Hall Sensor (HS)

General description 
      The hall sensor’s primary ignition signal is usually used in engines with a distributor, but nowadays distributor ignition is very rarely used.
If the ignition system uses the HS, it provides a primary signal for the ignition and for the fuel injection.

Working principle of the Hall sensor
      Hall sensor is usually mounted on vehicles with a distributor in which the Hall Switch is located. The engine ECU powers the sensor with voltage slightly lower than the battery nominal. The hall sensor circuit is closed by cable for the feedback to the ground. A magnet is located against the Hall switch whose field forces the switch to return low voltage to the ignition module. A shield with slots is attached on the optical axis of the distributor, whose number corresponds to the number of cylinders. Hall Switch is being turned on and off while the magnet passes between the shield and the sensor. A voltage is being send to the amplifier through a third signal cable, while the switch is against an optical slot. While a dense part of the shield is against the switch, returned voltage signal is being interrupted, because the magnetic field is being deviated. The number of returned pulses in a four-stroke engine is equal to the number of slots. It’s important to notice that the return signal is a voltage or its absence and has a rectangular form.

Procedure for checking the condition of the Hall sensor
 Fast verification of the Hall sensor 
(without starting the engine) 

NOTE: In most of the systems the Hall sensor is located in the camshaft. In some systems only (VW / Audi) the Hall sensor is located on the flywheel.

  • Disconnect the central high-voltage cable from the distributor’s cap common terminal and connect it to the cylinder head by an additional cable.
  • Disconnect the Hall sensor connector from the distributor.
  • Find which are the voltage supply, output signal and the ground terminals.
  • Short circuit the <0> and the <-> contact of the hall sensor’s cable harness for a little while by using an additional cable.
  • If a spark jumps between the additional cable, connected with a high voltage cable, and the cylinder head, ignition coil and ignition circuit-breaker are capable of firing a spark and the possible cause for failure is in the Hall sensor itself.

Check the Hall sensor with an oscilloscope

  • Move away the protective rubber cover of the Hall sensor’s connector.
  • Connect the oscilloscope ground probe to the chassis ground.
  • Connect the active end of the oscilloscope probe to the Hall sensor signal terminal.
  • Start the engine and left it idling.
  • You must observe the following signal (Fig. 2). This is waveform of a properly operating Hall sensor. Duty cycle is approximately 35%.

Fig. 2

If the ignition circuit breaker is not operating properly, you should see the following waveform (fig. 3):

Fig. 3

Fig. 4 shows how the signal from a defective Hall sensor looks like.

Fig. 4

Other possible damages: 
-- Lack of voltage signal or duty cycle

  • Stop the engine and remove the distributor cap.
  • When coupling of the Hall sensor is connected and ignition is on, connect the active end of the oscilloscope probe to the signal terminal of the Hall sensor. Set the oscilloscope voltage range to ± 15V.
  • Slowly rotate the engine crankshaft. 
    When the screen slot passes through the air gap, voltage should change from 10V¸12V to 0V.

-- Lack of voltage signal

  • Disconnect the Hall sensor connector from the distributor.
  • Connect the active end of the oscilloscope probe to the <2> (0) terminal of the connector’s harness.
    Voltage reading must be 10V¸12V.
  • If there is no voltage from onboard computer to the <2> terminal, check the conductivity of the signal circuit between the Hall sensor and the onboard computer with an ohmmeter.
  • If the circuit is good, check if there is a voltage on the relevant terminal of the onboard computer connector. If there is no voltage, check all power supply and ground terminals of the onboard computer.
    If connections are correct, the probable reason is the onboard computer itself.
  • Check for the presence of voltage (10¸12V) to terminal <1> (+) on the onboard computer. If the supply voltage is beyond the specified limits, check the conductivity of the circuit between the Hall sensor and the onboard computer with an ohmmeter.
  • Check the ground connection on terminal <3> (-) of the Hall sensor.
  • If the supply voltages and grounds are correct, the Hall sensor itself falls under suspicion.