Primary/Secondary Ignition Testing PRIMARY IGNITION The secondary ignition system voltage is dependent on the operation of the primary ignition system. Think of the primary system as the input to the ignition coil and the secondary system as the output of the ignition coil. If there is poor input to the coil, there will be poor output. So if testing of the secondary reveals a problem, then the next logical thing to check is the primary ignition system. If the secondary system checks out ok, then there is no need to check the primary. Below is an oscilloscope picture of a typical primary ignition waveform. Look familiar?
Primary kick
Dwell
What if you don’t have an oscilloscope? Is there any way to test the ignition primary? Not really. An oscilloscope is the only tool that can accurately show what is happening in the primary circuit with the engine running or cranking, but even with an oscilloscope the 8.1L primary circuit can’t be checked because most of it is internal to the ignition coil. With Volvo Penta’s ignition systems, the coil primary circuit is controlled by the Ignition Control (IC) signal input to either the coil (8.1L) or the ignition control module (ICM) from the ECM. So the coil primary only reacts to the IC input. No IC input to the coil or ICM, no coil primary operation. Since the IC circuit is an output from the ECM, there has to be some input that the ECM looks at to determine when to send the IC signal. That input is the crankshaft position sensor (CKP).
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