What I learned today - Saab 900 Air Mass Meter
All ye who are technically competent, please pass on to the next story. This is for the tech duds like me.
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Some of you may recall that I had some issues late last year with my 1985 Saab 900 Aero. The car would develop this tendency to jerk around a lot.
At first, we thought it was the fuel pump playing up, but $400 and a replacement later, we learned that wasn’t the case. Matt the fudgepacker correctly diagnosed the problem as being the Air Mass Meter. These are pretty expensive to buy new (around $800 here in Oz) so any sort of fix would do. Matt provided the simplest answer possible - A quick tap on the head of the unit and it seemed to right itself. It worked on the odd occasion it was needed after that, so I didn’t worry too much.
The problem was long forgotten until earlier this week, when the car developed it’s jerkiness once again and no amount of tapping on the AMM would set it straight.
Hesitating to pony up for a new unit, I rang a spare parts supplier in Melbourne and got them to ship over what they thought was a good second hand unit. I fitted the unit to the car today and it ran even worse than when I had my old part on it. Here’s the part, so you know what it looks like:
That’s actually upside down to how you see it in your engine bay, but the battery ran out in my camera before I could take another shot 
So whilst my part was off the car, I thought I’d take it into my local Saab specialist, Steve E, so that he could have a look. Steve tested the unit and found it had been adjusted at some point. Once it was adjusted back to the proper settings, I took it home, reinstalled it, and now the car’s running as good as gold.
So what was the adjustment?
Here’s a look at the unit from a different angle:
As you can see, there’s six pins in the plug for this unit. The test involved putting a multimeter on pins 3 and 6 and measuring the ohms. It should read 380 ohms. Mine was reading somewhere up past 530 ohms. The supposedly good replacement part I got from Melbourne was reading 995 ohms!!
You can see a gold-colored adjusting screw just to the right of the plug in that photo. That screw adjusts whatever it is that adjusts the ohms readout. Steve simply turned the adjusting screw until it read 380 on the multimeter.
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How the AMM works (my limited understanding) - the computer sends a current through a filament in the AMM, which is cooled by the air passing it. If there’s more air passing by and cooling the filament (and going on into the engine) then the AMM boosts the current in order to maintain a set temperature in the filament. The variations in current required tell the computers how much air is coming into the engine and therefore, how much fuel is needed to match the air and create an optimum mix.
I assume that if there was an incorrect setting, such as that on my unit (where it was 500+ ohms instead of 380), then it would be misreading the volume of air that it’s meant to measure, hence the rough running that I was experiencing from time to time.
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I believe the AMM is also commonly referred to as the Mass Air Flow sensor (or MAF)
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Swade the Super Saab Techie has spoken !
MAF was the first thing to fail on my Viggen.
Shaking the dice out of the Air Mass Meter probably helped, too.
Yahtzee!
Swade that is exactly how the ‘hot wire’ type mass air flow sensor (MAF) works, your description of the sensors operation is accurate (5 points). I have never worked with an adjustable type MAF sensor, so that is an interesting setup.
Hot wire sensors are very susceptible to contamination from cheaper air filters as fibers from the filter can attach to the sensor wire and cause it to send skewed airflow readings to the computer which causes the computer to incorrectly calculate fuel. Another thing to be carful with is aftermarket filters that need to be periodically oiled – the oil from the filter can ‘bake’ onto the hot wire which can permanently damage the sensor.
Systems that use an oxygen sensor for feedback can sometimes correct for a slightly skewed sensor and correct the air:fuel ratio, however gross errors would result in type of driveability complaints that you were experiencing.
Thanks for the top marks there, Prof. That’s Matt’s good teaching.
Ryan - Backgammon. We play, on average, around 6 games a day and have recorded who wins each 3-game series for each of the last 5 years. Sad, but true
swade, i replaced the AMM in my 88 900T yesterday, attempting to rid my car of a stuttering/hesitation that occurred under spirited acceleration. worked like a charm!
i remembered reading something about yours doing the same thing, and was actually planning to email this suggestion. glad to hear you ironed it out!
