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LAF Sensor Jim Moore ASPIRE Inc. Honda LAF Sensor Allows the stoichiometric 14.7:1 AF/R under some conditions But can monitor AF/R over a much greater range This allows it to operate at as lean as 23:1 Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 2 Zirconia O2 Sensor Operation The Zirconia O2 Sensor used on most vehicles operates in a very small window about .3% above and below 14.7:1AF/R A thimble shaped Zirconia element has two sides called electrodes The inside of the thimble is the reference air chamber or where the positive lead connects, vents to ambient air. The outside of the thimble is exposed to the exhaust side and connects to the sensor ground. Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 3 Zirconia O2 Sensor Operation In a chemical battery a flow of ions internally causes a flow of electrons outside the battery In the O2 sensor thimble the oxygen ions that flow through it create the return signal Oxygen ions are oxygen atoms with a charge Outside air has more oxygen than the exhaust creating an ion flow and eventually the O2 signal Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 4 By controlling the O2 in the diffusion chamber you control the operating range Chamber one is outside the chamber in contact with the exhaust Diffusion Chamber is between the Zirconia elements The air reference is at the other end Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 5 Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 6 Lean Air Fuel The LAF sensor has two Zirconia elements that share a diffusion chamber The LAF sensor is actually two O2 sensors in one One Zirconia element acts as an oxygen pump Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 7 What do we mean when we talk about an oxygen pump? Remember a flow of ions creates a flow of electrons The inverse is also true, a flow of electrons causes a flow of ions, charging the battery Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 8 4.3K Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 9 LAF Sensor Operation To distinguish the two Zirconia we will call them S1 and S2 The ECM monitors the voltage between S1 input and the ground lead It tries to hold the voltage difference between S1 and ground to 450 mV When the mixture goes rich, oxygen ions flow from the diffusion chamber to the exhaust Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 10 LAF Sensor Operation The voltage on S1 input increases The ECM detects the change and reduces voltage on S2 input This causes the voltage on S2 to go more negative than the ground voltage This causes S2 to pump oxygen out of the diffusion chamber to the air chamber At the same time the ECM drops voltage on S2 input, its also reducing fuel delivery Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 11 LAF Sensor Operation As the mixture goes leaner, oxygen ions flow from the exhaust into the diffusion chamber The voltage on S1 input decreases The ECM sees the voltage drop and increases the volt to S2 input The voltage on S2 is now more positive than the ground voltage Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 12 LAF Sensor Operation S2 now pumps oxygen into the diffusion chamber from the air chamber The voltage between S1 input and ground is held to 450 mV The ECM can determine how rich or lean the exhaust is by monitoring the amperage it takes S2 to maintain 450 mV on S1 input Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 13 LAF Sensor Operation On a standard O2, sensor 450 mV indicates 14.7:1 AF/R On the LAF sensor, the exhaust is at a known value when S1 is at 450 mV When S1 is at 450 mV and S2 is at zero volts, the mixture is at 14.7:1 AF/R If S2 is negative its richer than 14.7:1 If S2 is positive its leaner than 14.7:1 Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 14 Testing the LAF sensor Hook a DSO positive lead to the S2 (white wire) of the LAF sensor and the ground lead to the LAF sensor ground (black wire) Do not use a vehicle ground you will see nearly 2.7 volts when hooked up this way A DMM could be hooked to the S1 terminal and again to the sensor ground to monitor the 450 mV Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 15 Testing the LAF sensor DSO settings – – – – – Voltage Scale: 500 mV per division Time Base: 200 mS per division Trigger Level: Auto on Trigger Slope: Either way Put the zero voltage level in the middle of the scope display Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 16 Verifying the LAF sensor Be sure the engine is hot and the LAF sensor is fully warmed up run at 2000 RPM – Enrich with propane the LAF sensor voltage should go negative record the reading – Remove the propane quickly, the voltage goes positive Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 17 Authored by: Joseph Marchesani Copyright 1996 Aspire Inc. 18