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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

I won't even begin to try and explain how the computer processes this information, any more than I can explain how the computer on my desk turns my keystrokes into words on a screen. Suffice it to say it can and it does. Stored inside the computer is a set of values. These are the base values the computer uses to compare actual conditions to. Some computers have the ability to "learn." In other words, it keeps track of the way you drive and adapts itself to those conditions. It also uses these base values to check the sensors. If a sensor goes out of range of these values, it sees it as a malfunction and stores a trouble code. It will also turn a light on in the instrument panel to alert the driver that it has found a possible problem.

In the case of a sensor malfunction, the computer may go into a "fail safe" or "limp in" mode. For example, the Crank Angle Sensor dies and no longer sends a signal to the computer. The computer sees the signal is missing and notes the malfunction code and turns on the check engine light.

Now lacking that CAS, signal it goes to the base values stored in the computer and uses that value to control the ignition timing. It's not perfect, but it doesn't leave you stranded on the side of the road. Quite often the computer limits engine RPM to about 2500 RPM to prevent any possible engine damage and further alert the driver to a problem.

So what does the information from the sensors mean to the computer, and how does it affect the output? Let's go sensor by sensor and see what it means to the computer.

Mass Air Flow Sensor (MAF)
The mass airflow sensor tells the computer how much air is coming into the engine. The more air that enters, the more fuel is needed to get an ideal air/fuel ratio. At idle there is only a small amount of air entering so less fuel is required. At higher speeds, more air so more fuel is needed. To do this the computer will open the injectors for a longer period of time.

Intake Air Temperature Sensor (IAT)
The intake air temperature sensor tells the computer the temperature of the incoming air. Since cold air is denser than warm air, it needs more fuel to achieve the ideal air/fuel ratio. To do this, the computer will open the injectors for a longer period of time.

Camshaft (or Crankshaft) Position Sensor (CPS)
The camshaft position sensor tells the computer where the piston is. When the piston is in the proper position, the computer will fire the plug for that cylinder. It also uses this information to determine the sequence and times the injectors fire.

Coolant Temperature Sensor (CTS)
The engine coolant temperature sensor tells the computer what the operating temperature of the engine is. When the engine is cold, it needs more fuel to operate correctly, and if it is cold enough, it will inject a large quantity of fuel to start the car. This is called Cold Start Enrichment and replaces the function of a choke.

Knock Sensor
The knock sensor is attached to the cylinder block. It senses engine knocking using a piezoelectric element. A knocking vibration from the cylinder block is sensed as vibrational pressure. This pressure is converted into a voltage signal and sent to the computer, which will retard the timing to eliminate the knock.

Part 1       » Part 2       » Part 3

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

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