You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.
Note: This helpfile refers to a 10:1 attenuator. If you are using a 20:1 attenuator please adjust the Probe settings for the relevant channel. These settings can be found under the Channel Options button, then: Probe > 20:1 Attenuator.
Plug the 10:1 attenuator into Channel A on the PicoScope and plug a BNC test lead into the attenuator. Placing a large black clip on the test lead with the black moulding (negative) and a Back-pinning or multimeter probe onto the test lead with the coloured moulding (positive). Place the black clip onto the battery negative terminal and probe the coil's negative (or number 1) terminal with the Back-pinning or multimeter probe as illustrated in Figure 1.
The example waveform shows that the voltage seen during this test is relatively high and the scaling of the oscilloscope is therefore adjusted to suit. It is important that the 10:1 attenuator is used in all situations when a voltage exceeding 200 volts is to be measured.
Plug a BNC test lead into Channel B on the PicoScope, place a large black clip on the test lead with the black moulding (negative) and a Back-pinning or multimeter probe onto the test lead with the coloured moulding (positive). Probe each of the two connections until the larger waveform is displayed, the smaller waveform being the earth return.
Alternatively connections can be made using the TA012 breakout lead: connect the BNC test lead directly into both the shrouded plugs on the breakout lead. If a smaller than anticipated waveform is shown try reversing the connections. Connection of the break out lead is illustrated in Figure 2.
With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings.
In this waveform we can observe the output voltage from the Crank Angle Sensor (shown in red) at the same time as monitoring the Ignition's Primary trace (shown in blue). The main reason for evaluating these two waveforms together is to identify the cause of any impending misfires at higher engine revs.
The picture shows the 'missing tooth' reference point and the primary induced voltage. The offset between these two points will vary between different vehicle manufacturers as the 'missing tooth' is not always in the same position.
As the engine speed is increased, the distance between the reference point and the induced voltage alters due to the engine's ignition timing advance. The gap in the red trace is due to the 'missing tooth' in the flywheel or reluctor and is used as a reference for the Electronic Control Module (ECM) to ascertain the engine's position. Some systems use one reference point per revolution while others use two. The trace from the CAS should maintain a constant voltage at a given engine speed, while the primary ignition trace shows the firing of the ignition circuit.
Should the engine start to misfire at speed, ensure that the CAS signal is not breaking down: this may be seen as an intermittent trace or as a reduction in amplitude. If the CAS output remains constant, the primary picture may falter: this could be due to either a fault coil or amplifier.
Please see individual waveform topics:
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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