The purpose of this test is to evaluate the integrity of ignition amplifier ground circuit based on voltage drop during engine run conditions.
Plug a BNC test lead into Channel A on the PicoScope, place a large black clip onto the test lead with the black moulding (negative) and a back-pinning or multimeter probe onto the test lead with the red moulding (positive). Place the black clip onto the battery negative terminal and probe the amplifier's earth terminal with the back-pinning or multitester probe as illustrated in Figure 1. If you cannot reach the terminal or plug with a probe, then you may be able to use a breakout box or lead if you have one available. Either check with technical data or the earthing wire can be identified by a low voltage rising in a series of ramps.
As seen in the example waveform, the voltage seen during this test is very low and the scaling of the oscilloscope is therefore adjusted to suit. With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings. 'Hash' or RF interference may be seen on this waveform as the scale is so low it is susceptible to picking up interference from the vehicle's High Tension (HT) circuit.
The earthing on the ignition amplifier (also referred to as the module or the igniter) is vital to the operation of the ignition system and is often overlooked as an area for potential problems.
The earth connection, if not in good condition, can cause a reduction in the primary current that will affect the current limiting (or dwell control) circuit. It is therefore vital that this important connection is tested and rectified if it is found to be outside of its operational limits. An earth return circuit can only be tested while the circuit is under load and this therefore makes continuity testing to earth with a multimeter inaccurate. As the coil's primary circuit is only complete during the dwell period, this is the time that the voltage drop should be monitored.
Ensure that the 'voltage ramp' does not exceed 0.5 volts. The 'flatter' the resultant waveform the better. A waveform with virtually no rise, shows that the amplifier/module has a perfect earth. If the 'ramp' is too high, another earth wire can be soldered in parallel with the original wire and secured to a good earthing point.
The purpose of the ignition amplifier is to switch the relatively high primary current of approximately 8 to 10 amps to earth, when the component receives a signal from either the pick-up or Electronic Control Module (ECM). The earth path on this circuit plays a very important role in maintaining the correct operation of the primary ignition circuit.
The earth path is often overlooked as a problem area, the condition of the wiring and connections to ground can be checked using the Ohms scale on a multimeter. However, the reading may indicate good continuity under these conditions (no load), but this test does not demonstrate the circuit's ability to perform while operational.
A volt drop check is the only option available if the earth's path back to the battery is to be correctly assessed. When checking using an oscilloscope, the flatter the resultant waveform the better as this ensures a good earth path to the coil, through the amplifier. The length of the ramp is determined by the dwell angle and will expand as the engine speed increases.
Figure 2 shows a selection of ingition amplifiers.
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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