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What does this sensor do? -
Oxygen SensorOxygen Sensors are also commonly referrred to O2 sensors. Oxygen sensors are placed in the exhaust to measure the fuel mixture to tell the computer wether the motor is running rich or lean. Rich meaning the motor is getting to much fuel, or not enough air. Lean meaning the motor is not getting enough fuel, or getting to much air. With this data the computer can make necessary adjustments to the air fuel mixture. Oxygen sensors report the fuel mixture as a voltage, normal O2 sensors go from 0-1 volt. At an ideal air/fuel mixture an O2 sensor will put off 0.5 volts. If the mixture is rich it will be greater than 0.5 volts, the higher the voltage the richer the motor is running. If the mixture is lean it will be less than 0.5 volts, the lower the voltage the leaner the motor is running. Now there are three types of O2 sensors. Un-heated, Heated, and Wideband. O2 sensors have a minimum temperature they have to be to read properly, un-heated work fine once the car has reached operating temp, but can be a little inaccurate when cold. Heated pre-heat the O2 sensor so it reads properly instantly. Wideband are also heated and are the next generation O2 sensors, they are cable of much higher accuracy when reading the air/fuel mixture. Standard O2 sensors should be replaced every 30,000 miles, heated O2 sensors should be replaced every 100,000 miles. Typically a bad O2 sensor will only affect fuel economy and emmissions.
Throttle Position SensorThrottle Position Sensors are also commonly referred to as a TPS. The throttle position sensor tells the computer where the gas pedal is. If you have the gas pedal planted to the floor the ECM knows and makes the necessary adjustments to the fuel mixture. Newer automatic transmissions also use the TPS data to help determine both up shifts and down shifts. A TPS is nothing more than a rheostat thats mounted to the throttle body. Typically a TPS measures the pedal travel in both directions, both up and down, as the pedal travels the TPS changes in resistance. So as you are pressing down on the pedal one side of the TPS is increasing in resistance, while the other side is decreasing in resistance. Most TPS sensors have three pins, one 5 volt power in, one pin that goes from 0-5 volts, and one pin that goes from 5-0. volts. Idle measured on one side of the TPS might be 0.5 volt, while wide open throttle might be 4.9 volts, on the opposite side of the TPS idle might be 4.9 volts, while wide open throttle might be 0.4 volts. Its measured in two directions for accuracy. When a TPS sensor goes bad, you'll get read spikes or drops in resistance changes. For example, you might get to mid-throttle and all the sudden it reads wide open throttle, then it might go back to reading mid-throttle again. A TPS can be tested off the vehicle with a ohm-meter, or on the vehicle with a volt-meter. Symptoms of a bad TPS can be hesistation, erratic shifting, hard starting, sputtering, erratic idle, and poor fuel economy. My writeup on testing TPS sensors can be found here.
Manifold Absolute Pressure SensorManifold Absolute Pressure Sensors are also commonly referred to as a MAP sensor. The MAP sensor measures the pressure in the intake to help the computer make necessary changes to the fuel mixture to correct for altitude and and temperature changes. The MAP sensor is reading the barometric pressure minus the vacuum created by the motor. The main goal here is for the computer to have a way to measure the density of the incoming air. The more dense the in coming air is the more fuel the motor needs, if the air is less dense, less fuel is needed. MAP sensor is a critical piece in determining the proper fuel mixture, so if you have a bad MAP sensor, it can affect the way the vehicle runs drastically. I have a writeup on testing MAP sensors here.
Crankshaft Position SensorThe Crankshaft Position Sensor, or CPS, is nothing more than a hall effect sensor. The sensors have magnet inside that activates a switch. The crankshaft position sensor is mounted very close to the flywheel (or flexplate on automatic transmissions). The flywheel or flexplate has notches in it, that mark when each cylinder is at top dead center. As the motor rotates the CPS will reach a notch and lose the magnetic field and the switch will disengage. The ECM keeps track of these pulses in order, to determine which cylinder to fire, injector timing, and engine speed. When a CPS goes bad most vehicles will not run at all, and will have no spark. Some newer vehicles will guess until the motor fires, but for the most part Jeeps don\'t run without a CPS.
Pick-up CoilThe Pick-up coil, is nothing more than a hall effect sensor. The sensors have magnet inside that activates a switch. The pick-up coil is found inside the distributor. The distributor has a ring in it with notches in it, that mark when each cylinder is at top dead center. As the camshaft turns the distributor the pick-up coil will reach a notch and lose the magnetic field and the switch will disengage. On older vehicles this pulse goes straight to the coil to fire it. On newer vehicles this pulse goes back to the ECM and is used in conjunction with the CPS to determine proper timing, and injector timing. When a pickup coil goes bad most vehicles will not run at all, and will have no spark. Some newer vehicles will guess until the motor fires, but for the most part Jeeps don\'t run without a pickup coil.
Mass Air Flow SensorThe Mass Air Flow Sensor, or MAF, is used to measure how much air is coming into the motor. 95% of the MAF out there today are a hot wire setup. These hot wire MAF sensors, use a heated wire or grid, as air comes in it cools the wire. As the wire cools the voltage is increased to keep the wire at a constant temperature. This change in voltage is used by the ECM to calculate how much air is coming into the motor. The voltage should always be between 0-5 volts. Typically most sensors read about 1.0-1.5 volts at idle, and about 4.5-5.0 volts at wide open throttle. While not the most accurate method of testing a MAF can essentially be tested by measure the voltage at idle, then crack the throttle open, you don\'t have to let the motor actually rev out, but the voltage should jump with the throttle jump. Most vehicles will run with a bad MAF sensor, but fuel economy and power will suffer, as the fuel mixture will be off.
Knock SensorA Knock Sensor is used by the ECM to detect detonation. Detonation is also referred to as spark knocking, or pinging. Detonation happens when a second explosion happens internally, after the intial firing of the spark plug. This extra explosion creates a shockwave that causes a vibration that reverberates in the cylinder. A knock sensor detects the vibration created by the detonation. Since detonation can actually destroy a motor, as soon as knock sensor detects detonation the ECM pulls out ignition timing to prevent any further detonation. A knock sensor can fail in two ways, it can go bad and not detect detonation, this can result in engine damage if detonation occurs, but other than that the motor will run fine. On the other hand the knock sensor can continually tell the ECM that the motor is detonating, this will result in very poor engine performance. Most knock sensors can be checked with an ohm meter, one lead on the center pin, the other on the body of the knock sensor, if the sensor has been removed tap it on a table and you should be able to see it open and close. If the sensor is still in the motor, tap the block with a hammer near the knock sensor it should open and close.

 
Related Writeups: | Multi-Meter Basics | Testing TPS Sensors | Testing MAP Sensors |
 
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