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What it does (excrutiatingly long) Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Thu, 20 Dec 2001 10:31:32
In Reply to: Just Exactly What Does the AMM Do?, KevinD, Wed, 19 Dec 2001 16:22:15 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The ideal mixture of air to fuel is called the stochiometric ration (maybe misspelled) and is the ratio of fuel to air MASS. The mass of air determines how much oxygen is in it, and mass is determined by altitude and temperature.
The AMM works by heating a wire and having the air blow over it. The circuitry in the AMM tries to keep the wire at the same temperature all the time. If you have a lot of air flowing over the wire, it pulls more heat from the wire, so the AMM has to push more current through the wire to keep it hot. So that current is proportional to air mass. But electronics like to measure voltage, not current, so the AMM converts that current into a voltage by putting it through a resistor. That resistor consists of a fixed resistance and a variable one, the variable part being the adjustment screw. So the current is proportional to air mass, and the voltage signal out of the AMM is proportional to the current, as adjusted by the adjusting screw. That adjusting screw takes out variations in the rest of the AMM circuitry and other engine sensors.
OK, so what does the AMM do? The engine control needs to decide how much fuel to squirt into the engine. It has to know how much air is going in. It uses the AMM to get a coarse reading on how much air is going in - very little, medium, or a lot. But that's not good enough to meet modern emissions requirements. So it needs to 'fine tune' the fuel. It uses the O2 sensor for that. If the O2 sensor says the mixture is too rich, the ECU leans it out some. Too lean, the ECU tells the injectors to inject a little more fuel.
The O2 sensor can control about 25% of the fuel flow, but the AMM controls 100%. The AMM gets the mixture close, and the O2 sensor goes the rest of the way.
Why not use the O2 sensor to handle all the fuel flow? A few reasons - the big one is that the O2 sensor has a very narrow range of mixture operation. Perfect mixture is 0.5 volts, just a little rich is 0.9 volts, and just a little lean is 0.2 volts. But it only swings between 0.2 and 0.9 volts, so just a wee bit richer than a little rich (0.9) looks just the same as really, really rich (0.9 volts). So it's very hard to control an engine if allyou have is way off and just right. Also, Because O2 sensors fail,and they don't respond very quickly. If the O2 sensor fails, the emissions will go up, but if you have a signal from the AMM, you can at least drive the car. But what if the AMM fails? Then there is a throttle position switch - if you press the pedal enough, the switch closes and the ECU says 'heck with what the AMM says - give me gas!'.
More O2 info - the O2 sensor doesn't work until it's at least 200 degrees C. So when you start a cold car, the ECU doesn't look at the O2 sensor for the first couple of minutes - it just uses the AMM until the O2 sensor warms up. So if things work OK cold but not as good warm, start thinking O2 sensor.
More complication - the ECU doesn't just run the car at exactly the right fuel air ratio. Cars come with a 'three-way' catalytic converter. It's called three-way because it handles both hydrocarbons and oxides of nitrogen (yeah, that's only two, but bear with me). For the converter to work it needs a slightly rich exhaust to handle one, and a slightly lean exhaust to handle the other. Since the exhaust can't be both rich and lean at the same time, it takes turns. It does this by swinging the mixture between rich and lean, right around the 'perfect' point. This swing is around once per second.
The recommendation to disconnect the O2 sensor and set the AMM so the O2 sensor reads 0.5 volts doesn't work well. That's because if the ECU sees the signal from the O2 sensor isn't there, it assumes the O2 sensor is bad (a good assumption) and goes to a default fuel setting, which is on the rich side. It goes rich, because that will cause less damage than running lean. So if you disconnect the O2 sensor from the ECU and set the AMM to get 0.5 volts on the O2 sensor, you're actually setting the system too lean, because the ECU will lean out the system some when it sees a good O2 signal.
When O2 sensors go bad, they tend to slow down in their response. This can cause idle fluctuations, because the ECU sets the mixture a little rich, but the slow O2 sensor says 'still lean', so the ECU makes the mixture even richer. But now the 'slow' O2 sensor says, OK, too rich! and the ECU leans it out - you get overshoot.
If you have cleaned up every vacuum leak and you know the AIC is OK, I would think about replacing the O2 sensor to help with idle hunt.
Hope this was clear-
posted by 140.157....
, Thu, 20 Dec 2001 10:31:32 <-- Viewing This Message
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