Making Sense of Sensors: Part 2
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.
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.
