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Discussion Starter #1
When I acquired my 164 18 months ago it had a multitude of air leaks. All have now been fixed. I'm certain the previous owner had the air flow meter adjusted to compensate for the leaks. Annoyingly the arm of the AFM has been moved. I completely appreciate this is factory set and should never be touched. Can someone please advise me of the correct REST position for the arm in the 24V car? Here is my AFM attached. Should the arrow be right at the edge of the resistor strip or part way along?
 

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Dan there's no real answer. The AFM is individually laser trimmed to achieve the resistance curve needed. The wiper is set in mfg, then moved thru its arc and laser cuts on the resistors included in the circuit are made. Your best bet is a used, unmolested replacement
 

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But Mad North-Northwest
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As far as I know the AFM is only adjusted by changing the spring tension, not moving the arm.

Are you having some specific running problems that you think could be related to the AFM?
 

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Discussion Starter #4
Yes definitely still have an AFM problem, the occasional flat spot on moderate acceleration. I can improve it by incrementally reducing the spring tension but then the car ends up running too rich. I'm certain all other possible causes have been addressed - fuel pressure, air leaks, plugs, coils, various connectors etc. I was hoping someone had opened theirs and could give some indication as to where the wiper arm rests when the engine is off (at least on their meter) I figure they are all made by Bosch and the rest position at least might be common between meters. I'm certain the wiper arm position was fiddled with on mine, when I got the car it was resting right off the end of the resistor strip!


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If it helps, I am planning to strip a 164 twinspark AFM to try sliding the carbon track/pcb up a little to give it new bit of track to run on - as a test to attempting the same thing for my 164 24v. Have been putting it off but heard good things about the procedure - if done correctly - and will post pics when do it. Give me a couple of weeks.

Cheers,

Richard
 

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Discussion Starter #6
Thanks, that would be very helpful! I've done the same mod as you - it definitely helped. Take your time, best to take the circuit board right out of the AFM before you make the holes bigger.


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Thanks, that would be very helpful! I've done the same mod as you - it definitely helped. Take your time, best to take the circuit board right out of the AFM before you make the holes bigger.


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Good to know, thanks. Will be marking everything so can return to original positions in worst case.
 

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Discussion Starter #8 (Edited)
I seem to have made some progress. I set the arm default position to the very edge of the resistance strip which helped a lot, then I slowly increased the spring tension one stop at a time (ie rotating the wheel clockwise). It was initially running far too rich. Reached a point were it seemed to be running well (tested when cold and warm) then went one stop further which gave me slightly rough gentle acceleration. Figured I'd likely set too lean, so one stop back and all seems good. My neighbor has an old exhaust gas analyzer - so we may try that while driving.

If you are interested, these links are very helpful in diagnosing AFM faults (but not at resetting a messed with meter like mine!). The 24V 164 has the same meter as the later Porsche 944, so the input voltage is 5V (as per the document in the link). I appreciate this info is in the electrical manual on the 164 car disc, but I like the way this author has set it out... Turns out my AFM is good electrically, a nice smooth increase in voltage as the arm is moved. Make sure to disconnect the AFM when measuring the resistance of the temp sensor or you will get a dud result.

Electrically testing the AFM:
Air Flow Meter (AFM) Operation and Testing

If you are opening the AFM to modify it, it's easy to measure the voltages while the meter is in the car and connected, just use alligator clips on multi-meter leads attached to the inside of the AFM.

This details rejuvenating a worn AFM by bending the wiper arm- I think moving the circuit board by drilling larger holes is a better bet!
Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk
 

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But Mad North-Northwest
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Sounds like you're getting close. If you have a digital voltmeter you can tune it without a gas analyzer. You clip a wire to the O2 meter connector (without disconnecting it from the harness) and run it to a voltmeter in the passenger seat.

See step 12 in the L-jet procedure below for details.

L-jetronic Fuel Injection Technical Troubleshooting Article
 

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Hi,
Have opened the lid of my test AFM, from a twinspark 164, and found an issue getting the four arrowed screws undone.

Using a pozidrive number two bit, I can undo them about 30 degrees but then they seem to lock solid. Applying more force tends to make the bit jump out of the screw or begin to damage it. Anyone know of a better screwdriver bit size to use or able to give any advice?

Thanks.

Worth noting are the three very visible scored tracks in the material. This afm actually worked perfectly so just think how bad tracks must look on afms where they have been worn right through.
 

