Camshaft Position Sensor (CMP)

General description 
      Camshaft position sensor is also called cylinder identification sensor or phase detector.
In consecutive fuel injection systems, ECU must determine which cylinder to fire next. This information is provided from the cylinder identification sensor. During the engine rotation the sensor sends a signal to the onboard controller whenever the first cylinder is at the top dead center (TDC). Thus the duration of the pulse injection is estimated.
      In simultaneous fuel injection systems, the onboard controller does not identify the cylinders and the firing order, because this is not necessary for the system to operate. When a crankshaft or distributor advance ignition signal appears, the exact cylinder is detected by recognizing the mechanical positions of the crankshaft, camshaft, valves or the distributor shaft.

Fig. 1 shows the sensor for cylinder identification.

Fig. 1

Used types of sensors
In accordance to its production design there are three different types of sensors:

  • Inductive
  • Hall Effect
  • AC output

      The induction phase sensor may be located inside the distributor or on the camshaft. Therefore near the camshaft a device with permanent magnet is mounted. Every time the magnet passes through the sensor, its magnetic field is changed and the resulting pulse is sent to the onboard controller for processing.
      Hall Effect sensor may be located inside the distributor or on the camshaft. Screen with a slot and magnet is mounted on the shaft. When the screen goes between the magnet and the hall sensor, sensor is turned on and off. While a slot is in front of the sensor, a voltage returns to the amplifier through a third signal cable. As long as in front of the sensor is a solid sector of the screen, the feedback voltage is interrupted, because the magnetic field is deviated.
      AC output sensor is different from the others in that the output is an AC voltage signal. The onboard controller generates very high frequency (between 150 and 2500 cycles per second) to the exciter coil, which is located near the rotating disc. This disc is mounted at the end of the camshaft and there is a slot in it. When the slot passes the coil, it is excited by the mutual inductance and a signal indicating the position of the first cylinder is sent to the onboard controller. This type of sensor is used in some Vauxhall ECOTEC engines.

Synchronization of operation of the camshaft position sensor and the crankshaft position sensor

      It is very important how the signals are set by phase, between the CMP sensor and the CKP sensor for injection system vehicles. If the phases are not synchronized, engine and the onboard controller may enter an emergency operational mode with reduced power and increased concentration of pollutants in the exhaust gases. In the worst case the engine will not start at all.
Reasons for bad timing can be:

  • Incorrectly adjusted distributor (only for adjustable distributors);
  • loose timing belt (a common fault);
  • Wrong or improper engagement of the timing belt.

— Procedure to verify functionality of the inductive CMP sensor. Measurement with voltmeter and ohmmeter of inductive sensor 

  • Measure the resistance of the induction phase detector and compared it with the data relevant to the inspected car. Normal resistance must be in the range of 200Ω to 900Ω.
  • Turn off the ignition and disconnect the induction phase sensor or the onboard controller coupling.
  • Connect a voltmeter between the two terminals of the sensor or between the relevant terminals of the onboard controller.

NOTE: Better results usually are obtained by measuring the terminal "+" although the signal can be measured in the grounding circuit.

  • Crank the engine with the starter. 
    You should obtain the RMS value of the AC voltage (AC amplitude, multiplied by 0.707), not less than 0.4V
  • Plug in the inductive phase sensor or/and the onboard controller coupling.
  • Connect a voltmeter between the signal and ground terminals of the sensor.
  • Start the engine and left it idling. You should obtain a RMS value not less than 0.75V.

— Oscilloscope measurements of the inductive sensor  
      Connect the active end of the oscilloscope probe to the sensor signal terminal and the ground probe - to chassis ground. You should observe the following signal on the oscilloscope screen (fig. 2).

Fig. 2

Procedure to check the reliability of CMP sensor with Hall Effect

NOTE: The description below relates to a sensor located in the distributor. Sensor verification located on the camshaft is done similarly.

— Voltmeter measurements of Hall sensor  
Connect the negative probe of the voltmeter to the chassis ground.
Determine the output, signal and the ground terminals. They may be marked as follows:
   0      output (reference voltage)
   +      signal
  —      ground
Connect the positive probe of the voltmeter to the signal terminal of the Hall sensor.
NOTE: The onboard controller coupling should be plugged in. Let the engine idle. The average value of the resulting voltage should be approximately 2.5V, and the working duty cycle - about 50%.

— Oscilloscope measurements with Hall sensor  
Connect the active probe of the oscilloscope to the signal terminal and the ground probe - to chassis ground. You must observe a similar signal as in fig. 3.

Fig. 3

— Possible damages in Hall sensor  
-- Lack of signal voltage or lack of operating cycle signal

  • Stop the engine.
  • Remove the distributor cap.
  • Plug in the Hall sensor coupling and switch the ignition on.
  • Connect the positive voltmeter’s terminal to the signal terminal.
  • Slightly crank the engine. When the screen slot passes through the air gap, the voltage should vary between 0V to 5.0V.

-- Lack of signal voltage

  • Disconnect the Hall sensor coupling from the distributor.
  • Connect the voltmeter’s positive terminal to the output terminal <2> (0) of the sensor coupling.
  • If there is no voltage on terminal <2> coming from the onboard controller, you should check the signal wires between the sensor and the onboard controller.
  • Measure the voltage at the output of the onboard controller again. If there is no voltage, you should check all power supply and ground connections of the onboard controller.
    If they are present and correct, the reason may be the onboard controller itself.
  • Check the supply voltage (5.0V) of terminal <1> of the Hall sensor.
    If supply voltage is abnormal, check the wires between the Hall sensor and the onboard controller.
  • Check the connection to the ground terminal <3> (-) of the Hall sensor.
  • If the presence of voltage and ground are correct, the reason may be in the Hall sensor itself.

Procedure to check reliability of CMP with AC output

— Oscilloscope measurements of the AC output sensor  
Connect the active end of the oscilloscope probe to the signal terminal and the ground probe - to chassis ground. You should observe the following signal - fig. 4.

Fig. 4

Move the active end of the oscilloscope probe to the other terminal. You will observe the same signal as in Fig. 4.
In practice both signals are shifted by 180 degrees, but this can be observed only in a simultaneous measurement of both signals with a dual channel oscilloscope.