold days, they'd be getting a big bargain because any gas is denser when it's cold than when it's hot -- in essence, you'd be giving them a baker's dozen and then some. Sales would go way up (and profits way down) in winter as soon as your patrons realized that fact of physical reality. And they'd always try to buy at sea level in dry weather because altitude and humidity affect density, too. To keep from losing your shirt, you'd still have to use a formula that varied the price per cubic foot according to temperature, altitude, and moisture content. Wouldn't it be a lot easier, faster, and more precise if you had a meter that read out in the actual weight instead of just the volume? Sure it would. Back to engines. Speed-density EFI systems, such as you might find on your typical older Honda, use computer power to calculate the mass of intake air from input on rpm, vacuum, throttle position, and intake air temperature sensor input. But, as we said, a direct reading by means of an air flow sensor aids swiftness and accuracy, which are both critical if optimum performance, high fuel efficiency, and low emissions are to be achieved. Since air/fuel ratios are by weight (stoichiometric is 14.6 lbs. of air to one lb. of gasoline -- in gallons it would be about 2,000 to one), however, measuring mass makes even more sense, so for 15 years we've had sensors that do just that. *Keeping it hot Regardless of the type, a mass air flow sensor has some other advantages besides its ability to account for density: no moving parts, restrictions, or compensating sensors. A typical MAF has a wire or film element that's kept heated to a specified temperature above ambient (180 deg. F. in Bosch units) and is exposed to intake air. Through a Wheatstone bridge circuit and dedicated electronics, the amount of current required to maintain that temperature becomes the signal to the computer. High air flow obviously has a greater cooling effect than low, but so does the denser air of cold days and low altitudes, so the PCM gets the true data on mass it needs to provide the longer injector pulse width that extra oxygen needs to fire dependably. Bosch versions and GM 5.0/5.7L V8 units produce an analog voltage output, but Hitachi and most AC Delco MAF's send out a varying frequency digital signal -- a square-wave. The Karman-Vortex air mass measurement principle first showed up on the Lexus LS 400 and Toyota Supra. The speed of vibration of a thin metal mirror in the intake vortex is monitored optically to generate a frequency signal that varies according to the mass of air entering the engine. Various Mitsubishi units use sound waves to exploit the vortex idea. *Broken? From various authorities, we've compiled quite a list of probable symptoms of MAF trouble. Bosch tells you to expect starting problems both hot and cold, hesitation, stalling (especially under load), rough idle, and low power output. GM says the engine will fire up, then die. Nissan gives stalling, poor idle, black smoke, and engagement of the fail-safe mode (perhaps limiting rpm to 2,000) as evidence of air mass measurement problems. With all makes, contamination of the sensing element, which slows response, will result in stumble, which brings us to the most prominent logical effect of a bad or non-existent MAF signal: transient throttle glitches, including stalling, sagging, and missing. If it's far enough out of range to cause the PCM to shift to LOS (Limited Operating Strategy), overall performance and driveability will be lousy. But don't let these symptoms cause you to jump to an unfortunate conclusion and automatically replace that expensive sensor. Plenty of other malfunctioning components can result in the same kinds of annoying and inefficient engine behavior. So, as we say over and over, check the basics first. That means ignition, compression, fuel pressure and volume, etc. A simple problem that would be embarrassing to overlook is a hole or rip in the duct between the sensor and the throttle body, which admits unmeasured or "false" air and leans out the mixture. An open PCV can do the same thing. A restricted air filter element may make trouble, too. Then there are all the other sensors and the wiring and connections that are part of every electronic engine management system. In short, don't blame the MAF right off because most of them have proved to be pretty dependable. *On the other hand Of course, there's still a considerable percentage of cases where the MAF is indeed the culprit. The early GM/AC MAF, for example, garnered a poor reputation for reliability. Some cars thus equipped were factory retrofitted to speed-density systems by means of a PROM change to keep reoccurrences and warranty costs within reason. By the way, the higher-frequency 10kHz Hitachi unit used on later model GM cars has a much lower failure rate. So if the basics check out and you suspect the EFI, how do you determine if the air mass meter is shot and not some other sensor? Well, most late models have on-board diagnostics, so you might as well use them first. That's why they're there, after all, and now mandatory under OBD II regulations. *SOS As you may have noticed, the fault codes of the car makers' proprietary self-diagnostic systems vary (as opposed to the OBD II situation), so you'll need the specific service info for the car at hand, but we'll offer a couple of examples for illustration. With a typical GM product, Code 33 means the grams-per-second signal is too high, and Code 34 means it's too low. On a garden-variety Ford EEC-IV, Code 26 tells you that the MAF (or, for that matter, the earlier VAF) is out of self-test range, Code 56 means the signal is above the max allowed, and Code 66 indicates that the signal is below the minimum. OBD II continually examines the MAF under the comprehensive component monitor. It looks for an out-of-range