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Discussion Starter #11
Sorry I don't have any bright idea to remove the screws, but I'll pass this on to my mechanic (automotive and aero) neighbour to see if he has any bright ideas. The 24V AFM does not have this additional plastic guard over the circuit board and looks to be simpler to dismantle. I note the rest position of the arm is already quite a way along the resistance strip. It will be interesting to see where the arm in your 24V AFM sits once it is open.
 

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Sorry I don't have any bright idea to remove the screws, but I'll pass this on to my mechanic (automotive and aero) neighbour to see if he has any bright ideas. The 24V AFM does not have this additional plastic guard over the circuit board and looks to be simpler to dismantle. I note the rest position of the arm is already quite a way along the resistance strip. It will be interesting to see where the arm in your 24V AFM sits once it is open.
Thanks. I'll take it to a hardware shop tomorrow and match up a screwdriver. If that fails I'll just schedule in a day to go straight to the 24v afm. Cheers.
 

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Hello folks!

I've got a TS with a messed up AFM too. Have been having erratic idle and CEL 1223. NO air leaks. good plugs, good leads, good coil etc. So i took to adjusting the tension on the AFM potentienomenter and have been able to bring the idle to as smooth as I can achieve without special tools. No more bad idle or CEL.
Good thing I tumbled upon this thread! Great links, especially the one @Gubi posted!

Also, RIchard, I think the pic you posted will be able to help me adjust the AFM to as close as possible to the factory setting. Thank you for that ;)

Zaid.
 

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Hello folks!

I've got a TS with a messed up AFM too. Have been having erratic idle and CEL 1223. NO air leaks. good plugs, good leads, good coil etc. So i took to adjusting the tension on the AFM potentienomenter and have been able to bring the idle to as smooth as I can achieve without special tools. No more bad idle or CEL.
Good thing I tumbled upon this thread! Great links, especially the one @Gubi posted!

Also, RIchard, I think the pic you posted will be able to help me adjust the AFM to as close as possible to the factory setting. Thank you for that ;)

Zaid.
Glad to help. Did you get my troublesome screws undone OK? I think the best screwdriver bit is a number 2 pozidrive rather than a number 2 philips. Can I ask which you used?
Thanks
 

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Are all the screws acting the same way? Sounds suspicious.

Don't know if this is true from this unit, but in some installations, the screws turn just maybe a few degs to unlatch, and then the unit can be lifted off. Try that, but of course, do not force.
 

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But Mad North-Northwest
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Glad to help. Did you get my troublesome screws undone OK? I think the best screwdriver bit is a number 2 pozidrive rather than a number 2 philips. Can I ask which you used?
Thanks
If the screw has the lines marked between the crosses it's Pozidrive, if it doesn't it's Phillips.
 

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Are all the screws acting the same way? Sounds suspicious.

Don't know if this is true from this unit, but in some installations, the screws turn just maybe a few degs to unlatch, and then the unit can be lifted off. Try that, but of course, do not force.
Del, they all do that and lock quite hard after 30 degrees or so. Definitely worth testing your option of part turn= unlock. Will test and post. Thanks.
 

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No, mine were very hard to remove too. Didn't want to force as it looked like it was damaging the head of the nut. So I ended up using WD40 to clean up the insides and then wiped it dry with some tissues before blow drying. The resistor strip in mine is a little worn out at the point of idle. So I took out the Wiper arm (mark position before removing), carefully bent the 3 contact points to ensure a tighter contact with the resistance strip and fitted it back in place. Haven't had any problems since. No dead spots, sluggish idle etc. Might have to check exhaust gas emissions though, to ensure its not too rich or too lean.
 

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If the screw has the lines marked between the crosses it's Pozidrive, if it doesn't it's Phillips.
Thanks, Gubi.

The screw heads indicate pozidrive, number 2 fits best - but feels no different to Philips no 2. Given the way they loosen then bite after 30 degrees or so, going to see if 30 degrees releases the AFM plate. If not will have to think again.
 

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No, mine were very hard to remove too. Didn't want to force as it looked like it was damaging the head of the nut. So I ended up using WD40 to clean up the insides and then wiped it dry with some tissues before blow drying. The resistor strip in mine is a little worn out at the point of idle. So I took out the Wiper arm (mark position before removing), carefully bent the 3 contact points to ensure a tighter contact with the resistance strip and fitted it back in place. Haven't had any problems since. No dead spots, sluggish idle etc. Might have to check exhaust gas emissions though, to ensure its not too rich or too lean.
Thanks. Will add some WD40 before trying again. :)
 
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