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How does your A/F Ratio Gauge work?

Discussion in 'Modified Explorations!!' started by B94Sport, October 3, 2002.

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    1. B94Sport

      B94Sport Well-Known Member

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      For those who have them... How does it display the A/F ratio to you? I have a 10 segment bar graph version that moves up and down constantly and will never stay at just one reading (unless I floor it - in which case it goes all the way to rich and stays there). I always interpreted mine as taking readings every so often, rising to the segment that represented the reading, and then falling down until another reading was taken, at which point it would rise back up again. I assumed this was normal. Anyone else?

      The reason I ask is that I have mine wired to a toggle switch so that I can switch between the driver- and passenger-side cylinder bank O2 sensors... I noticed today that the reading from the passenger-side remained at a constant position (not all the way rich) and never fell or rose while I was accelerating/cruising, but resumed its (perceived to be) normal rise and fall readout when I stopped at a light and idled. When I switched to the driver-side sensor during the same time that the passenger-side was remaining constant, the readout rose and fell like normal.
       
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    3. Alec

      Alec Elite Moderator Moderator Emeritus

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      From Autometer:
      Hope the following helps. I don't know why your 2 banks are reading differently. Did you tap into the sensors at the computer or at the sensors themselves? The only thing I can think of is that you might've tapped into the wrong wires. Either that or the O2 is going bad and can't react fast enough to change it's signal. Hmmmmm, I'm thinking. Anyway, the Autometer stuff:


      A/F RATIO METER & OXYGEN SENSOR OPERATION

      This is normal operation for most computer controlled vehicles.

      The A/F Ratio Meter is a voltmeter with a range of 0 to 1 Volt. The meter displays the output voltage of the vehicles oxygen sensor through 20 LED's. The first LED will come on at a voltage of .050V, the second at .100V, the third at .150V, etc.

      LEAN RANGE:

      Four red LED's

      (.050 to .249V)

      STOICHIOMETRIC RANGE:

      Ten yellow LED's

      (.250 to .749V)

      RICH RANGE:

      Six green LED's

      (.750 to 1.000V)

      The stoichiometric (STOICH) air/fuel ratio is the chemically correct ratio, theoretically all of the oxygen and all of the fuel are consumed. The mixture is neither rich nor lean. However, due to the fact that combustion is never perfect in the real world, there will always be a small amount of oxygen left in the exhaust. This small amount that is left is what the oxygen sensor measures. The smaller the amount of oxygen that is left in the exhaust, the richer the A/F ratio is, and the higher the oxygen sensor voltage is. The on-board computer or Powertrain Control Module (PCM) monitors the voltage from the oxygen sensor. If the PCM sees an oxygen sensor voltage greater than .450V, it immediately starts to reduce the amount of fuel that is metered into the engine by reducing the on time to the fuel injectors. When this happens, the A/F ratio starts to go in the lean direction, and the oxygen sensor voltage starts to go down. When the voltage drops below .450V, the PCM immediately starts to increase the fuel metered to the engine by increasing the on time to the fuel injectors to produce a richer A/F ratio. This occurs until the oxygen sensor voltage goes above .450V. This repeating cycle happens very fast (many times per second). The PCM is said to be in closed loop. It is constantly monitoring the oxygen sensor voltage and adjusting the on time of the fuel injectors to maintain a stoichiometric A/F ratio. This A/F ratio produces the lowest harmful exhaust emissions, and allows the catalytic converter to operate at peak efficiency, therefore reducing the exhaust emissions further.

      Since the oxygen sensor output is non-liner and very sensitive at the stoichiometric A/F ratio it will cause the A/F meter LED's to bounce back and forth rapidly. A very small change in A/F ratio causes a large change in oxygen sensor voltage as can be seen on the graph. This causes the A/F ratio meter LED's to rapidly cycle back and forth, and is normal operation when the PCM is in closed loop and trying to maintain a stoichiometric A/F ratio.

      The oxygen sensor is very accurate at indicating a stoichiometric A/F ratio. It is also very accurate at indicating an A/F ratio that is richer or leaner than stoichiometric. However it can not indicate what exactly the A/F ratio is in the rich and lean areas due to the fact that the oxygen sensor output changes with the oxygen sensor temperature and wear. As the sensor temperature increases, the voltage output will decrease for a given A/F ratio in the rich area, and increase in the lean area as shown on the graph.

      During wide open throttle (throttle opening greater than 80% as indicated by the throttle position sensor) the A/F ratio will be forced rich by the PCM for maximum power. During this time the oxygen sensor outputs a voltage that corresponds to a rich A/F ratio. But the PCM ignores the oxygen sensor signal because it is not accurate for indicating exactly what the A/F ratio is in this range. The PCM is now in open loop, and relies on factory programmed maps to calculate what the on time of the fuel injectors should be to provide a rich A/F ratio for maximum power. The A/F ratio meter should indicate rich during this time.

      During hard deceleration the PCM will command an extremely lean mixture for lowest exhaust emissions. This may cause the A/F ratio meter not to indicate anything. The A/F ratio is so lean that it is outside the range that the meter will indicate.
      See our A/F Gauges:
       
    4. B94Sport

      B94Sport Well-Known Member

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      I tapped into them at the sensors themselves... I will admit, at first (when I installed the gauge over a year ago) I tapped into the wrong wires, but quickly figured that out when the gauge did not change at all. Since (as your info from AutoMeter stated) the gauge is just a voltmeter, tapping into the wrong wire - which would be either a power or ground - would just give a constant lean or rich readout. In my case, it was a constant lean readout. But I fixed that and had not had any problems since. Thanks for the info, so I guess that the bouncing is normal... I am still wondering about the abnormal readout from this morning though. My O2 sensors only have about 15K on them, so I don't think that is the problem. I am having problems with rough idle and decreased fuel economy as well... Perhaps they are somehow related.
       
    5. Alec

      Alec Elite Moderator Moderator Emeritus

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      Hmmmm, how long have you noticed that the 2 sensors aren't the same?

      I'd try cleaning the MAS.
       
    6. B94Sport

      B94Sport Well-Known Member

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      I just noticed it this morning since I normally just keep the reading set to one sensor and don't both to switch back and forth and compare the two... But I did install that switch thinking it could help with diagnostics :) I usually have it set to monitor the driver's side but I switched it this morning just to see and sure enough the passenger's side was acting odd.

      I cleaned the MAS about 3,000 miles ago... I have a thread about my rough idle/loss of power problems here, where I have detailed those and am taking suggestions for fixing them... Right now it is looking like I will get new plugs and wires, clean the IAC, replace the PCV valve, retorque the lower intake manifold bolts, and replace my fuel filter.
       

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