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Re: So analog is the way to go...Q for Ari Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 14 Dec 2001 13:10:01
In Reply to: So analog is the way to go...Q for Ari, JohnK, Fri, 14 Dec 2001 12:21:26 Members do not see ads below this line. - Help Keep This Site Online - Signup |
There are two different places to pick up the signal, and you need to treat them differently.
The O2 sensor output is very high impedance - that means it can't push much current. A standard analog meter presents too much of an electrical load, and will screw up the signal to the ECU. It won't damage anything, just mess up the O2 sensor signal while the meter is connected. A good analog meter presents a load of about 20,000 ohms/volt - so for the range you'd use for looking at an O2 sensor, you're presenting a load of about 20K ohms, which is too much for the sensor. For that, you use a digita voltmeter. They usually have an input impedance in the megaohm range.
The signal at the test connector has been processed by the ECU already, and the output is enough to drive an analog meter. It's a different kind of signal.
The output of the O2 sensor varies between 0.2 and 0.9 volts. The ECU takes that signal and 'squares it up' - the test connector has something like 5 volts out (ON) when the O2 sensor is above 0.5 volts, and 0 volts out (OFF) when the O2 sensor is less than 0.5 volts.
Because a digital meter takes samples of the voltage and updates the display at discrete times (usually only a few times a second, if that), it gets pretty jumpy looking at varying signals. An analog meter does a much better job. Ideally, you hook up the analog meter to the test connector output, and tune the AMM so that the meter spends half the time at the ON voltage, and half at the OFF. Technically, you're looking for a square wave with a 50% duty cycle.
Hope this helps.
posted by 140.157....
, Fri, 14 Dec 2001 12:07:46, Fri, 14 Dec 2001 13:10:01 <-- Viewing This Message
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