jmech

My 911 has recently started to misfire when put under any load.
I have been advised to check the air flow meter for correct operation by the following procedure:
Check resistance between pins 2 and 3 whilst moving vane through its full range of movement, service manual states that resistance should fluctuate throughout range of movement.
Porsche also advise to check for voltage change by metering from pin 2 to ground whilst moving the vane from closed to fully open, readings should be 0.26v closed up to 4.6v fully open.

When I checked for resistance values my results were 0.68 ohms closed which increased to 1.11 half open the droped back to 0.8 when fully open. As the manual does not give actual values, but merely states resistance should fluctuate, i am not sure if I have a fault. Should it proportionally increase or decrease with opening as one would expect?
I also checked the voltage from pin 2 to ground, my results were a 0 volt reading at any position, the voltage at pin 3 was 5 volts as specified.
From these results I would assume a faulty unit, but would the car still run if pin 2 was not sending a voltage signal back to the DME?

As the unit is quite expensive to replace I would be gratefull for any advice from someone with experiance in this field.

Lorenfb

It's unlikely that the AFM is the source of your misfire. Many (shops included) mis-diagnose
problems and assume the AFM. The vane type AFM is very reliable if maintained
with a light lubricant. If the AFM flap does not move smoothly, it can cause a lean
misfire or a flat spot under load, not a common problem, though.

For info on checking a 911 3.2 AFM, check out this ( www.systemsc.com/tests.htm ) under Digital ECM.
The voltage output of the AFM increases as the flap moves as does the resistance (with respect
to ground). You can check pin 7 of the Motronic unit for AFM voltages.

For diagnosing your misfire problem, check out this ( www.systemsc.com/diagnostic.htm ).

jmech

Thanks for your reply Loren. I have checked out your site which is very informative.
I am still trying to check the afm for correct operation. As i had no voltage back on pin 2 something must be wrong with it. I have removed the unit from the car and removed the black cover to expose the carbon wiper assembly to check for any wear, I assume that the function of this wiper is to decrease the resistance and therefore allow more volts to pass through, the further open the afm vane is moved. When checking the resistance from the input to the various tappings along the wiper this appears to prove this theory, but when taking the resistance from pin 2 to pin 3 whilst moving the vane readings are erratic and inconsistant and not at all linear.
Does this prove a fault with the carbon track and wiper?

Lorenfb

Yes, the resistance should decrease between the two pins which results in an increasing voltage
to the Motronic input. The resistance should change basically linearly as does the voltage
as the vane moves. You should not measure any jumps in resistance or voltage. The carbon
should not be scratched nor grooved.

The best test is to measure the voltage as you move the vane and comapre the reading with
those indicated at ( www.systemsc.com/tests.htm ).

jmech

Hi Loren, when you say no scratches or grooves, there does appear to be two noticable tracks in the carbon that correspond to the wiper contacts although they look to be equal in size to the small radial tracks that go out from the carbon to the tappings on the pcb, if that makes sense.
Are these normal? These marks are only noticable when light is reflected off the carbon.

Lorenfb

Unless the tracks are really grooves and not just smoothing marks, then they're O.K.
Spray some WD40 all over the inside of the AFM over the thick-film ceramic carbon piece.

Steve W

You can't test an air flow meter straight with just an ohm meter. You'll need to connect a 9 volt battery across pins 3 and 4, and measure the voltage change off pin 2. As you sweep the door, the output voltage should be about 0.45 volts at full closed, and smoothly build up to the about 8 volts at full open, assuming a 9 volt input from a 9 volt battery. Carrera afms output a linear voltage, from 0 degrees to the max opening of about 95 degrees. Therefore at mid sweep of about 47 degrees, it would read 4 volts; 25% open about 2 volts, and 75% open about 6 volts. The key here is to get a smooth and consistent voltage output without any jumps or voltage gaps. If there is wear on the resistor track, it most commonly can be seen on the first 10-20 degrees or so of the AFM track. You can move the sweep arm on the shaft up or down to give the arm a new resistor track to sweep on, thus making it like new.

jmech

Hi Steve, I have just checked the afm using the method you stated and I get a smooth linear change of voltage. The only thing is the voltage decreases as the vane is opened as opposed to increasing. Does this make sense!

Many thanks for your and Lorens help. Look forward to hearing from you.

Steve W

Sounds like you placed the ground lead of the voltmeter on the + pole of 9 volts, and while reading the signal off pin 7. Everything sounds fine, but if you want to recheck this properly, place 9 volts on pin 3, ground to pin 4, with your voltmeter, ground lead to pin 3, and read the voltage off pin 2.

jmech

Hi Steve. I have just retested the afm with an analogue volt meter set in the 0.1-1volt range and when i move the wiper slowly throughout its initial range i notice that the voltage increase is erattic i have double checked this and it definatly appears to be the case. When the meter is set to 0-12v and the wiper is slowly moved throughout its full range the meter needle is also showing signs of this. We are only talking around about a 0.2-0.3 voltage fluctuation at certain points as the voltage increases but nonetheless the volt increase does not appear to be really smooth. Does this signify a faulty unit?

Steve W

Possibly. I'd reposition the wiper arm down the shaft to give the arm a new resistor track to sweep on. Then retest.

jmech

I have moved the wiper arm contacts by slightly bending the arm and the readings are now stable. I will put it back on the car tommorrow.

Many thanks for taking the time to reply, its guys like you that make these forums such a useful tool. Checked out your website, no doubt be looking into one of your chips soon!

Thanks again!

911 2

jmech how did you get on did it work

nathanUK '81 930 G50

A big thank you to Loren and & Steve for their posts here on this thread. I was able to test my AFM and am happy that it is working 100%

Here is how I did it.

First I had to remove the AFM, second I cut up a crimp terminal as I had to get it to fit the AFM pins. I only had to make one as I could get a clip to do one of the connections.
Here is a whole one on the left and a modded one on the right. You can get the right conenctors but I didn't have any to hand...


Next I got a power supply, an old mobile phone charger hooked up and took a voltage reading.


Here is the reading fully closed.


I then moved the meter for more accuracy, but it wasn't necessary.


I compared my readings to the chart here http://www.systemsc.com/tests.htm and considering my test voltage I used my AFM is spot on with no problems. I also checked the temp sensor which was within spec too.