Do I have a bad ignition coil? Measurements inside

Question

I'm troubleshooting what I believe is an engine misfire on my LS1 Chevy and want to rule out if I have bad ignition coils.

In order to do this I took out the ignition coils and measured the resistances across every possible combination of the four pins. The thought here was that any measurement that registers as an outlier in comparison to the rest would manifest itself in the readings.

I also purchased a new coil and took similar measurements.

The terminal nomenclature corresponds to what is in the diagram below:

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Here are the raw measurements taken with a Fluke 87 V multimeter. All measurements are in kilo-ohms.

    | Coil #1 | Coil #2 | Coil #3 | Coil #4 | Coil #5 | Coil #6 | Coil #7 | Coil #8 | New Coil #1
----+---------+---------+---------+---------+---------+---------+---------+---------+--------------
A-B | 31.71   | 29.30   | 29.64   | 34.19   | 28.14   | 31.47   | 34.08   | 32.89   | 26.77
A-C | 31.50   | 29.85   | 29.55   | 34.17   | 28.67   | 31.35   | 34.02   | 32.89   | 26.59
A-D | 33.67   | 33.7    | 33.56   | 34.09   | 33.52   | 33.89   | 61.00   | 62.10   | 31.29
B-C | 12.05   | 11.96   | 11.95   | 12.17   | 11.90   | 12.03   | 12.14   | 12.10   | 11.83
B-D | 88.70   | 83.10   | 84.80   | 94.00   | 82.40   | 87.20   | 95.10   | 95.00   | 73.9
C-D | 88.50   | 83.70   | 84.80   | 94.00   | 82.90   | 87.1    | 95.00   | 95.00   | 73.8

I realize that logging resistances may not reflect whether something is "bad", so I'd appreciate if someone could explain what the circuit diagram shows.

More information on the LS1 coil can be found here.

Answer 1

Unfortunately, there aren't any terrific ways of testing these things with a meter because of the internal electronics. In addition, at least one measurement you make might (will probably) be polarity-sensitive, especially if you use the "diode check" function on your multimeter - that's the A-D reading. Using "diode check", reading one way your meter should read something like 0.7 and reading the other way your meter should read a far higher number.

Of interest is the A-to-sparkplug reading. That's reading directly across the coil's secondary winding, assuming that the little grey box inside the igniter isn't some unspecified component but is instead just a connection. That reading should read the same both directions whether you use "diode check" or not, and all coils should read about the same.

Regardless of the meter settings, all the A-B-C readings may yield all manner of inconsistencies because they read through the igniter's electronics... and the igniters themselves may have been built by any number of companies using parts with varied tolerances, and those tolerances may be cumulative so it's possible to get quite a range of readings. The larger grey box inside the igniter is what's known in the electronics field as a "black box", meaning that nobody except the manufacturer will ever know exactly what it contains.

All that said, coils #7 and #8 do stand out significantly in the A-D readings. That may be because the internal NPN transistor (a bipolar) or its bypass diode has developed a higher-than normal internal emitter-to-collector(s) resistance, or may simply be indicative of those cumulative tolerances.

You may want to try running the engine & taking careful note of how (in terms of misses) it runs under all conditions (cold/hot/slow/fast), then swapping out #7 with your new coil and comparing notes, then moving the new coil to #8 position and comparing notes again. If you see an improvement with the new coil at either #7 or #8, then the original coil for that cylinder is suboptimal. If you see an improvement with the new coil both and #7 and #8, then you'd do well to buy another replacement coil & swap them both out.

Answer 2

The answers you have had so far only go to show just how important a scan by an experianced mechanic is. The scanner will show if a cylinder is misfiring, or a particular cylinder is not firing - misfire count feature. The injectors can be seen to be firing and their operation on acceleration - opening durations and fuel pressures. You can do a balance test precisly and determine cylinder condition. On top of this you will have fault codes, that allow you to investigate areas the engine ECU thinks is wrong - back door coding. Multimeters are extremely useful with your vehicle, but most if not all can be better seen on an oscilloscope or graphing scanner because of the speed of operation and low voltages - milli volts and milli amps. In passing, because of power and switching diodes, 'testing' of pencil coils cannot be tested effectively other than by wave forms.