Tedjs - Good information, but also high mileage/kilometre engines with poorly maintained breathing systems create oil mist which contaminates the hot wire, particularly when a hot engine is shut down The oil mist flows back up the inlet tract and ‘poisons’ the MAF sensor.
Yep, ran into this problem a lot when I was running with my mustang and the guys of the drags…. Gotta’ keep those MAFs cleaned otherwise it’ll make everything else not work.
The MAF meters are indeed expensive, even the “rebuilt” ones. I think the one in the ‘85 900T is different than in subsequent years as well, so its rarity might increase the cost even more.
Does anyone know if the filament can be cleaned safely with some kind of spray cleaner? If not, can the ‘85 MAF be replaced with one from a later model year? Is a wiring change required?
Gripen - we used to use brake parts cleaner to clean them off if we suspected a problem with them.
We would let the sensor cool off for a bit and hit the sensing wire with a quick shot of cleaner which is a key point here - a very small amount would clean the sensor if it was contaminated. I only say this because I watched someone use half a can on a sensor and I think it damaged the sensor as brake parts cleaner is a strong solvent. Unfortunately some people live with the adage if a little is good – a lot must be better. Don’t get me wrong – it was entertaining watching him, but that was kind of his method of operation. The guy could not fix a sandwich.
Sometimes if you would clean them the problem would come back and the sensor would have to be replaced. The sensor wire generally gets about 100 degrees Fahrenheit (37.78 degrees Celsius) higher than ambient air temperature so I think things would get cooked on it occasionally.
CRC recently introduced a mass air flow cleaner here in the USA that is for sale at most auto parts stores. It is less caustic than brake clean and is less likely to damage the sensor housing and attack the electronics. I know some shops are selling it as a maintenance service as newer cars are so reliable they have little to do in terms of routine maintenance, but I have mixed feelings about that. That sensor is expensive and cleaning a clean one might damage the filament. Sort of a ‘if it ain’t broke – don’t fix it’ approach if you know what I mean.
I had the AMM go on my 900T shortly after purchasing it. Tapping on it as the car would start would make it run great for a few miles and then I’d go back to zero boost.
Interestingly, the car ran better when my right front corner light fell out. ??
tedjs: LOL! “…couldn’t fix a sandwich”. Thanks for the info on the brake cleaner.
I had my Mass Air Meter replaced on my SAAB 900- 92 when it hit 150,000 miles. It indicated a fuel pump problem and so a fuel pump was put in as a first solution at the first shop, a foreign car shop. That did not solve the problem. As I was in Washington State and the dollar conversion was good in those years, I drove to Vancouver, BC with it not sounding right not knowing if I would make it. I ordered the Mass Air Sensor on advice of the mechanic in advance, (a pricy item), from Ottowa ( to take advantage of the dollar conversion saving, and then had it shipped to the states where I then drove with it to Canada to install. Yes, the pressures in the lines were adjusted when this thing was put on indicating this was the problem but the problem persisted and was supposedly alleviated when all the fuel lines were replaced too or so they thought. It was very costly. Then when I went back to the states, the car died in the “fast lane” in Portland on the freeway. I just told the tow truck to take it to any SAAB place. The problem was fixed by this “new blood line” SAAB shop as it was noted in my case (after $4200 work), that the first mechanic that was not so familiar with SAABS had put in a rebuilt fuel pump. I kept my SAAB, though, and now have a competent mechanic. I praise the heavens! I could not resist commenting on this travisty as a non- techie. Love your site. Am hoping for that plug -in hybrid SAAB in the works. I have 200,000 miles on my SAAB and it sounds and runs like new thanks to my mechanic in Santa Monica.
Esther, et al: It is important to note that the later C900 models have a different AMM. It works on the same principle, but they aren’t adjustable and they are a little less finicky, or, more likely, the controllers are able to work with a slightly wider input range. Just FYI.
That was one of the questions I asked of Steve E while he was adjusting it - “why didn’t they just set it up at 380 ohms and remove the adjustment capacity?
I’m no techie, you see.
Thanks effsngrits and everyone else for the expanded view on the MAF as it was called when mine was replaced, I think. Nice to know the newer models use less finicky ones. I remember my first beloved car, a 1966 3 stroke and SAAB has come a long way since then.