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Most manifold pressure sensors have a couple of cor-rective features to account for differences in temperature and altitude. The Intake Air Temperature (IAT) sensor signals the PCM to correct injector pulse width for colder, denser air. Altitude compensation is achieved by venting one cell to the atmosphere. In a part-load range, both manifold pres-sure and atmospheric pressure are reduced as the altitude increases, so the fuel injection signal must be adjusted for thinner air. Some manufacturers no longer regard manifold-pres-sure sensing as a sufficiently accurate measurement of engine load. Many modern systems now measure airflow or air mass instead. But manifold pressure sensing is still widely used by GM, Chrysler, Honda, Toyota and other manufacturers. And some manufacturers equip their engine management systems with both MAP and MAF sensors.

Mass Air Flow (MAF) sensor The MAF sensor (see illustrations 5.33a, 5.33b and 5.33c), also known as a hot-wire sensor, is a black plastic or cast aluminum housing located in the intake tract somewhere between the air filter housing and the throttle body. (Some manufacturers, such as Mitsubishi, refer to the MAF sensor as the Volume Air Flow (VAF) sensor, but its the same device.) If you remove the sensor for a better look, youll see small platinum resistance wires suspended inside the cylinder. The wires are smaller in diameter than a human hair - about 70 micrometers, or less than 1/10 of a millimeter. Each wire looks - and is - delicate. It looks like it could break rather easily just from normal vibration. But in actual service, this type of sensor has proven more reliable than the old Bosch vane-type airflow sensor. An ingenious suspension system prevents the wires from snapping in two and a pair of mesh protective screens at either end of the sensor cylin-der protect the wires from damage from incoming particulate matter as well as from backfires. In the unlikely event that a

wire should break, the engine will run - albeit in a limp-home mode - well enough to get you home. (You can simulate limp-home mode by unplugging the air-mass sensor con-nector on a warmed-up engine, then driving the vehicle). The MAF sensor is completely electronic - there are no moving parts. It measures airflow as a function of the amount of current flowing through heated wires. It gets its hot-wire name from this heated-wire design. The MAF sen-sor has several significant advantages over the vane-type airflow sensor used on pre-OBD-II systems. First, it measures air mass, or weight (in physics, mass and weight arent exactly the same thing, but theyre propor-tional, so for the purposes of this discussion, well use them more or less interchangeably). The air-fuel mixture ratio is really the ratio of the masses of the substances involved. A certain mass of fuel is mixed with a certain mass of air.

5.33a The Mass Air Flow (MAF) sensor is located either on the air filter housing . . .

5.33b . . . between the air filter housing and the air intake duct . . .

1 Electrical connector 2 Mounting screws

3 MAF sensor 1 Electrical connector 2 Hose clamps

3 MAF sensor

5.33c . . . or somewhere in the air intake duct itself

1 Electrical connector 2 Hose clamps

3 MAF sensor

Powertrain management basics

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Unlike a MAP sensor, which measures the absolute pres-sure inside the intake manifold, measuring the air mass also eliminates the need for compensation sensors for air tem-perature and altitude. And eliminating compensation correc-tions simplifies the PCM program. An air-mass sensor has no moving parts. The system monitors the cooling effect of intake air as it moves across the heated wires. Imagine a fan blowing across an electric heater. If the fan motor is set to its Low position, the cool-ing effect on the heater wires is minimal; turn the fan motor to its High position and the cooling effect increases. The control circuit uses this effect to measure how much air is passing over the hot wires. The wires are heated to a spe-cific temperature differential that is 180-degrees F. above the incoming air when the ignition is turned on. As soon as air begins to flow over the wire, the wire cools. The control circuit applies more voltage to keep the wire at its original temperature differential. This creates a voltage signal that the PCM monitors. The greater the air flow, the more the wire cools and the greater the voltage signal. A hot-wire sensor responds much more quickly than the hinged flap inside a vane-type airflow sensor. Changes in air mass are followed by corrected measurements within 1 to 3 milliseconds. The wire in a MAF sensor offers virtually zero resistance to the air moving past it. Even at maximum air-flow, the drag on the wire, measured in milligrams, is insig-nificant. Air mass measurement by hot wire improves drive-ability, stability and reliability. Even racers use it!

Output shaft speed sensor See Input Shaft Speed (ISS) and Output Shaft Speed (OSS) sensors.

Oxygen sensors The oxygen sensor (see illustration 5.34a), once referred to as a Lambda sensor (Bosch), an exhaust gas oxygen (EGO) sensor (US manufacturers) or simply an O2 sensor, is the most important information sensor on a fuel-injected vehicle. The oxygen sensor compares the differ-ence between the amount of oxygen in the exhaust and the amount of oxygen in the ambient air, and it expresses the results of this comparison as an analog voltage signal that varies between 0 and 1 volt. An oxygen sensor is a galvanic battery that generates a small variable voltage signal in proportion to the difference between the oxygen content in the exhaust stream and the oxygen content in the ambient air. The PCM uses the volt-age signal from the upstream oxygen sensor to maintain a stoichiometric air/fuel ratio of 14.7:1 by constantly adjust-ing the on-time of the fuel injectors. There are two oxygen sensors for each catalytic converter: one upstream sensor (ahead of the catalyst) and a downstream oxygen sensor (on or right behind the catalyst). The oxygen sensor is based on the Lambda concept pioneered by the Robert Bosch Corporation. Lambda is the Greek symbol which engineers use to indicate the ratio of one number to another. When discussing the control of the air-fuel ratio, lambda refers to the ratio of excess air to stoi-chiometric air quantity. (Thats why Bosch calls the oxygen sensor a lambda sensor). When the maximum amount of available air is com-bined with fuel at the stoichiometric (ideal) air-fuel ratio of 14.7:1, theres no air left over, and theres no shortage of air. Lambda, therefore, equals 1. But if the mixture ratio is lean, say 15, 16 or 17:1, theres air left over after combustion. The lambda ratio of excess air to the ideal amount of air is now

5.34a Typical oxygen sensor, also known as a Lambda sensor,

Exhaust Gas Oxygen (EGO) sensor or simply 02 sensor:

1 Contact 2 Supporting ceramic 3 Sensor ceramic 4 Protective tube (exhaust side) 5 Electrical lead 6 Disc spring 7 Protective sleeve (air side) 8 Housing (-) 9 Electrode (-) 10 Electrode (+)