Hall Effect Pickup / Ignition Reference Sensor in 2.2 / 2.5 liter engines
To monitor engine speed and crankshaft position, the logic module uses a sensor in the distributor called the ignition reference sensor.
This sensor sends a pulse signal to the logic module each time a shutter passes through the sensor, which indicates the position of base timing or ignition timing. Ignition timing is usually set at 12 degrees before top dead center (BTDC) on the 2.2 and 2.5 liter engines. The information received from this sensor is used by the logic module to fire the ignition coil, which delivers high voltage to the spark plugs, making them spark.
In turbocharged engines, there is also a fuel injector sync pickup sensor which the logic module uses to time firing of the fuel injectors. Both the ignition reference sensor and the fuel injector sync pickup sensor are housed in the ‘Hall Effect Pickup’ (HEP).
To clarify: for the Throttle Body Injector (TBI) engines there is only one sensor in the HEP. For turbocharged engines the fuel injector sync pickup sensor and the ignition reference sensor are both in the HEP.
Each of these sensors consists of a permanent magnet on one side and a coil on the other side. The magnet creates a field which is sensed by the coil. As the shutter passes through the “sides” of the sensor, it breaks the field generated by the permanent magnet, generating a signal which is sent to the logic module. The shutter is mounted to the distributor shaft which turns as the engine turns. For turbocharged engines, one of the four shutters has a hole in the center which indicates the #1 cylinder.
The open collector output of the sensor is pulled up to 5v by the logic module as the shutter passes through the sensor. When the shutter is not between the magnet and the coil the field is detected and the output is pulled down to 0v. The distributor shaft is directly driven by the intermediate shaft. Because the intermediate shaft and the cam both have the same size sprocket the distributor turns one time for each rotation of the cam. The crankshaft sprocket is half the size of the other two sprockets. This means that the crankshaft spins two times for each turn of the cam and intermediate shaft.
On turbocharged engines one of the shutters has a hole in it. This passes the fuel injection sync pickup one time for each rotation of the distributor shaft. The logic module interrupts this as an indication of the #1 cylinder. The logic module then fires the proper injector circuit based on this signal.
If the ignition reference sensor’s signal is not received by the logic module during an attempt to start the engine, the logic module will trigger and store a code 11 and the engine will not start. If the logic module receives a signal from the ignition reference sensor as the engine is cranked (started), the logic module will clear the code and the engine will start.
Should the signal be lost while the engine is running the engine will stall. When this happens no code will be stored. If the sensor has failed the logic module’s memory must be cleared and an attempt to start the engine again. If at that time there is no signal from the ignition reference sensor a fault code 11 will once again be stored.
Why the Hall Effect Pickups (HEPs) fail; other symptoms
One of the greatest early HEP-killers were cheap distributor rotors that didn't have the reluctor vanes grounded through a center tab to the distributor shaft. Inside the distributor on a running engine, it looked like a lightning storm with sparks scattering around. Some of this high-voltage electricit hits the reluctor vanes and should be taken right to engine ground — but without a grounding tab on the rotor, some jumps from the rotor vanes to the HEP, which can eventually kill it. The high voltage power can also travel up from the HEP to the computer; while it has some protections against transients, reverse polarity, and over-voltage, it might get through the protections and damage sensitive things.
If you pull the rotor off, look at the underside where it fits over the distributor shaft. Make sure that there is a metal tab that connects the reluctor vanes to the hole that slips over the distributor shaft for a ground connection. The rotor should fit snugly to the shaft. Later distributors mounted the vanes underneath the HEP and that greatly reduced this failure from happening.
Non-Mopar parts may not meet the specs or standards required to operate long, or at all. This has happened to me. Your replacement Hall Effect Pickup doesn’t have to be Mopar brand, but it should definitely be described as an OEM-replacement part somewhere on the box.