After watching this video on voltage drop, it makes me feel like testing the resistance of various sensors and accessories with my multi-meter isn't really a reliable test of anything, since I can see correct resistance numbers with the system off, yet under load there can still be a problem with high resistance. For example, I could see OK numbers testing the primary and secondary coils on my coil pack, yet there could still be a voltage drop due to excess resistance under load. Same thing with testing the resistance of the plug wires. I'm exactly in this situation now. I strongly suspect a problem with the coil pack is causing some hesitation and rough idle problems, yet the resistance on the primary and secondary coils show up fine. I suspect this because swapping the standard plugs with fine tip, low voltage ngk g-power plugs caused about an 80% improvement in the problem but didn't make it go away completely.

1 Answer 1


Voltage drop problems are very common in multi component, multi interface systems.

For instance the starter motor has a cable that runs from the battery to the starter. The cable has ring terminals on the ends and those terminals are bolted to the appropriate component. High resistance at any of the aforementioned connection points is a recipe for voltage drop. Also the very large gauge multi stranded wire can be prone issues as well. Something specific to mention is that the test only checks the supply and return lines of the device and not the device itself.

Inside an ignition coil a solid core wire is used to create the windings. The chances of damaging the thin solid core wire in the coil to create a voltage drop is tremendously small. The common failure mode for coils is a fracture of the wire which will cause the resistance to spike.

You may be able to check the primary side if you so wish. Apply power to the coil and measure it's current consumption. Compare the current consumption of the two coils. Checking the secondaries for anything besides resistance or conductance to the secondaries would be fruitless because they are meant to operate at a high voltage. Trying to check then at operating conditions would be extremely difficult if not outright dangerous.

A similar problem happens with spark plug wires. They are prone to fractures of the conductor. When that fracture happens the resistance shoots up tremendously. There is no good way to check them besides just resistance.

  • Are there common places where voltage drop usually happens and common places to check for it? Is it a problem that could affect the amount of current arriving at the ignition coils? Commented Mar 23, 2016 at 19:53
  • @RobertS.Barnes The ignition switch, fuses, relays... the list is long. The ignition coils don't tend to draw a lot of juice. What you can do is ground the control side of the coil and check the power side. The control side is more difficult to test because of the switching. You could unplug the pcm and ground the coils from there. Don't ground the coils too long, they can overheat.
    – vini_i
    Commented Mar 24, 2016 at 2:46

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