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Wed Jul 29, 2009 8:39 am

Here's an iddy biddy viddy I took while testing an AFM that was giving some trouble. Symptoms were hesitation off-idle, and intermittent missing while under load in low-mid-range rpms (under 3200rpm).

The Bentley test confirms circuit integrity, but doesn't tell you what you really want to know, which is that the ECU is receiving a variable signal between 0V and 5V that is smooth, uninterrupted, and linear with relation to the air vane position. This test shows the actual signal as the ECU would see it. By the way, this customer had already done the Bentley tests himself with an ohmmeter and found nothing that was outside specs.

You want to use an analog meter; a digital one has a periodic refresh rate so it doesn't do a good job of showing a steadily changing reading, when constancy and steadiness is what you need to confirm. Basically, it's an analog signal, so it is best represented with an analog instrument. You could do the test with a digital meter, but you would want to move the vane very very slowly so you could see the parts where voltage drops out, if it does.

I removed the AFM from the filter box to have access to the inlet so I could deflect the air vane itself. This also obviates the need to remove the cover simply to test the part (although you will need to remove it to repair the meter if it does test as bad). On this particular AFM, moving the wiper hub at top did not show any problems, while moving the air vane did, so even slight pressure on the wiper hub can alter the pressure of the wiper on the track enough to mask a problem.

After unbolting the AFM from the filter box, I plugged it back into the vehicle wiring, and switched on the ignition so the ECU would be feeding it's regulated 5V output into the potentiometer. That voltage is reduced variably by the resistance of the wiper board, and what comes out is the 0-5V signal the ECU needs to calculate air mass. If you have a regulated 5V supply, you could bench test the AFM by inputting the 5V at pin 3.

The voltage signal is on pin 2 (pins are 1-4 left to right when viewed from the rear as installed; they are also numbered on the male connector flange of the AFM), and 4 is grounded inside the ECU, so you want to test V between 2 and 4. Use a low range on the meter; I have this meter on a 2.5V range. Even though the actual reading across the entire scale will be from just above zero to about 4.5V, I'm most interested in seeing the lower half of the range because that's where the wiper spends most of its time, so that's where wear will occur. At idle the signal will normally be around 1V, so at every opening of the throttle it has to sweep thru the area just above one volt. Consequently that is where the most wear will be found, and that is the range that will cause the hesitation on tip-in and load hesitation that this customer complained of.

Clicking on the pic will take you to the Photobucket site. Have the sound up because I'm explaining what I'm doing and what you're seeing.

Here's the URL if clicking on the image doesn't play it for you:

http://s105.photobucket.com/albums/m218/tencentlif...V02923.flv

*****
Moderator edit: Updated video info:
https://intrepidoverland.com/afm-testing-with-video/

*****

An AFM that shows dropouts can usually be repaired. First remove the cover, and then I first try spraying the wiper track with electrical contact cleaner while moving the wiper across it a bit to also clean the wiper tips. Then test again.

If cleaning doesn't restore smooth signal, there are two ways to reposition the wiper to run on a different radius of the track. First choice and simplest is to loosen the screws that hold the wiper board down and slide the board toward or away from the wiper hub. In this AFM I couldn't get the screws to break free, so I used the second choice solution. I used a bent scribe and a small screwdriver to make a slight double bend in both sides of the copper wiper arm itself, just enough to move the wiper tips a tiny bit closer to the hub, so it will run on a fresh line on the track. This restored smooth signal output in a subsequent test, and the van now runs smooth and the customer is very happy.

The other test to be done is the Intake Air Temp sensor, which is just an exposed NTC thermistor wired between pins 1 and 4. An ohmmeter and knowing the ambient temperature are all that's needed to check that, according to the temp/resistance table in Bentley. The IAT is very rarely bad, whereas the AFMs physically wear so after 20 years of use it's not at all uncommon to find them in this kind of condition.

The test will be exactly the same with both 2.1 and 1.9 WBX AFM's. It could also be done on earlier L-jet injections, but the pin positions will vary. I think that the signal output on L-jets would be #7, but you would need to verify.

Wed Jul 29, 2009 8:59 am

That is fantastic. Simple explanation of a very common problem with a "how to".
:D Titties :D

Wed Jul 29, 2009 9:23 am

videos like this are very helpful!

Wed Jul 29, 2009 9:47 am

What a useful video and explanation. Displays quite clearly how a dirty and/or worn track would affect the voltage. And interesting/important point regarding moving the arm via the vane. Makes total sense.

