Crankshaft Position Sensor Testing
There are a number of common symptoms of a bad crankshaft position sensor that you can look out for to help you in determining whether or not your sensor has failed including engine misfires, rough idling, stalling, or hard starting just to name a few. If you are experiencing problems like these you might need to replace your crankshaft position sensor.
However, these same symptoms can be caused by problems with your ignition or fuel injection systems, too. So, before you replace your crankshaft position sensor you might want to run some tests to determine that the crank sensor really is the source of the problem. Here is how to test a crankshaft position sensor:
Onboard Diagnostic Codes
If your check engine light is on, then your ECU has recorded some sort of trouble code. You can check these codes with a diagnostic scan tool. Codes between P0335 and P0338 correspond to crankshaft sensor problems.
This is probably the most straightforward and the most certain way to test for and identify a crankshaft sensor problem. Unfortunately, the crankshaft sensor is usually pretty far gone by the time that the check engine light is on. You might already be experiencing crankshaft sensor problems before the ECU has noted any sign of trouble. If you want to be proactive, there is some other testing you can do.
This next crank sensor testing technique also uses the diagnostic scan tool. One of the settings of the scanner allows you to read the engine speed in revolutions per minute (RPM). Now, can you guess where the scanner gets that information from? If you guessed the crankshaft position sensor, then you would be correct. Set up the scanner to read engine RPM and crank the engine. The scan tool should read between 100 and 500 RPM. A bad reading indicates that the crankshaft sensor isn't working right. A reading of zero indicates that the crankshaft sensor has failed completely.
Of course, not everyone has access to a scan tool (although they can sometimes be rented from parts stores). A multimeter is a more common tool and a very useful one for diagnosing many electronic components in your vehicle. A multimeter can measure voltage, current, and resistance.
You can remove the sensor, and then test the resistance. Attach one end of the multimeter to each wiring lead of the sensor. Resistance of zero means that the there is a short circuit. Infinite resistance means there is an open circuit. Either one of those readings indicates that the sensor is not working. For any other reading, check it against the manufacturer's specifications. If your reading is not close to the recommended resistance, then you should replace the sensor.
Another way to test the crankshaft sensor with a multimeter is by checking the output voltage with the engine cranking. You will need an assistant to do this. Be very careful around moving parts as you do this. Probe the wiring connectors and measure the output voltage in AC millivolts. Typically, this reading is around 200 millivolts, but this can vary from vehicle to vehicle. Check the manufacturer's specifications. If there is no output voltage, then, obviously, your sensor is not working.
These tests will help you pinpoint the source of your problem. They will either prevent you from making a repair you don't have to, or confirm that the repair you're about to make is the one you really need. If the testing has indeed confirmed that your sensor has failed, check out this article on how to replace a crankshaft position sensor.