The digital motor electronics (DME) engine management systems in BMW E83 X3 vehicles with an M54 engine use the engine control module (ECM) to control fuel injection, ignition and other important drivetrain functions. The ECM sequentially triggers the fuel injectors to spray fuel into the intake ports and then, a split-second later, triggers the ignition coils to fire the spark plugs. This sequence of events is timed by using the crankshaft position. Using the signal from the crankshaft sensor, the ECM "knows" which cylinder is ready for fuel intake and then, later, for ignition.
The crankshaft position sensor is located near the starter motor, mounted in the engine block. The sensor reads a toothed reluctor wheel mounted to the end of the crankshaft. It then sends a signal to the DME used to identify cylinder location. If this signal is missing, for example from a faulty crankshaft sensor, then the engine will not start. You can consider the signal from the crankshaft sensor the RPM signal for the DME.
There are many reasons an engine may refuse to start. Before you decide to replace the sensor, it's a good idea to test it to make sure it's the reason the engine won't start. This article will show you how to test the crankshaft sensor.
Keep in mind that when your car was serviced before, parts may have been replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have, so be prepared with different size sockets and wrenches.
Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Do not work on your vehicle if you feel the task is beyond your ability.
Vehicle models change and evolve as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.
The crankshaft position sensor (green arrow) is located near (below) the starter motor (yellow arrow), mounted in the engine block (crankcase).
Terminal 1 is the sensor supply voltage (red arrow). Terminal 2 is the sensor reference voltage / signal wire (yellow arrow). Terminal 3 is the ground for the sensor provided by the DME (blue arrow). The wiring color and DME terminal locations may vary. Check your model against a wiring diagram.
Turn the Key ON, but do not start the engine. Connect your DVOM black lead to battery negative. Place the DVOM on D/C volts (red arrows). The red lead will be used to probe the electrical connector (red arrows). Pin 1--12 volts. Pin 2--5 volts, signal. Pin 3 ground, around zero volts.
Monitor the DVOM for voltage to fluctuate from 5 volts (blue arrow) to zero volts (yellow arrow) as the engine rotates. If voltage stays at 5 volts or 0 volts while you rotate the engine, and does not fluctuate, the sensor is faulty. Be sure your backprobe (red arrow) doesn't fall out during testing. This will give you false results.
Testing with sensor removed: You can also test the sensor with it removed from the engine. Turn the Key ON, but do not start the engine. Remove the sensor from the engine. See our tech article on crankshaft sensor replacing. Connect your DVOM black lead to battery negative and place the DVOM on D/C volts. The red lead will be used to probe the electrical connector. Connect the camshaft position sensor electrical connector. Insert a backprobe into terminal 2. Then connect the red lead of your DVOM to backprobe. Hold the sensor while moving a wrench across the tip. Monitor the DVOM. The voltage should fluctuate just as it should when rotating the engine by hand. In this photo the wrench (blue arrow) is away from the sensor (red arrow), so the DVOM reads 5 volts.
Testing with sensor removed: With the sensor removed, plug the electrical connector back into the sensor. Then connect a BMW scan tool, turn the key to run but leave the engine off. Navigate to engine data that displays engine RPM. Use a large wrench and move it toward and away from the tip of the crankshaft sensor, avoiding contact but coming close. Move the wrench quickly, you will feel the magnetic field. As you move it, the scan tool RPM parameter should go from 0 RPM when wrench is static to an RPM reading of 20 up to 1500 RPM (red arrow). Depending on how well you break the magnetic field. You can also test using a BMW scan tool with the sensor installed or removed. Installed, cranking RPM should be about 300 RPM or actual cranking RPM (depending on battery voltage).