MAF sensor: Produce RPM or voltage output?

2002 Subaru Legacy L non-turbo wagon
I suspect the MAF sensor needs to be cleaned or replaced.
In the car while the MAF sensor is still connected to the wiring harness
and with the car running, I can check the RPM or voltage output from the sensor output. Another test is to tap the sensor to check if RPM or voltage bounces around.
Out of the car, I can measure resistance across the sensor output and ground pins on the sensor. I can also hookup 12V across the 12V and ground inputs to the MAF sensor, put the meter (RPM or voltage) across the sensor and ground inputs, and blow across the wire to see if sensor output changes. The intermittent test by tapping the sensor can also be performed with the unit out of the car.
The following are what I don't know what to expect for sensor output:
- Does this MAF sensor generate an RPM or voltage output?
- For either RPM or voltage, what is the normal operating range for the device (when it is powered)?
- When tested after removal from the car, what should be the resistance across sensor to ground inputs?
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If the Subaru MAF semsor is the hot wire type you cannot test it that way off the car. A predetermined voltage is applied to the wire, and the current is measured. The computer knows what temperature the wire is at by the current drawn, and adjusts the voltage to maintain a given temperature. It knows by the amount ov voltage required to maintain the temperature how much heat is being removed from the wire by air flow, and calculates the air flow from that.
If the hot wire is fouled, the temperature response to air flow changes, and the MAF is out of calibration. Properly cleaning the MAF wire restores the calibration - improperly cleaning it can destroy it.
If it is a "cold wire" type, (sounds like it likely is) it will have a +, -, and "F" terminal, and will produce a variable frequency square wave output between the F and -, These must be powered by FIVE VOLTS - NOT TWELVE or you will toast them in an instant.
Tapping them with a screwdriver handle while operating is a recommended test (on GM vehicles). If the frequency changes, the MAF is toast. Changing frequency SHOULD cause a change in the engine idle - but may not be noticeable.
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clare wrote:

Below are pictures I took of the salvaged air box with MAF attached (same model as what is in my 92 Subaru Legacy L wagon):
pinout:
http://imageshack.us/photo/my-images/585/sub92mafpins.jpg sensor:
http://imageshack.us/photo/my-images/837/sub92mafsensor.jpg
Sorry for the debris in the pic for the sensor. When I turned it over after removing from the box, some crap that landed in the box got inside that I'll just blow out. This is NOT the unit in my car. I didn't have the time today to dismantle the air box or remove the MAF sensor to see what the one actually in my car looks like. The in-car and salvaged units are the same but the in-car unit could be in worse or better shape than the salvaged unit. I have never used an oil-based air filter since the oil would coat the MAF sensor and Subaru says to use paper-type air filters.
There are 5 pins on the connector. None of them are labelled. There is a "3" marked on the locktab side of the connector shroud but it doesn't really line up with a pin. The sensor looks to be thin metal strips which means it is the cold-air type that vibrates as the air passes across the sensor. So it looks like the output (whatever pin it is) is an RPM signal sent to the ECM.
While I've heard carburetor or brake cleaner spray can be used to clean a MAF, I already picked up some MAF cleaner spray. I can't really see there are any deposits on the metallic sensor strips but my eyes aren't that good anymore and the residue might not look like old varnish. I'm not even sure the cold-air type even accumulates any residue so there would be nothing for the MAF spray to clean off.
I'd have to get a voltmeter to check out the voltages delivered to the unit but it looks like I'd have to get an RPM meter (I don't have one) to measure the sensor output to check it was within range and didn't bounce around when tapping the sensor's body.
From my reading, it looks like the MAF is supposed to get replaced every 60K miles. This vehicle has 140K on it and I don't ever recall the MAF got replaced (nor remember ever having the shop or dealer mention it should be replaced). I may get stuck having pay the big bucks to get a new MAF unit but I'd like to first see if a home-brew repair gets it working okay.
I didn't see much point in testing the unit out of the car since any hookup would emulate its operation in the car (i.e., supplying the correct voltage, moving some air, and checking the sense output to the ECM). I don't know what voltages to expect on each connector pin or which is the sensor output. There are no "F", "-", or "+" markings on the connector. Wire colors were red, white, black, black, blue. I've also read that there may be an air inlet temperature (IAT) sensor inside the MAF unit so both the air volume and temperature get reported to the ECM. That would explain the 5-pin for MAF+IAT versus 3-pin connector for MAF-alone.
Since this MAF sensor looks to output an RPM value that varies with the air flow then I won't be able to test it produces output with some min/max operating range. So I'll just give it a shot with the MAF cleaner to see if the cold-damp bogging problem goes away or reduced.
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No such animal The third type is the Karman Vortex - which has THREE outputs - a frequency, a voltage relating to pressure, and a voltage relating to temperature. The frequency is related to air SPEED, not mass - so the temperature and pressure are REQUIRED to calculate mass. The Vortex sensor has a MIRROR and a LED/sensor pair, and the mirror vibrates with the frequency of the vortex created by the air flowing past the cone shaped vortex generator in the center of the air flow. The Karman Vortex sensor is not REALLY a MAF sensor - it is an airspeed sensor, more analogous to a vane type airspeed sensor coupled with a MAP sensor and an IAS sensor in one unit. These were used by Mitsubishi and a few Toyota models

The picture you show is DEFINITELY a "hotwire" type MAF sensor - a slightly different configuration than what I had originally described, in that it uses a wheatstone bridge type circuit, with the hot wire and a thermistor as 2 elements of the bridge. What you see in the "eye" of the sensor is the hot wire and the thermistor.
100% POSITIVE identification. Your outputs will be voltage between 0 and 5 volts and will basically not mean anything as raw readings.. I imagine there are specifications somewhere on what the resistances of the different elements should be, but measuring them accurately enough to tell you anything would not be a trivial endeavour.