Not sure if there's a similar arm on the TPS of my Jetta ABA swap, but if ever needed, I'd be curious to know if a similar test could be done.

Thanks .10!

Neil.

Wed Jul 29, 2009 9:54 am

Quote: Not sure if there's a similar arm on the TPS of my Jetta ABA swap, but if ever needed, I'd be curious to know if a similar test could be done.

The throttle position sensor? It's a pot just like the AFM, where the ECU puts 5V in and a variable V comes out. You just need to find out which pins are which, and then you could test it the same way. Chances are there is a procedure to do just that in whatever manual covers that, I've seen the tests explained in manuals for other cars that have a TPS, VW shouldn't be any different. But lacking that you could surely figure out which pin is which on your own.

Wed Jul 29, 2009 9:58 am

Awesome write up & video! While an analog meter is definitely the tool for the job, all is not lost if you just have a digital meter. Using the AC setting you should have a higher sample rate and if you watch the display you should be able to pick out the glitch. This is just a workaround to not have an analog meter

Wed Jul 29, 2009 10:46 am

tencentlife wrote: Quote: Not sure if there's a similar arm on the TPS of my Jetta ABA swap, but if ever needed, I'd be curious to know if a similar test could be done.

The throttle position sensor? It's a pot just like the AFM, where the ECU puts 5V in and a variable V comes out. You just need to find out which pins are which, and then you could test it the same way. Chances are there is a procedure to do just that in whatever manual covers that, I've seen the tests explained in manuals for other cars that have a TPS, VW shouldn't be any different. But lacking that you could surely figure out which pin is which on your own.

Yes. Throttle position sensor. Thanks. Kind of figured it was the same or similar. Have swapped one out, just haven't cracked one open. Jetta Bentley does specify voltage of 5v. (IIRC) Just not sure if it specifies measuring for voltage throughout entire range. Should be easy. Only 3 pins. (manual tranny) Thanks again.

Neil.

Wed Jul 29, 2009 12:21 pm

Very nice!

I've bookmarked your photo site. Great stuff in there.

I like the spring compressor design for on the engine valve spring work. I take it you came up with this to address fixing AMC keepers while the head is still on the engine? Very slick!

Wed Jul 29, 2009 2:10 pm

No, I do those before the engine is built. I was just fooling around with shimming valve springs so that's what that was built for. Kinda nice tool as it turns out, though, you can hold the spring compressed and have both hands to do the keepers and such. It would be useful to the aircooled crowd, too. I should make up a bunch and sell them.

Neil, if your TPS has only three wires, then it's easy to find out which is the signal pin. Probing the back of the connector with everything hooked up as normal, with ignition on or off one pin should be common with ground. Ignition on, another pin should show 5V. The last pin is the signal output, just test with ignition on and measure V between that one and the grounded one and work the throttle open and closed. You will see a signal V between 0 and 4.5 on the ouput. Just like the AFM, it should change linearly with the degree of opening.

Wed Jul 29, 2009 3:02 pm

Thought it would be hard for you to top your well written, thoughtful and technical posts

I stand corrected

We owe you... big time

Wed Jul 29, 2009 3:17 pm

Well if a picture is worth a thousand words, video makes you a word billionaire. I would use it more, it's easy to take, but such a bitch to upload.

Quote: We all you... big time

You all me.... what?

Wed Jul 29, 2009 3:23 pm

all i can say is .... long day

fixed

Wed Jul 29, 2009 3:28 pm

tencentlife wrote: Well if a picture is worth a thousand words, video makes you a word billionaire. I would use it more, it's easy to take, but such a bitch to upload.

Chris, if you ever wanted to run for President of the Internet now would be the time.

8)

Wed Jul 29, 2009 3:41 pm

shadetreemech wrote: tencentlife wrote: Well if a picture is worth a thousand words, video makes you a word billionaire. I would use it more, it's easy to take, but such a bitch to upload.

Chris, if you ever wanted to run for President of the Internet now would be the time.

8)

Who would want that job? It doesn't even come with free healthcare. And have you seen where you have to live?

Wed Jul 29, 2009 11:55 pm

tencentlife wrote: ......

