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  Air-Fuel Ratio Monitor The following information was taken from the Auto Meter website (www.autometer.com/hp/). Auto Meter makes our Air Fuel Ratio Monitor.
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.
A bit of clarification on how they work and what they look like when they are working.
We get tons of calls, emails, and letters about what this gauge does and how it works, and what it is supposed to look like. In its simplest form, the gauge is a color coded voltmeter, which lights up the LED(s) that correspond to the voltage that the gauge is sent via its signal wire. On most late model vehicles, the sensors are quite sensitive, and capable of responding very fast to a change in A/F ratio. This makes it appear that the LED's flicker in the different ranges, and this is normal. The gauge is showing the voltage it is receiving from the sensor, so more often than not it will appear that more than one of the LED's are lit, or that the readings are being shown to be in more than one section. Provided the gauge is connected properly (good power and ground) and that the connection to the O2 sensor signal wire is SOLDERED, this operation is normal.
Usually, the only time the gauge will not show so many LED's is when the engine is at WOT. Instead, the gauge will pin the last few green LED's of the rich section. With time, as the O2 sensor degrades in sensitivity, the readings may be more spread out on the gauges display. Also, this gauge is not in any way designed to be used as a tuning tool, but more of an aid for the overall tuning of the engine. This gauge, when used in conjunction with either an EGT gauge or even 'reading' the condition of the plugs will help you determine the optimal A/F mix for your vehicle. Keep in mind the above only pertains to computer controlled, later model vehicles. For the rest of the carb fed engines out there, you will not see the exact same operation. Instead, the readings will be less diverse, as in the gauge will not illuminate a number of LED's, actually only a few or even one at a time will be lit. This is due to the lack of a computer in place to both monitor and adjust the A/F mix, so the readings are not quite as sporadic, and thus aiding with the overall tuning of the engine.
A couple of things to look for when connecting your A/F ratio gauge to your vehicle are the O2 sensor itself, and it's current condition. Normally, an oxygen sensor is designed to last about 50,000 miles. However, its life can be shortened by contamination, blocked outside air, short circuits, and/or poor electrical connections.
The oxygen sensor can become contaminated by, but not limited to the following:
Leaded fuel - leaded fuel is the most common cause of O2 sensor contamination. Lead particles can coat the ceramic element and the sensor cannot produce enough voltage output for either the computer or the gauge.
Silicone - Sources are antifreeze, RTV silicone sealers, waterproofing sprays, and gasoline additives. Silicone tends to form a glassy coating.
Carbon - Carbon contamination results from an excessively rich fuel mixture. Carbon in the fuel can coat the sensor, too.
With this in mind, you may want to visually inspect the O2 sensor itself. When doing so, check to make sure that the outside of the sensor and its electrical connection(s) are free of oil, dirt, undercoating, and other deposits. If outside air cannot circulate through the O2 sensor, it simply will not be able to function.
Also keep in mind that the O2 sensor only generates anywhere from 0-1 volt, and averages around .5 volts. A poor or deteriorating electrical connection is not only frustrating but could also prevent this small voltage from reaching the computer and the gauge, too. Always be sure to check the electrical connections as well.
Testing the O2 sensor output is another way to ensure proper operation. A digital voltmeter can be used to test the output of an O2 sensor. WARNING! Be sure to only use a high impedance digital multimeter to measure the O2 sensor voltages. A conventional analog or low resistance meter can draw too much current and potentially damage the sensor itself. So, for testing, be sure to warm the engine to full operating temperature to shift the computer into closed loop. The sensor must be hot(at least 600 degrees or hotter) to operate properly. You may have to warm the engine at fast idle for 5-10 minutes with some cars to reach the target temp. Note that a few systems can drop out of closed loop at idle, so you may have trouble keeping the engine at a hot enough temp to see the readings you are looking for.
Just to reiterate, we STRONGLY recommend that when you make termination to the A/F ratio input wire on your vehicle that you solder the wires together, so that the connection is permanent, and also there is no potential for wire breakage or contamination of the signal transfer. PLEASE also be sure to test the signal wire BEFORE you solder the wires together, by twisting the wiring together; this will work for testing to ensure proper connection prior to the final termination.
Note from IAP: We are in the process of testing Air Fuel Ratio Monitors in carbureted and injected Alfa Romeos and Fiats, but for now, we refer you to the above Tech Tips from Auto Meter. |
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