testing and ign. coil

[John S.]

How do you test and ign. coil to see if it is up to standard? I have no way to test it on a engine right now. Thanks for any help. john

[William C.]

Re: testing and ign. coil

With an ohmeter check the resistance, typically the spec is around 1.5 ohms on the primary side and 9500 ohms on the secondary side, plus or miinus 25% will be in the ballpark.

[Albert P.]

Re: testing and ign. coil

A quick test can be made of the primary and secondary resistance; disconnect the wires on the pos and neg side; the primary resistance should be between 1-2 ohms; secondary 7500-11000 ohms; check your manual for actual values.

[Jack W.]

Re: testing and ign. coil

how would this catch a coil that fails only at hot temperatures, alternative phrasing of that question, how do you test for a coil that is possibly failing when hot? (Me, I have "tested" for this by putting another $35 coil in)

[Jack H.]

It won't...

The static resistance measurements of primary/secondary are simply necessary but NOT sufficient tests. They tell you about the integrity of each winding, but say nothing about the quality of the magnetic coupling through the transformer's iron core.

Also, an ohm meter probes the circuit under test using its built-in battery (typically 1.5 VDC), measures the resulting current that flows, and 'translates' that reading into an equivalent resistance value. Things often change when you use higher voltages, more current and actually get the target part up to operating temperature.

Unfortunately, in order to really test a coil, you need a wave form generator to exercise the primary side of the coil and a HEALTHY size, high power load for the secondary side with an oscilloscope to watch the energy transfer process.

You can get a bit more sophisticated with ordinary static bench tests by blowing off the primary/secondary resistance measurements and replacing them with actual inductance measurements IF you have an RLC meter. There you'll the coil's actual step-up components with primary side readings in the milli Henry range and secondary side readings around 50 Henry.

The ratio between the two is the coil's step-up/step-down factor. Plus, if the coil has suffered catastrophic damage (core melt-down), you'll get 'funky' inductance readings where mundane winding resistance measurements can show good...

Actually, the easiest way to validate an ignition coil is take the car to a decent auto shop, attach an engine analyzer, and physically observe the spark waveforms on the scope with the engine at temp, running. Why guess, when you can put science to work?