*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'* GM experts says it'* almost 100% accurate. You might even want to try it before pulling codes.

But there'* another quick check that'* almost as fast. With the key off, unplug the MAF'* harness connector, then start 'er up. If the engine runs appreciably better now, it'* 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'*. 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'* 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'* 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'* 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."