Afm Resistance Linear Or Not?

[KaspaCti 0]

Hi,

 

Quick question,

 

When moving the flap and measuring the resistance on an AFM (1.6) should the resistance change linearly i.e. increase and decrease progressively between the specified lower and upper resistance or does it jump around (within the lower and upper resisitance figures) dependant on position?

 

Both of mine jump around within the range i.e reads 200 ohms then jumps to 600 then down to 300 etc etc as the flap is moved. I haven't got it in front of me but I think Haynes just says check reading varies between the lower and upper resistance figures quoted (can't remember what the figures are at the mo'), which could be read either way.

 

I have done a search but couldn't find this info specifically. My gut feeling is that it should be linear and that the jumping around indicates wear on the carbon track, but nothing would surprise me!

 

I have thought about getting an analogue meter as I know digital meters aren't that great for showing a changing reading. However as the resistance is jumping around so much I presume the digital meter is giving me a fairly accurate indication of what's going on. Be interested to know what others are using and what results you get.

 

Many thanks

 

Sean

[welshpug 1,627]

it should be linear, pop the cover off and you'll see if there's wear on the tracks, there are a few threads covering taking the AFM apart on here I think

[paulh 0]

Test the airflow meter as per this link here

 

http://frwilk.com/944dme/afm.htm

 

You can then use your digital meter. This test worked fine on mine.

[KaspaCti 0]

Cheers guys thanks for your help and the link.

 

Sean

[Rob_the_Sparky 9]

it should be linear, pop the cover off and you'll see if there's wear on the tracks, there are a few threads covering taking the AFM apart on here I think

 

Please be careful with you info, a Motronic AFM is linear but a Jetronic is not. On a Jetronic the AFM track froms the closest thing that Jetronic has to a fuel map. On Motronic (and all digital ECUs that I'm aware of) the map in stored in memory and the AFM output is just an indication of air flow.

[KaspaCti 0]

Please be careful with you info, a Motronic AFM is linear but a Jetronic is not. On a Jetronic the AFM track froms the closest thing that Jetronic has to a fuel map. On Motronic (and all digital ECUs that I'm aware of) the map in stored in memory and the AFM output is just an indication of air flow.

 

 

Thanks Rob, you,ve saved me butchering one of my AFMs! Mines definitely Jetronic so I guess as long as it moves around within the quoted resistance range it should be ok.

 

Thanks again

 

Sean

[welshpug 1,627]

no matter which system its from it shouldn't do the following.

 

jump around within the range i.e reads 200 ohms then jumps to 600 then down to 300 etc etc as the flap is moved.

[Rob_the_Sparky 9]

Agreed, the earlier link shows how it should respond. Not linear but not jumpy either. Be careful when checking it though as I've seen some misleading readings before. If the engine runs anything like OK then it isn't jumping around in reality as this would throw the fuelling out all over the place.