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Sense of Sensors: Part 2

    There are a lot of things going on in your engine as you drive. A lot of information is taken in and processed. But how is this information gathered, and what happens to it once it's collected?
» Part 1: Making Sense of Sensors
» Part 2: The Sensors
» Part 3: More Sensors

Heated Oxygen Sensor (HO2S)
The HO2S is placed into the exhaust manifold. It detects the amount of oxygen in the exhaust gas compared to the outside air. The sensor has a closed-end tube made of ceramic zirconia. The zirconia generates voltage from approximately 1V in richer conditions to 0V in leaner conditions. The sensor signal is sent to the ECM. The ECM adjusts the injection pulse duration to achieve the ideal air-fuel ratio. The ideal air-fuel ratio occurs near the radical change from 1V to 0V.

Throttle Position Sensor (TPS)
The throttle position sensor tells the computer where the gas pedal is. The computer determines the load being applied to the engine and adjusts injector and ignition timing as required.

Vehicle Speed Sensor (VSS)
The vehicle speed sensor tells the computer how fast the vehicle is moving. The computer then adjusts injector pulse to maintain the proper fuel mixture at any speed and load. It also sends a signal to the speedometer and, in most cases, to the cruise control computer.

Power Steering Pressure Switch (PSPS)
The power steering oil pressure switch sends a signal to the computer that there is an extra added load on the engine, and the computer will raise the idle speed to compensate. The air conditioning switch will also send a similar signal to the computer, and the computer will raise idle speed to compensate.

Manifold Absolute Pressure (MAP)
The Manifold Absolute Pressure sensor measures changes in the intake manifold pressure resulting from engine load and speed changes. The computer sends a 5-volt reference signal to the MAP sensor. As pressure changes in the intake manifold occur, the electrical resistance of the MAP sensor also changes. By monitoring the sensor output voltage, the computer can determine the manifold absolute pressure. The higher the MAP voltage output, the lower the engine vacuum, which requires more fuel. The lower the MAP voltage output, the higher the engine vacuum, which requires less fuel. Under certain conditions, the MAP sensor is also used to measure barometric pressure. This allows the computer to automatically adjust for different altitudes. The computer uses the MAP sensor to control fuel delivery and ignition timing.

Cranking Signal
The control module uses this signal to tell when the vehicle is in the STARTING mode. This information is used to allow enrichment and cancel diagnostics while engine is cranking.

The computer does a lot more than these basic functions I've mentioned here. It sends signals to control transmission shift timing, cruise control and other things. All I have attempted to do here is give you a basic understanding of how the inputs, the computer and outputs relate to each other.

It is important to keep in mind that when a trouble code indicates a component has failed, it does not necessarily mean that it is indeed the component itself. You still need to check the whole circuit to find out if it is a bad part or bad wiring. The most common part to be replaced needlessly is the O2 sensor. When a sensor goes out of range and sends bad information to the computer, the O2 sensor will try to compensate for it. This often results in a malfunctioning O2 sensor code. In cars with early self-diagnostics and OBD I, this was easy to do. With OBD II it has become a little more difficult to fault a good O2 sensor since the OBD II codes are more specific.

Part 1       » Part 2       » Part 3

Additional Information provided courtesy of and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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