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.
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:
Saab AMM
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:
Saab AMM
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.
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.
I believe the AMM is also commonly referred to as the Mass Air Flow sensor (or MAF)