signal, and compares output to values calculated from input on rpm, manifold absolute pressure, throttle position, and intake air temperature, then sets a generic DTC if there's a discrepancy. But codes can get you into trouble. All they should really be used for is aiming your diagnostic efforts in a general direction, not as the final word on what's wrong. You'll need to do some more troubleshooting and rely on your experience before you can be sure. *Tap test and quick check We'll bet most of you have heard of the basic GM MAF test: When tapped with a tool (the handle of a screwdriver, not a two-pound ballpeen) at idle, a bad sensor will not only produce a dramatic change in frequency, it may also cause the engine to stumble or stall. This is certainly a convenient check, and one of MOTOR SERVICE's GM experts says it's almost 100% accurate. You might even want to try it before pulling codes. But there's another quick check that's almost as fast. With the key off, unplug the MAF's harness connector, then start 'er up. If the engine runs appreciably better now, it's time for a new sensor. Several service bulletins have been issued on the power and burn-off relays used with GM 5.0L/5.7L V8's. Dig them out if you're presented with a hard start, rough running, surging, or stalling complaint. *Scan and measure MAF diagnosis is yet another area where a scan tool works well. Errors in sensor calibration are magnified as air flow increases, so being able to test during road load can be helpful. In some cases, both the air flow value the PCM is using and the actual signal from the sensor can be displayed. If these two numbers don't match, the computer is probably reverting to a substitute value from memory because the MAF info is faulty. By the way, GM units should produce a signal that results in a reading of 4-7 grams per second at idle, or 100-240 gps at WOT (naturally, depending on the displacement and horsepower of the particular engine). To check out a Bosch hot-wire air mass sensor, first look for battery voltage at the appropriate terminal, then measure output. A typical unit should read about 2V at idle, rising to almost 3V at 3,500 rpm. Greg McConiga, former NAPA/ASE Tech of the Year, tells us, "Diagnosing Bosch sensors can be trying, in that as little as 250 millivolts means the difference between perfection and rolling black smoke." To give you a few more ballpark references, common output specs for Ford hot wire units are 0-0.5V key on/engine off, 0.5-1.0V at hot idle, 1.5-2.5V at hot cruise, and 3.0-4.7V at WOT. Also, you should see the voltage change when you blow air through the sensor. If you're using a lab scope (also known as a DSO for "Digital Storage Oscilloscope"), set it for two volts and 200 milliseconds per division, then tap into the sensor's signal wire. Any spikes or jagged areas in the pattern are cause for replacement. If your DMM can measure frequency, you can use that mode to check AC, Hitachi, and any other unit you run into that produces a frequency signal. Set the meter to read Hz or kHz, and connect its leads to the sensor's signal and ground wires. An ordinary AC MAF as found on a 2.8L Chevy V6 should show you about 45 Hz at 1,000 rpm and 72 Hz at 3,500, whereas the high-frequency type of a later-model 3800 will read 2.9 kHz and 5.0kHz at those same speeds. Record the readings at various rpm and compare them to specs. You should see a linear frequency rise with no dips or jumps as speed increases. *More better But since these are high-frequency sensors, you might not catch a glitch with your DMM, so using a lab scope makes sense. Start out by setting amplitude to 5V per division. Changing the amplitude to one volt per division will give a different view of this signal. The timebase setting varies according to the range of the MAF. Use 0.1 milliseconds per division for a 10kHz Hitachi unit, for example. You should see a square waveform with even frequency pulses. As engine speed and load are varied, the pulse-width frequency should change smoothly and evenly. If you see any gaps in the pattern while tapping the sensor or driving the car, a new part is in order. You can measure a Toyota/Lexus Karman-Vortex signal using a lab scope set at 1V and 10 milliseconds per division. At idle, you should get a nice even rectangular waveform. Another test, which Lexus gives in the 1990 service manual, is that of resistance compared to temperature. Unplug the sensor from the harness, then check the resistance between terminals THA and E2 of the meter's connector. At 68 deg. F., you should see 2-3 ohms. This should fall to 0.9-1.3 at 104 deg., and 0.4-0.7 at 140 deg. At the other end of the scale, 10-20 ohms is specified at -4 deg., and 4-7 ohms at 32 deg. A final note from our friend Greg: "You might want to mention that cheap, poor quality air filters that shed fibers are causing MAF problems, particularly on GM/Hitachi units," he says. "They don't apply 'glue' to the element surface, and fibers escape and wrap around the sensing element, skewing the output." |
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Article
by: 
Details:
Designers
actually use a laser Doppler system to precisely measure the flow
characteristics. Designing a single basic design that can be used
across many applications is the goal so costs can be minimized.
Suffice it to say that a MAF from identical cars will not be
identical. The variations in the electronics that drive the sensor
have some influence on the way the car runs. As a model year
progresses other variables, like part lot numbers and such can
cause one car to run much better than the next. If all of the
variation of the sensors and devices like injectors are taken into
account, you could have a marginal car to begin with. This is why
some people see great gains with slight mods and others see
nothing or it gets worse. 
