Problem with the air flow sensor
 

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

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

In article >,
Ulf > wrote:
> 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.

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.

--
*Half the people in the world are below average.

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
> In article >,
> Ulf > wrote:
>
>>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.
>
>
> 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.

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:

http://member.rivernet.com.au/btaylo...rCalibrat.html

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

http://www.bmwe34.net/e34main/mainte.../upper_arm.htm

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

In article >,
Ulf > wrote:
> > 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.

> Thanks for the info. I'll see what my old air flow sensor has for
> voltages after I try to repair it.

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

--
*TEAMWORK...means never having to take all the blame yourself *

Dave Plowman London SW
To e-mail, change noise into sound.