Testing coil packs

[Regimof3]

I'm chasing down a low end stutter/stalling issue and I want to test the coil packs. Is there a way to do this with the plenum on and just go off the wires or is it better to take the plenum off and get at the actual packs? What numbers am I looking for on the meter?

[XfireZ51]

Originally Posted by Regimof3

I'm chasing down a low end stutter/stalling issue and I want to test the coil packs. Is there a way to do this with the plenum on and just go off the wires or is it better to take the plenum off and get at the actual packs? What numbers am I looking for on the meter?

I just went thru this looking for the cause of mid-range knock. I started out testing each coil pack end to end by measuring the resistance from plug end to plug end of coil pack pairs, ie 2 and 3.
Pulling the plenum is really the way to inspect the packs both electrically and visually. That's when I found my coil packs had cracks in pack case which had carbon tracking on them. Electrically they tested OK.

[Regimof3]

Originally Posted by XfireZ51

I just went thru this looking for the cause of mid-range knock. I started out testing each coil pack end to end by measuring the resistance from plug end to plug end of coil pack pairs, ie 2 and 3.
Pulling the plenum is really the way to inspect the packs both electrically and visually. That's when I found my coil packs had cracks in pack case which had carbon tracking on them. Electrically they tested OK.

What kind of resistance numbers should I see if it's good/bad?

[XfireZ51]

Originally Posted by Regimof3

What kind of resistance numbers should I see if it's good/bad?

Partly depends on which plug wires you are using. With the Magnecores, I got
~ 13.5-14K end to end. What you want to see is consistency. I had one coil pack that measured 16.9K which is still supposed to be OK ie < 20K but given that it was significantly different than the other 3, it raised suspicion. Subsequently I found that the plastic casing was cracked on that pack, and that carbon tracking could be seen.

[Paul Workman]

The issue with measuring resistance on high voltage devices like coil pack (in particular) is the internal insulation cannot always be detected with the standard VOM. But, certainly, a standard VOM can often detect other issues such as open windings or even shorted windings - in some cases.

Measuring through the plug wires can be a bit of a crap shoot, far as specific results go. Although you can sometimes measure continuity (as Dom showed), but as for values, I contend that it is ALWAYS better to measure across the coil terminals themselves to isolate the readings from any issues that may or may not also be present in the plug wires. I'll get to them later.

For grins, I measured a pair of AC Delco coils I took out of my car - which were working normally when removed. The results were as follows:

Primary side: 0.7 and 0.8 Ω
Secondary (spark side): 5.8k and 6.1k Ω

Depending on which coils you have, the readings may vary compared to my readings, and that's OK. But, as Dominic said, the key is uniformity: They all should read relatively close to one another. The ODDBALL reading that is different by an order of magnitudeis reason enough to replace them ALL with a quality unit: if one goes, it is indicative that the rest have also reached their service life and it is better to not get stranded somewhere when the next in turn cashes in its chips!

Plug wires have "linear resistance". In other words, their resistance increases with their length. Different brands have different linear resistances, AND one brand or type will often vary when compared to another brand/type.

However, the linear resistance should be proportional and uniform to length - whatever that value is: IOW, a wire twice as long as another should measure twice the resistance.

Getting back to the coils again: It is not unusual for a coil to check out on a standard VOM and yet be bad. The insulation (in particular) can have developed a pin-hole in it causing the high voltage to arc, thus shorting out the high voltage (spark). The result is a miss in the ignition (and b/c two plugs share the same coil, both cylinders my likely misfire - simultaneously or at random between them. And, unless there is other evidence such as carbon tracking (as Dom discovered), the only other way to test the insulation is with a high voltage ohm meter called a MEGGER.

However, not many people or shops either have a MEGGER. But, if your testing eliminates all other components, you can try swapping the suspected coil with another and see if the problem follows the coil*. But, of course it may be more practical to simply install a new one and see if that fixed the problem. If it does, then replace the rest of them.

*NOTE:
T

1. he spark plugs - especially on the cylinder(s) that are miss firing - must be beyond suspicion too! Replace them with fresh ones if the least bit questionable. AND...never assume!
2. It is prudent to "ohm out" the new coil(s) before installing them. Replacing a part with a new one does not always fix the problem, due to the fact the chance of failure called "infant mortality" is extremely highest immediately when a device is first put into service!
3. It is common for coils of all kinds to be prone to failure after they operate for a while (due to heat sensitivity). The result is (whatever) works initially until it warms up before failing. Just a point to keep in the back of your mind...