Ignition amplifier - earth volt drop

The purpose of this test is to evaluate the integrity of ignition amplifier ground circuit based on voltage levels during engine run conditions.

How to perform the test

View connection guidance notes.

1. Use manufacturer data to identify the amplifier earth terminal.
2. Connect PicoScope Channel A. to the amplifier earth terminal.
3. Start the engine and allow to idle.
4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
5. Start the scope to see live data.
6. Vary the engine RPM and look for waveform changes.
7. With your waveform on screen stop the scope.
8. Stop the engine and turn the ignition off.
9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

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.

Further guidance

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.

GT041