Distributor pick-up (Hall effect) - voltage

The purpose of this test is to check a distributor pick-up’s Hall effect signal during engine cranking or running conditions.

How to perform the test

View connection guidance notes.

1. Use manufacturer’s data to identify the distributor pick-up circuit.
2. Connect PicoScope Channel A to the distributor pick-up signal circuit.
3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
4. Start the scope to see live data.
5. Crank and, if possible, run the engine at idle.
6. With your waveform on screen stop the scope.
7. Turn off the engine.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• A cyclic digital waveform switching between two voltages; a low voltage just above 0 V and a high voltage just below 2.5 V.
• The switching frequency increases with increasing engine speed.
• There are no drop-outs or anomalies within the waveform.

Further guidance

The function of a distributor pick-up is to provide a timing reference signal to the Ignition Control Module (ICM) or Engine Control Module (ECM) to indicate the primary circuit dwell period for each ignition event.

The mechanism uses a rotating assembly consisting of a set of rotating vanes (one for each cylinder) passing between a stationary magnet opposing a stationary Hall sensor. As the vanes pass in and out of the gap, the magnetic field is disturbed. Each disturbance causes the Hall sensor output to switch. Therefore, the sensor output is a digital signal with a low voltage at around 0 V and a high voltage up to the sensor supply voltage. An output around 0 V indicates dwell, or coil on time.

As distributor rotation is mechanically linked to engine rotation, an increase in engine speed reduces the dwell period but the dwell angle remains constant. At increased engine speeds, dwell is advanced by a centrifugal flyweight mechanism within the distributor.

The ICM or ECM, dependent on type, use the Hall sensor output as a (switched-earth) trigger of the primary circuit driver transistor. However, the module retains control of the actual dwell period and other charging parameters, such as current limitation, peak coil charge, and current cut-off.

This additional control ensures that an ignition coil is not overheated by too much dwell or that the spark duration is not reduced by too little dwell. Similarly, the actual dwell period can be increased during cranking to compensate for lower system voltages (to maintain the same overall coil charge).

For engine specific ignition dwell periods, consult manufacturer’s data.

Typical system faults will include:

Mechanical:

• Mounting and drive issues (e.g. excessive air gap or wear) associated with distributor.
• Physical damage or excessive detritus around the rotating vanes, magnet and hall sensor.

Electrical:

• General electrical external circuit problems, open, short, high resistance.
• Hall sensor damage due to heat and vibration.

These faults would manifest themselves as:

• Engine cranking but non-start.
• Engine difficult to start / prolonged cranking.
• Engine misfire.

GT020

Suitable accessories