Ohms drop on a crank shaft position sensor when heated up

Tiny
MEL BARTZ
  • MEMBER
  • 2000 JEEP GRAND CHEROKEE
  • 150,000 MILES
Turn the key and sometime the gauges will not work and truck will not start. Will shut off when running. Took out the crankshaft sensor and heated it up in water. The ohms dropped down to 1.003. Was on the understanding it was supposed to be between 700 and 900. Not sure.
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Tuesday, July 11th, 2017 AT 7:34 PM

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Tiny
CARADIODOC
  • EXPERT
Please list the engine size for an engine-related problem. In this case there are only two, but they use entirely different ignition systems.

The test you are trying to do has absolutely no relevance to the sensors used on these engines. Magnetic sensors develop a voltage pulse dependent on speed. They have two terminals in their connectors. You can measure the resistance of two-wire sensors, but no one does that other than a few educators who have their students do that to learn how they work. No professional would ever waste a customer's time and money testing a sensor because half will measure outside the specified value and work just fine, and half will test fine but not develop the necessary signal. The results of the test are usually meaningless. What you care about is if the signal is showing up at the engine computer, and the computer will tell you when it is not.

These magnetic sensors have a real long piece of fine wire wrapped around a magnet. Since the resistance of that wire is a factor of its diameter and its length, it can be expected to change a little when its temperature changes. How much it changes has no meaning, except if you find it suddenly goes open circuit. That would be caused by the wire breaking off one terminal inside when it or the core it is wrapped around expands.

Your cam and crank sensors have three terminals. That means there is a bunch of electronic circuitry inside. You cannot measure the resistance of the coil of wire or any other part inside the sensor. If you want to tell if it is working, you would need an oscilloscope to view the waveform it generates while cranking the engine. Even then, it will usually not be a nice clean square wave. You will not find a drawing of what the waveform should look like in any service manual or text book, because no one goes through that much work to learn nothing of value. With position sensors, the voltage of the pulses can vary a lot, but that is not the information that is used. It is the timing of when that pulse goes up or down that is used by the computer.

The other problem here is you observed the dash gauges stop working when the engine stalling or no-start occurs. The crankshaft position sensor has nothing to do with the instrument cluster. That means you have some other electrical problem related to something both systems have in common. I can think of two places to start looking. The first is to follow the smaller battery positive wire to the under-hood fuse box, and be sure that connection is clean and tight. Follow the smaller battery negative wire to the body and be sure that is tight and not rusty. The second is to see if anything else stops working when the problem occurs. The ignition switch is a good place to find an overheated contact, and two darkened terminals in the connector. If you find that, I can describe how to repair it.
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Tuesday, July 11th, 2017 AT 9:04 PM

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