Distributor pick-up (inductive) - voltage

The purpose of this test is to investigate the operation of an inductive distributor pick-up based on voltage and frequency output during engine cranking and 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 waveforms 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

Cranking conditions

Running conditions

Waveform notes

These known good waveforms have the following characteristics:

• A cyclic pattern with a smooth upward concave curve to around 4 V, followed by a steep drop to around -3.5 V and a smooth convex curve back towards 0 V.
• A peak to peak voltage around 8 V, whilst the engine is cranking.
• When the engine is running, the waveform amplitude and frequency increase.
• 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 mechanical dwell period for each ignition event.

The mechanism requires a rotating reluctor and a stationary pick-up coil with a magnetised core. The reluctor rotates at camshaft speed and has one activation pole for each engine cylinder. As a reluctor pole approaches, passes, and moves away from the stationary pick-up, a cyclic waveform is produced.

The rotational period between each cycle represents the maximum dwell period, during which the primary coil can be charged. The dwell advance, required for higher engine speeds, is controlled by a mechanical centrifugal flyweight assembly within the distributor.

As this system is mechanically driven and produces an inductive voltage (which increases with increasing magnetic field disturbance), the waveform amplitude and frequency increase with engine speed; however, the mechanical dwell rotational period (angle) remains constant.

The ICM or ECM, dependant on type, will rely on either positive, negative, or both pick-up signals to trigger the primary circuit driver transistor; however, the modules retain control of the actual primary dwell period and other charging parameters, such as current limitation, peak coil charge, and current cut-off.

These system design features can ensure 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 primary dwell period can be increased during cranking to compensate for the lower system voltage (to maintain the same overall coil charge).

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

Typical system faults include:

Mechanical:

• Mounting and drive issues (e.g. excessive air gap or wear) associated with distributor.
• Physical damage or excessive detritus around armature and pick-up within the distributor.

Electrical:

• General electrical external circuit problems, open, short, high resistance.
• Pick-up coil 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.

GT019