Troubleshooting a Crankshaft Sensor of a Pontiac Grand Prix GT

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Introduction to Crankshaft Sensors

  • The crankshaft position sensor on a Pontiac Grand Prix is a small electro-magnetic device that is used to determine relative position of an object. It does this through the use of a trigger wheel composed of a series of raised points, similar to a star shape, but with a multitude of different shapes in place of the tips of the star. Every time the raised steel point comes close to the sensor, the magnetism created causes an electrical pulse. As the raised point passes the sensor, the pulse subsides. It acts much like a switch turning on and off. The computer uses this pulse to determine the relative position of the number 1 cylinder in an engine and all remaining cylinders, as well as the RPM of the engine. This determines the firing time for the spark plugs. These same type of sensors are used in every walk of life in millions of applications from a CT Scan machine in the hospital to an elevator to determine what floor it is on.

Failure of the Crankshaft Sensor

  • The sensor in the Grand Prix is a 3-inch cylindrical sensor located on the side of the engine facing the firewall, directly under the exhaust manifold. It is inserted into the block and held in with a 10-mm bolt. It is a 3-wire sensor with one purple, one light green and one light blue with a black stripe wire. This sensor has a tendency to fail around the 95,000-mile mark, although that's not written in concrete. It could fail for various reasons at any point. When this happens, the computer will not get a reference for the timing of the ignition and the vehicle will either run rough or not at all, depending on the degree of failure of the sensor. This out-of-range pulse will be identified by the computer and considered a failure of the sensor. The computer will turn the check engine light on to alert the driver of an impending or out-and-out failure.

Diagnosis and Reading Codes for the Crankshaft Sensor

  • Any time the check engine light comes on, a code scanner is needed to interrogate the computer and access the codes for the problem. The computer will set a code for anything that it operates that gives a signal that is out of the normal. These codes are 4-number codes that must be translated into a definition of the problem. A code sheet comes with the code scanner and explains all the codes. The code scanners can be purchased at any auto parts store and are very inexpensive. One scanner will cover most all cars.

    To use the scanner, simply plug it into the connector under the driver's side dash panel. In most cases, the connectors are easy to see and made easily accessible. Once this is done, turn the ignition key on with the engine off. Press the "Read" button and a number code will appear. Cross reference this code with the code sheet and the explanation will come up on the screen.

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