Problem with the air flow sensor

May 22, 2005 5 Replies
U

I've finally figured out what the problem is with my newly bought '92 BMW 535. The air flow sensor is messed up. I removed it and measured the resistance while opening the vane and it wasn't linear at all. Closed it was 680 ohm, opened a little about 1100, almost fully open 110, and completely open 420 ohm. I cleaned the circuit board and readjusted the pick up to get 1100 ohm completely closed and then the drop was pretty much linear, except for a few spikes up to 900 ohm, down to 100 ohm at about 70% open. More than 70% the resistance would increase and at about



90% it would go off the circuit board and into infinity. Figured if I drive gently I'll be okay until I get a new air flow sensor. I thought it would be best to reset the ECM too, so I removed the minus cable off the battery and waited a while. However, now the car runs *really* rich and the idle will oscillate until it stalls. So far I've only driven for
10-15 minutes, but how long does it take before the ECM relearns what it needs to know? Also, what should the resistance be for the air flow sensor, and is it possible to buy a new circuit board instead of the whole "box"?


Ulf


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S

Did you try cleaning the MAF first? You may not really need a new one.

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U

After calling my dealer to see what a new one would cost, I ended up buying one from a local scrap yard for $100. Now the car runs great, no hesitation anywhere in the rev range, and it definitely feels more powerful too. I did think the car was a bit sluggish when I bought it, but I figured with a weight of two tons and only six cylinders under the hood that's the way it was. Of course, it's still not as quick as my Camaro, but that can't be expected either. :-)

Ulf

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D

The correct way is to look at the output voltage with the ignition on with a scope.

You should start out with a supply voltage of 4.3 volts.

With the flap at rest, the output should be 3.7. slowly move the flap and look for irregularities. Fully open should be 1.6.

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U

Thanks for the info. I'll see what my old air flow sensor has for voltages after I try to repair it. Found a few ideas on this page:

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Either way, it runs great now. Next step are the thrust arm bushings.

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I know the discs are slightly warped, but the worn bushings are probably amplifying the vibrations in the steering wheel, and they're cheaper to replace. :-)

Ulf

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D

I forgot to say this was from a generic Bosch repair manual - not from one specific to your car.

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