I have had MAF sensors fail while under warranty, and others last over 240,000 Km - would not generally consider them to be "consumeables" - and MAF cleaner, properly used, is the way to go on most hot-wire MAFs when they start acting up.
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clare wrote:

Too vague to know what you think there is "no such animal". Based on the position of your reply, I'm guessing that you refer to the "ECM" acronym. Whether it's ECM or ECU depends on who you talk to as to what the car's computer is called: "module" versus "unit".
http://encyclopedia.thefreedictionary.com/Engine+Control+Module http://encyclopedia.thefreedictionary.com/Engine+Control+Unit
I might use either acronym and even bounce between them within the same discussion.

Yeah, I've read varying stories as to what the resistance (cold) should be with unit out of the car. Seems a megaohm is what's about what is expected. I don't have Subaru's books for this model year to have the specs for it.

I've never before thought of MAFs as consumables, either. It wasn't until I decided to address the cold-damp bogdown issue and read up on it that I'd see some folks claiming there was something like a 60,000 mile recommendation replacement schedule for the MAF unit. Yet it's not listed in the maintenance schedule (but then lots of parts aren't listed there) so I always figured it was had a replacement schedule of whenever it happen to break, like your alternator.

Other than squirting the MAF cleaner onto the metal strips/wires inside the unit, what else is there to do? How would one improperly use MAF cleaner spray? I figure you punch a couple blasts at the wires, wait a minute to make sure the cleaner works, and follow with a couple more blasts to move off any residue. The cleaner itself evaporates without leaving a residue. I hear it's even good as a contact cleaner. This is the second time you said "properly" in using MAF cleaner implying there's some way to improperly use it. I figure on hitting it with a couple short blasts of MAF cleaner, let it sit, and a couple more blasts and then let dry before use.
http://www.youtube.com/watch?v=vPT8rL0noYg
is a video by CRC on using their own MAF cleaner spray. For me, the tech shown spraying the MAF cleaner (at the 1:30 time mark) seems intent on emptying the whole can to clean just one unit. I would think it would only take a couple blasts on the wires, wait, and then a couple more blasts.
I've read some users that spray and then use an ear swab or pipe cleaner wetted with the spray to work it on the sensor to remove residue but that just seems way too dangerous. This metal looks rather fragile (and probably why it's after the air filter to make sure nothing big hits it). My eyes don't see any residue on the metal wires but then I don't have a new same-type MAF against which to compare.
As a side note, this MAF unit has 5 pins for its connector. That leads me to believe it incorporates an IAT (intake air temperature) sensor, too. I suppose that could also effect the cold-damp bogging that I occasionally experience. A defective IAT would report a wrong value and the ECM (or ECU or whatever you want to call the car's computer) would produce a wrong (over-rich) gas mixture. How likely is it that the IAT is bad?
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On Sat, 23 Jul 2011 15:36:52 -0700, "Nobody > (Revisited)"

The cold-wire sensor output CAN be checked with a tachometer ( at least SOME tachometers will read the frequency - not sure which range you would need to set it at any more - it's been over 23 years since I checked one that way) but you are correct - the reading will have very little to do with engine RPM. The frequency will go up with increased airflow -

Correct
And correct.

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Nobody wrote:

Revolutions per minute, cycles per second, same thing: a measure of changes per unit of time.

Could be increased vibration of a thin metal strip. Could be a vane that spins faster and uses an optical sensor to measure the speed up of the vane's rotation. Could be a hot wire whose resistance goes up when the wire is colder. Different mechanisms to measure the same effect of increased air flow.

The last tachomoter I ever had (which was over a couple decades ago) was one that used light pulses to measure physical rotation. That's been lost over several residence moves since. It's been a very long time since I had an oscilloscope. I'd have to get a multimeter includes measuring frequency (i.e., electrical impulses) rather than a physical tach. Instead of blowing the money on a meter that can read RPM (as electrical impulses), like $100 for http://tinyurl.com/3tq5ku8 , I'd rather save the money towards a new MAF unit.
From other users of meters that read RPM and online videos, these are not "go, no-go" testers. They show you the actual RPM reading so you can tell if the measurement is within specs (provided you have the specs for reference). It's not an idiot tester to see if there is or is not some electrical impulse but give you the actual RPM value. Maybe there is a go, no-go RPM meter but I haven't seen one (but then before this I haven't been looking around for one).
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