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How to Know Whether to Replace Your Camshaft Position Sensor or Crankshaft Position Sensor

by Don Bowman

The camshaft sensor and the crankshaft sensor, although both hall effect sensors, serve different purposes. They both can be diagnosed using similar procedures. Neither sensor should have any glitches or dropouts in their respective sine waves when checked on an automotive oscilloscope. These sensors cannot be accurately diagnosed as to their function without this equipment. It checks the length of the signal, the strength, purity, fluctuations and dropouts.

Pull the codes to check for any recognized failures stored in the computer by plugging the code scanner into the OBD (on board diagnostics) port. Nine times out of 10, it will be located to the left side of the steering column under the dash. If by chance it is not there, look behind the ashtray or cointray, such as in a Toyota. Turn the key to the "On" position without starting the engine. Press the key marked "Read," and the computer will interface with the computer and relinquish any stored failure codes. These codes will be displayed on the code scanner in a five-digit format, a letter followed by four numbers.

Cross-reference these codes with the code decipher sheet and an explanation of the code will be presented along with the particular sensor plus variables, such as the time of failure in engine starts since failure and type of failure. If a cam or crank sensor has failed, the check engine light should have illuminated and the computer should have set a code to that effect.

Watch the performance on a sensor scan tool. Turn the scanner on, and insert the year, make, model and engine size to bring up the correct sheet. The next question the tool will ask is what type of test desired -- press "component test." Scroll down to "Camshaft Sensor" and press the button marked "Enter." A list of tests on the camshaft sensor will be displayed. Scroll down to "Waveform Viewer." The oscilloscope is operational once the connections are made. When the waveform viewer is picked, it will give a description of the location of the best place to tap into the wires and which wires are which by color code.

Connect the black lead on the scanner to a good ground, and attach the red lead to the wire on the harness connector as described by the scanner. Start the engine, and observe the screen. The spikes should be very straight with no raged edges, the distance between spikes unison, and it should not show any obvious dropouts (screen goes blank). If by chance the vehicle does not start at all, scroll to the "No start condition test." The scanner will tell you where to find the wire to test the sensor for a complete failure and the color of the wire. Since the engine will not start, there is no waveform. It will check the sensor in terms of frequency and duty cycle as the engine turns over unable to start. If the frequency is there, the problem is elsewhere. If the signal is not there, the sensor has failed. The same procedure applies for the crankshaft sensor.

Diagnose the problem through the process of elimination. Taking into consideration that the cam sensor has a direct effect on the operation of the fuel injectors and the crank sensor on the ignition, check for spark at the plugs. Pull one of the spark plug wires off one of the plugs. Install an extra spark plug in the plug wire and lay the plug on the engine so it will get a good ground.

Have a helper crank the engine over, and watch for a spark at the plug. If there is spark, the crank sensor is working well enough to start the vehicle. If there is no spark, then the sensor is bad. If the engine will start but with difficulty and the power is very sluggish and the engine runs rather uneven (cannot hold a steady rpm), the camshaft sensor is likely the culprit. This is the best that can be determined without the proper tools. To narrow this down further, check the plugs, wires and vacuum hoses. If there are no problems found, the odds are excellent that the sensor is bad.

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About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).

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