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AFM out of spec

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Posted (edited)

Hi all,


I know this is a topic covered to death but I haven't quite found the answer to what I'm looking for.


I found the AFM testing guide on the old 944 forum via web archive here and the guide on our forums here.


Neither guide is very clear on what to do if the testing is out of spec, other than the 944 forum saying to replace it if the air temperature readings are out.


From our forum:



- For Jetronic: Disconnect the 5-pin connector from the AFM, and measure the resistance on the unit between pins 5 and 7. It should be between 60 ohms (flap closed) and about 1 KOhm (fully open), with a smooth transition. Check the resistance between 8 and 9, it should be between 160 and 300 Ohms. Check the resistance between 5 and 8, it should be between 350 and 450 Ohms.


The tests between 8&9 and 5&8 are correct but the resistance between earth and the output is all over the shop. It's around 160ohms at rest, goes up to around 1.2k mid stroke and back down to about 800ohms fully open. I checked it with both an analog and a digital meter and the readings are erratic on both meters, jumping up and down throughout the sweep and not at all "smooth". 


I've tried to move / re-track the arm and it's still the same. I wonder if the spring has been played with in the past. It put up a fight to open and looks clean, except the screws inside are all corroded. If it was still sealed from factory I wouldn't expect rusty screws inside a sealed unit?


Can retensioning the spring fix this, or is it a case of once the metallic strip is giving erratic readings, the AFM needs to be replaced? I think I have a spare 1.9 AFM down the unit which I'll dig out and test over the weekend but in the meantime, anything I can do to salvage this one?


For context - the reason I'm digging into this is the car is running rich and lumpy - at MOT the testers really struggled to get it under 4% CO as adjusting the pot doesn't really have any effect. I had this same problem with CO at the last MOT two years ago, since that MOT I've swapped the engine and throttle body, so the AFM is the only common item left!

Edited by artois7

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Does the air bypass channel adjuster make any difference?

If the flap in the AFM has the right tension the adjustment for the the bypass has a noticeable effect.


Flap tension spring adjustment is tricky without any markings for the original setting.

Perhaps someone has some info on a testing method to get back to the correct tension.  A certain weight hanging from a measured length lever to give a certain torque and the tighten the spring tension to balance it might be a place to start.

Or maybe no preload on the spring and then count up x number of notches to reach the right tension.


I have seen resistance figures for the individual segments on the circuit board.   I can't recall where though. If I find them I will post here.


Do you have the correct AFM?  1.6 and 1.9 have different OE numbers. I presume the resistance of the segments and possibly flap tension are altered to suit the difference in power curves between the two engines.

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You quote Jetronic testing  yes mention adjusting the Potentiometer,  which is only on Motronic.


I have managed to replace the Potentiometer itself on an mp3.1 setup and I believe the component is the same.


However  it sounds like your carbon track on the board is just worn out.

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