Ignition Hall Effect Sensor: Description and Operation

MPI Start-Up Reference Signals:

The hall sensor, located in the ignition distributor, generates a square wave signal which the ECU monitors. This signal, along with the crankshaft reference sensor signal and engine speed sensor signal, is used to identify TDC of the firing stroke for #1 cylinder. This is necessary to calculate the ignition timing, and fuel injection timing for the engine to start.

Hall Sensor & Distributor (Engines w/Sequential Fuel Injection):

To positively identify the #1 cyl. in the firing order, only one window is provided in the trigger wheel. When the hall sensor signal and the crankshaft reference sensor signal coincide, the ECU begins counting the pulses from the engine speed sensor and calculating the changing position of the crankshaft. From this information, the ECU can determine the exact position of the crankshaft at any given time for precise control of ignition and fuel injection timing. Once the engine is started, the hall sensor signal is no longer needed, and the ECU calculates the ignition and fuel injection timing based on signals from the engine speed sensor and a reference position value stored in the computer memory.

THE HALL EFFECT

The Hall Effect:

The hall sensor contains a semiconductor wafer element, and a permanent magnet. When the current flowing through the sensor element in the direction of one axis (call it the X axis) is subjected to a magnetic field whose lines of flux cross perpendicular to the direction of current flow (call it the Z axis), electrons (charge carriers) in the current flow are deflected along an axis (call it the Y axis) perpendicular to the "X-Z" plane defined by the direction of current and lines of flux. The resulting difference in the concentration of electrons along opposite edges of the element creates a voltage potential (hall voltage) which can be measured. When the distributor is turning, vanes of a metal rotor, called a trigger wheel, alternately pass and block the lines of flux from the permanent magnet. When a vane passes between the magnet and sensor, the lines of flux permeate the metal vane and do not reach the sensor element. The flux density through the sensor element is negligible and the hall voltage is too small to measure (effectively zero). When no vanes are present between the magnet and sensor, the lines of flux pass through the hall sensor creating the hall effect. Since there is only one window in the trigger wheel, the signal generated by the hall sensor describes the exact position of the crankshaft. At the instant the hall sensor voltage goes high, a specific number of degrees of crankshaft rotation remain before TDC of the #1 cylinder power stroke. The ECU uses this information to calculate the begining of the fuel injection sequence, and ignition firing sequence.

SELF DIAGNOSIS
By comparing the crankshaft position reference sensor signal with that of the hall sensor, the ECU can determine if the distributor basic setting is correct or not. If the signals do not agree, then a fault code is stored. For this reason it is important for the distributor to be set properly, even though the ignition timing is not actually adjustable.
There is no substitute signal for this sensor. If the hall sensor fails or the signal is lost due to a wiring problem while the engine is running, the engine will continue to run (based on signals from the engine speed sensor and a position reference stored in the computer memory) until the ignition is turned off. A failed sensor or lost sensor signal will result in a "no-start" condition.