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Date:         Sun, 3 Oct 1999 21:52:44 -0700
Reply-To:     "Tom L. Neal" <jneal@NETCOM.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         "Tom L. Neal" <jneal@NETCOM.COM>
Subject:      AFM pathology (no legal or medical content)
Content-Type: TEXT/PLAIN; charset=US-ASCII

Subject: Thoughts about a couple of AFM failure modes, developed from studying a used digijet and new digifant AFM, reading the service note, list messages, some archive search, personal experience, etc. Conclusion: AFMs will wear out and fail in unpleasant ways. The capacitor fix is dubious. Preventative measuring and digitool monitoring the AFM are very smart. Should be replaced about every 90K miles. ---------------------------------------------------------------------- AFM Description: A laser trimmed resistor stick with eleven taps over to a parallel thick film resistor. Presumably the resistor stick is trimmed to give a linear output voltage relative to air flow. The wiper has two contacts, presumably for better reliability since they're both measuring the same voltage. A voltage is put across the stick, and the wiper which is hooked to the airflow gate senses the voltage on the film resistor as it moves along it. OK, this is all pretty obvious.

Also obvious is the most likely failure mode: The thick film resistor is worn by the contacts and the resistance between the contact and the resistor climbs up to levels that can affect the ECM, due to a high source impedance. I actually measured this by checking the resistance from the end of the stick to the moving wiper and watched it go up smoothly to several thousand ohms, then take about a 10K ohm leap, then come back down. The 10K ohm leap was about one third of the way up the resistor and seems about where the contact would be at cruising speed. The obvious repair is replace the AFM because the material is worn out. I was able to make the resistance well behaved by squirting with cleaner as some have done, but a concern is that the base resistive material is still worn and will soon fail in the same way. Bending the wiper to contact different places on the film resistor, as one clever person did, would probably also work for a while but the contact pressure is uncalibrated, maybe too much, maybe too little. It seems these repairs may have questionable long term reliability. The AFM wiper contact will deteriorate with usage over time.

The other failure mode of record is the alleged mysterious wiper vibrating that usually takes place in cold weather after driving for about 30 minutes. Volkswagon recommends the capacitor fix for it. Frankly, this whole wiper vibrating concept seems farfetched unless it's associated with resistive wear indicated above. Averaging the signal with a capacitor merely mitigates the problem somewhat, it doesn't get rid of it. Seems like it's new AFM time again. Or is it? The AFM-caused-hesitation-after-30-minutes-of-cold-weather-driving MAY have happened to my van at 15K miles, well before the resistive film should have worn. So this is still puzzling. I have the capacitor but don't feel that it helps much.

To the hyper-prevention types like I'm becoming, the most conservative approach seems to be to treat AFM's as wear and throw out devices (keeping a spare used one around) and they should be monitored as follows:

1) every 30K miles, measure the stick end to wiper resistance for monotonic behavior, and when the behavior gets significantly non-linear, replace the AFM. This is a much better measurement than looking at the voltage at the wiper while moving the gate with the key on.

2) monitor the AFM on the digitool while driving, and if the AFM signal starts bouncing around at a constant throttle setting, replace the AFM. Seems like a simple way to address this known wearout device that causes so much fear, uncertainty, and doubt, not to mention $150 for a wiring harness of dubious merit. Save the money and give it to Darrell Boehler for a digitool with the capacitor included! New AFM's cost about $100 from list vendors.

The old AFM can be kept as a spare for troubleshooting with or without attempting to "fix" it (which is fun, by the way).

Other information or opinions about AFM pathology? I'm really interested in some credible theory about what really might cause wiper vibration.

Cheers, Tom Neal


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