The functional objective for the crankshaft position sensor is to determine the position and/or rotational speed (RPM) of the crank. Engine Control Units use the information transmitted by the sensor to control parameters such as ignition timing and fuel injection timing. The sensor output can also be related to other sensor data including the cam position to derive the current combustion cycle, this is very important for the starting of a four stroke engine.

A bad crank position sensor can worsen the way the engine idles, the pistons fire, or the acceleration behavior. If the engine is revved up with a bad or faulty sensor, it may cause misfiring, motor vibration or backfires. Accelerating might be hesitant, and abnormal shaking during engine idle might occur. In the worst case the car may not start.
From Wikipedia, the free encyclopedia

What is wrong if your 1990 Chevy Lumina won't crank?


My 1990 Lumina 3.1 had the same problem and it was caused by the crank sensor.
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How to Diagnose a Loss of Spark in Your Car Engine
Some newer cars don't have one,and if it doesn't then the ECM or computer sends signal via the crankshaft position sensor and the camshaft position sensor. ...
How to Test Crankshaft and Camshaft sensors

Sometimes the sensor may become burnt or worn out. The most likely causes of crankshaft position sensor failure are exposure to extreme heat when you have a gasket or crank problem, corruption from oil or other engine fluids that leaked onto the sensor, or wear over a long period of time. When it goes bad, it stops transmitting the signal which contains the vital data for the ignition and other parts in the system.
Chevrolet Lumina Wont Start