Neil, if your TPS has only three wires, then it's easy to find out which is the signal pin. Probing the back of the connector with everything hooked up as normal, with ignition on or off one pin should be common with ground. Ignition on, another pin should show 5V. The last pin is the signal output, just test with ignition on and measure V between that one and the grounded one and work the throttle open and closed. You will see a signal V between 0 and 4.5 on the ouput. Just like the AFM, it should change linearly with the degree of opening.

Cool. That makes total sense. Thanks TC.

The pinout you're describing certainly rings a bell, even though it's been a while since "dealing with" my engine swap wiring..... several times.... which included, in part, repairing, and correcting my wiring, to the TPS. ;) (though what's shown in pic obviously doesn't need to be done to repair wires to TPS)

Out she comes again. D-oh!

Thu Jul 30, 2009 4:24 pm

Please add this to the"stickys"

Thu Jul 30, 2009 6:00 pm

this solved my hesitation completely... THANK YOU!

I readjusted my throttle position switch and verified my coolant temp II was working properly but still had a slight hesitation from idle.

I used a digital multimeter. If you push the gate under the AFM slow enough you can see the dead spots - but as stated above an analog multimeter is best.

my screws were also epoxied in or something, because they wouldn't budge- so I very gently bent the contact arms with a small needle-nose.

installed and retested and it was good. I could feel the difference immediately. my voltage drop out was around 1.5-1.7 volts. After I altered the arc - it disappeared.

At full deflection my sensor was only putting out slightly over 4v (never got to 5v) - so i hope that isn't a problem - the van has been driving fine though.

Thu Jul 30, 2009 7:21 pm

That's right, you'll never see 5V, at most around 4.5, so no worries.

Glad it helped. It's such an easy test and allows you to verify without any doubt whether the thing is working or not, and that your fix worked if that was needed.

I looked at the AFM in the vid with a digital as well, and if you move the vane really really slowly you can in fact see the dead spots clearly enough to be sure they're not just a meter artifact, but nothing paints such a clear picture as a constant-reading analog meter.

I haven't tried the ac setting as RR suggested, but that seems like a good idea if all you have is digital. Me, I find that 6 VOM's isn't enough. Each one does certain things better than the others. The one I used in the vid was bought specifically for it's 2.5V range and being analog, so I could see an O2 signal 0-1V wave pattern.

Fri Jul 31, 2009 5:11 pm

Hey Tencent:

That's a great video and we are all thankful to you for that kind of help. I wanted to help others NOT make the dumbass mistake I made on my AFM on my '86 syncro 2.1. I had running problems that, after eliminating all other possible sources, pointed squarely to the AFM. For me, tinkering with the AFM was not a big deal because, at the time, I had all my ducks lined up for my SVX transplant and had everything except the time to actually do the transplant. So I got a little careless.

I opened up my AFM and saw that the circuit trace under the sweeper was covered with some sort of liquid which I sagely assumed was part of the problem. Being careless and impatient, I never tested the AFM. I used some commercial electrical contact cleaner and cleaned the circuit trace until there was no liquid left.

I reassembled, being careful to silicone the lid shut, and test drove. The driveability problem had disappeared and I was an unsung genius - - for about three days.

Then, it began belching black smoke on start up and under all running conditions. Then I realized I was more dumbass than genius. I immediately realized that the liquid was there for a reason - - lubrication. Without lubrication, my 200,000 mile old AFM had scores ripped into the circuit trace by the unlubricated sweeper arm.

Soooo . . .

To others less careful and less knowledgeable than Tencent, when you clean the circuit trace, don't forget to add a proper lubricant afterwards. What to use, I can't possibly say because I wimped out. I called VM Autohaus in Burnaby BC and they had the last (I'm told) brand new OEM AFM in Canada of the 100 or so they got from Volkswagen Canada some years ago and I bit the CAD $340 bullet, figuring I could always sell it later when the SVX goes in.

While we all learn from our mistakes, I think it's better to learn from others' mistakes. I hope this helps.

Fri Jul 31, 2009 7:42 pm

That's funny. I've opened dozens of these over the years and all the boards are dry, dry, dry. The wiper trace is a graphite printing that has conductivity but low friction. Never seen anything like lube in one. Maybe someone had got there before you and put vaseline or dielectric grease on it, or who knows what.

I'll spray the board with contact cleaner and run the wiper over it while it's still wet, just to clean off film from air pollution that may have accumulated. There shouldn't be much of anythig since the box is sealed up pretty tight, but little molecules get into everything given enough time. That hardly ever actually helps, but if it did I wouldn't need to take the next step of adjusting the contact pattern.