Re: O2 sensor question - Saab 900 Bulletin Board - Saabnet.com
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Re: O2 sensor question
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Posted by Perry (more from Perry) on Tue, 27 Sep 2005 18:35:25 Share Post by Email
In Reply to: Re: O2 sensor question, TC, Sun, 25 Sep 2005 15:19:49
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Todd is aware of this but I'll add it to the thread for future searchers...the O2 sensor plum doesnt due its job unless you have the VOM hooked up taking a reading off the black signal line from the sensor. its as if the ehater circuit isnt working so its not heating up the O2 and thus not functioning properly, I ran this buy my Electronics genius buddy and this was the official diagnosis.
It sounds like the input of the computer is floating. Input gates need a certain amount of leakage to ground to bleed off voltages induced by electrical noise. Many chips have internal resistors and some rely on external resistors. These are called "pull-down" resistors. Maybe it came loose or went bad. Your VOM has an internal resistance of between 20,000 ohms and 15 meg ohms depending on how it's designed. Old analog meters were low resistance and digital meters are always more than 15 meg. Higher is better because they upset the circuit value to a lesser degree thus provide more accurate measurements.
Your meter is providing that needed leakage to drain off the static buildup. The easy cure would be to replace the meter with a resistor. Usually 10,000 ohms, 10k is about what they use but anything over that will probably work too. Just hook it from that hot lead to chassis and see what happens. The higher the value, the less it will upset your desired voltage reading.

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