Ignition coil

[Jeffrey S.]

Hello all. Is there any way to field test an ignition coil to see if it is functional using a digital multimeter? I have seen many coils at swap meets that I might be willing to buy but without a way to test it's a crap shoot. Thanks for your help.
Jeff

[Jack H.]

Necessary but NOT sufficient...

With a multi-meter you can verify the primary and secondary winding resistance values. That simply tells you if the transformer's (an ignition coil is a step-up transformer) windings are good to go. The test is necessary but not sufficient...

Why? In addition to being able to pump current into the coil and get it out of the coil, you have to be able to transfer energy from the primary winding to the secondary. That process occurrs when magnetic fields collapse and magnetic flux circulates in the transformer's core moving energy from the primary to the secondary winding.

If the core is damaged (prior over heating resulting in core de-lamination), the magnetic coupling path can lose efficiency. Hence, to REALLY test a coil, you need to dynamically pace it it by firing the primary winding electrically and measuring the output of the secondary.

Unfortunately, we're talking about multiple Henries of inductance and kilovolts of output. This is typically beyond the limits of your ordinary hand held multi-meter.

But, rule of thumb; if the primary and secondary windings ohm out OK and you can't hear oil 'sloshing' when you shake the coil (loss of internal cooling oil is what typically creates overheating and core delamination), you've probably got a +80% probability of the coil being good...

[Duke W.]

Re: Ignition coil

Yes, you can test primary and secondary resistance (and also check for ground faults) - values listed in appropriate service manuals, but some coils can be flaky at operating temperature even though they show in-spec static resistance and no ground fault at ambient temperature.

You take a chance anytime you buy a used electrical part, but doing these simple checks is certainly better than buying a used coil on pure faith.

Duke