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Re: O2 , ECU damage follow-up
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Wed, 26 Oct 2005 06:36:23 Share Post by Email
In Reply to: O2 , ECU damage follow-up, John Daikh, Wed, 26 Oct 2005 01:28:46
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Again, I really doubt you damaged the ECU with the meter. Whether a meter uses 2 1.5v batteries or a 9 volt, it works by supplying a small current to the load. Yes, it does that by applying a voltage, but in all cases a small one. Most DVMs produce well under 0.7 volts, because that's the voltage it takes to turn on a diode. Better DVMs actually have a diode range which apply more voltage to turn on diodes. And again, the ECU isn't going to die because you hit it with 2 or 3 volts with little or no current.

What tells you all is right? Yes, if a warmed up O2 sensor (running at least 5 minutes) is swinging back and forth between about 0.2 and 0.9. BUT, the O2 sensor can be just fine and not see that. Huh? The O2 sensor is part of a system.

The O2 sensor has no circuitry - it's just two electrodes and an insulator. Specially coated electrodes and a special insulating material, but that's it. There is a hole in the back where the wires come out, that lets outside air into one side of the insulator. The other sees exhaust. If there are the same number of oxygen atoms on both sides of the insulator (exhaust is rich in oxygen - lean), it produces no voltage. If there are more oxygen atoms on the back side than on the exhaust side (running rich), the difference produces a voltage. The materials are 'tuned' so that when the intake mixture is perfectly stochiometric (exact right ratio of fuel to air), the amount of oxygen left in the exhaust gives 0.5 volts out of the sensor. Just a little rich, and the voltage goes up to 0.9. Even richer, and it stays at 0.9. Just a little lean, and the voltage goes down to about 0.2. Really lean, still just 0.2 So it is sensitive, and it saturates quickly.

The ECU doesn't put a voltage out. Any voltage read coming out of the ECU is just an artifact of the input circuitry. The ECU reads the O2 sensor voltage and adjusts the fuel mixture. The AMM is the main way the ECU adjusts fuel; the O2 sensor is for fine tuning.

Why does the voltage swing? IF the mixture control is OK, the ECU actually swings the mixture from slightly rich to slightly lean and back again. It does this to make the Catalytic converter work properly. Don't ask why.
But the ECU only adjusts so much fuel based on the O2 sensor. If you have a vacuum leak and are running lean, the O2 sensor will read lean. The ECU will add fuel. If the leak is big enough that this doesn't bring the mixture into balance (stochiometric), the O2 sensor will still read low. And it will be right. As mentioned before, the ECU will only add so much more fuel before it says 'enough'.

I don't know what car/year you have. If you have an adjustable AMM, that is the first step. The O2 sensor controls only about 25% of fuel flow; the AMM controls 100%. So the AMM needs to be adjusted to get the O2 into range; the AMM is the coarse adjust, the O2 is the fine adjust. In later years, the system is self adjusting. But if there is a big vacuum leak, you may be stuck lean. If the FPR is leaking fuel or the vacuum hose to it is leaky/off, you'll be running rich, no matter what, and the O2 sensor output will be stuck rich.



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