Curious if there is a recommended procedure to bench test if an ignition coil is good, or at least reasonably effective test?

I have a pile from the salvage yard, and I'm relatively certain one of mine on the car is slowly starting to fail.

So it would be nice to have a method to test these salvage coils before trying them out on the car, although admittedly that is the real test.

.

 

I've bench tested a slew of them.

The only spec I've ever found is for primary resistance--- .75 ohm.

Problem is, as you allude to, being within that spec is no guarantee that the coil will perform well on the road....as I've learned thru experience myself.

If either the primary or secondary resistance shows 'open' then it's fair to say the coil is junk. That's about the only conclusive bench test I can think of.

Cheers
DD

 

Does one test the primary coil by ohm meter leads on the two terminals in the plug?

 

It seems like a simple and inexpensive bench test for coils could be devised using a coil-on-plug (COP) tester like one of the following:


SG Tool Aid 23950 Coil On Plug Ignition Spark Checker - Walmart.com

In the car, 12V is always present at the coils. To fire a coil, the ECM briefly connects the coil to ground. It seems likely that you could set up a 12V source, such as an old battery, battery charger or jump starter, connected to the input terminal of the coil (the terminal to which the White/Pink wire connects when the coil is installed on the engine). Connect a wire to the negative terminal of the coil to serve as the grounding path. Connect the COP tester to the discharge end of the coil and attach the clamp to the negative terminal of your 12V source.

To test the coil, you would briefly touch the wire connected to the coil's negative terminal to the ground terminal of your 12V source. Or better yet, connect a momentary ON switch in the circuit so you could just briefly depress the button to connect the coil to ground.

If the coil is good, either the lamp will illuminate a spark will jump across the electrodes, depending on which type of COP tester you have.

Could this work?

Cheers,

Don

 

Yes, but you PROBABLY would want a capacitor across the switch to shape the primary firing current. Remember the "condenser" in cars with points?

 

Hi Ross,

Good thought. My assumption has been that the coil firing circuit in the ECM is based on power transistors used as switches to connect the coils to ground for a controlable duration. I guess it could also be done with small relays. Is it your view that whatever devices the ECM uses for connecting the coils to ground would each be bypassed with a capacitor?

Thanks,

Don

 

I have not looked at the circuit, but there is almost certainly some method of controlling the electrical pulse created when you open a circuit with a large inductive load. Remember, on a conventional automotive coil, the coil "fires" when the circuit is interrupted. It is the collapsing field from the primary winding that creates the high voltage pulse in the secondary. I ASSUME the circuit is similar for the electronic version and the waveform I see with my scope gives me no reason to think otherwise.

 

Has anyone succeeded yet? This would be a very valuable test. The shop I spoke to never tests coils, they just replace suspect ones. I have a flaky coil, and while swapping in good coils, I got a backfire into my intake manifold that prevents starting and further diagnosis. So I'm in a quandry.

 

Assuming we are talking about an X-300, make sure you have the rear two coils connected correctly. The wiring to the rear coil SHOULD have a small wrap of tape, identifying it for cylinder 6. The way the wires "fan out" from the wiring bundle make the correct connection sort of counter- intuitive!