September 1999 Articles & Features

Launch Tech

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Foreign Service

By Dan Marinucci | September 1999

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This month, Dan begins a series on Honda's lean air/fuel ratio sensor, a smarter, more fuel-efficient variant of the conventional oxygen sensor.

You may hear your Honda buddies talk about lean-burn engines and think, Lean burn? That's a blast from the past! But the systems they're describing go far beyond Chrysler's electronic spark advance system of the mid-1970s.

Rather, lean-burn Hondas boast cutting-edge technology you'll probably see on other engines in the future. These powerplants use a variation of the traditional oxygen sensor called a lean air/fuel ratio sensor, or LAF. The LAF's flexibility allows engines to run anywhere from a 12:1 air/fuel ratio up to 23:1 while in closed-loop! This means better efficiency over a much wider range of engine rpm and load. The LAF, a distinctive five-wire sensor, threads into the same spot as the traditional oxygen sensor.

Furthermore, these high-efficiency Hondas may have already rolled into your shop. The 1992-95 Civic VX and 1996-98 Civic HX are equipped with an LAF sensor. Honda specialists can think of these lean-burn Civics as descendants of the Civic HF sold in the 1980s. The HF, which we covered in previous columns, was a legitimate 50-mpg contender that also performed well.

In this and subsequent columns, we'll explain LAF operation and show you how to test it.

A Little Background

The LAF is really one traditional oxygen sensor piggybacked upon another. So it makes sense to review basic O2 sensor operation before we continue.

Robert Bosch created the familiar zirconia oxygen sensor back in the '70s. This common O2 sensor generates a voltage signal based on oxygen differential. The outside of the sensor is immersed in the exhaust stream; the inside is vented to the outside air. Outside air contains about 20% oxygen, the exhaust about 1% to 2%, but this percentage varies according to operating conditions.

Ions are electrically charged particles. The way a conventional O2 sensor is constructed, oxygen ions flow through the sensor, where there's a difference between the oxygen inside and outside. When the ions flow, a voltage is induced. The greater the difference between the inside and outside oxygen, the greater the ion flow and thus the stronger the voltage signal the sensor supplies.

For instance, a rich mixture leaves little oxygen in the exhaust because the engine consumes most of it trying to burn up the fuel. The relatively big difference between the inside and outside oxygen creates a relatively high sensor signal-approaching 1.0 volt.

A leaner mixture, on the other hand, leaves some unburned oxygen in the exhaust, reducing the difference between the inside and outside oxygen. Reducing the inside/outside difference causes a weak sensor signal-near 0 volts.

A conventional oxygen sensor is relatively limited because it works only within a narrow range of air/fuel ratios close to stoichiometric (14:1). In other words, it doesn't respond outside this range. In fact, the zirconia sensor's signal drops to 0 volts when the mixture goes slightly leaner than stoic. Likewise, its signal jumps to nearly 1.0 volt when the mixture goes slightly richer than stoic.

We all know that fuel burns most efficiently at stoic and catalytic converters operate most efficiently when the air/fuel mix is held at or near stoic. But an engine won't perform well under all driving conditions or give optimum gas mileage if we hold its mixture at stoic full time. When driving conditions call for something beyond stoic, a typical PCM goes open-loop and reverts to preprogrammed air/fuel ratios.

LAF Sensor Operation

The LAF sensor improves overall efficiency by keeping the fuel control system in closed-loop during a much wider range of driving conditions. So instead of using preprogrammed, open-loop air/fuel ratios in many situations, the ECM fine-tunes the mixture much more closely based on the actual oxygen readings from the exhaust.

The illustration on page 18 shows how the LAF piggybacks two sensors, which we'll call Sensor 1 and Sensor 2. The outside of the entire sensor (the outside of Sensor 1) senses the exhaust stream. The inside of the sensor (the inside of Sensor 2) is exposed to the great outdoors. Meanwhile, the blue-shaded diffusion chamber, which is sealed off from outside air, separates the two sensors.

Using this approach, Sensor 1 doesn't compare exhaust oxygen content to outside air anymore. Instead, it compares it to diffusion chamber oxygen, which the PCM controls.

Let's look at the three wires on an LAF: The Cell Voltage Input wire works just like the signal wire on a conventional oxygen sensor because it carries the signal from Sensor 1 to the PCM. Furthermore, Sensor 1's signal reacts just like a standard O2 sensor: A rich condition drives voltage up, lean drives it down. Just remember the critical difference between Sensor 1 and a regular oxygen sensor: Sensor 1's voltage comes from the oxygen difference between the exhaust and the diffusion chamber, not the difference between exhaust and outside air!

The Pump Cell Control wire, which the PCM uses to control the oxygen content of the diffusion chamber, goes from the PCM to the outside of Sensor 2. Pump refers to electrically increasing or decreasing diffusion chamber oxygen. By applying voltage to this wire and pushing current through the diffusion chamber, the PCM adjusts oxygen content as needed to maintain a .45-volt signal at Sensor 1. This also means the PCM is holding the difference between the Cell Voltage Input wire and the Reference wire as close to .45 volt as possible.

The Reference wire appears to be a common ground linking Sensor 1 and Sensor 2 with the PCM. But as you'll find out later, it behaves more like a reference voltage wire than a true ground wire. Never confuse this wire with chassis ground, either!

Now let's follow the LAF sensor system as it responds to a richening mixture. As the mixture goes rich, Sensor 1 signal voltage increases on the Cell Voltage Input wire. (Remember, the PCM wants to hold the voltage on this wire as close as possible to .45 volt.)

The PCM responds by reducing the voltage applied to the Pump Cell Control wire. It may force voltage on this wire into the negative range to adjust diffusion chamber oxygen to the desired level. To simplify, lowering voltage on the Pump Cell Control wire reduces diffusion chamber oxygen. In turn, this lowers Sensor 1's signal. Sensor 1 voltage then drops back to the preferred range of .45 volt and the closed-loop cycle continues. (By the way, increasing the voltage on the Pump Cell Control wire corrects a lean condition.)

So how does the PCM know what the air/fuel ratio really is? It monitors the Pump Cell Control wire voltage/current required to hold Sensor 1's signal at .45 volt. If this technique sounds remotely familiar, it should. Some PCMs sense air volume by tracking the current needed to keep an airflow sensor wire at a specified temperature.

Next month we'll cover diagnosis and show you both scan tool readings and oscilloscope patterns from a LAF Civic. See you then!