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Ignition Coils: Dead, Deader, Deadest

2023-01-19 - Paul Bertucci

Ohm's Law: V = A x O

V = A x Ω is a formula familiar to anyone who's done a little electrical work: Voltage = Current X Resistance.

In spark ignition (gasoline) engines, this formula is an important one: to take our fixed amount of current from the battery/alternator and overcome the high resistance of the spark plug's air gap, we need a lot of voltage to make a spark that'll jump the gap. This is where ignition coils come to the rescue.

Essentially miniature transformers, ignition coils use a high current primary (input) winding of thick copper wire and a high voltage secondary (output) winding of many loops of thin copper wire, all wrapped around an iron core:

  1. Electricity flows through the input winding's coils of wire
  2. The electricity traveling along the multiple loops of the input coil winding makes a magnetic field
  3. The loops of the output winding capture the magnetic field and turn it back in to electricity

The ratio of output winding to input winding coils determines voltage, so ignition coils typically have a handful of input winding coils but dozens or hundreds of output winding coils to step up the voltage to a high enough level to make a strong spark. A usual ignition system voltage might be anywhere from 20000 to 40000 volts, with greater voltage needed to keep the spark from blowing out in high compression or high boost engines.

Ordinary failure: dead coils

The primary coil -> magnetic field -> secondary coil transformer process is not a 100% efficient conversion, with the balance turning in to waste heat. Add this to the already toasty environment of an engine bay, and heat-related failure is the most common ignition coil killer, particularly for coil-on-plug types sitting directly on the hot cylinder head. The heat embrittles connectors and melts wire & coil winding insulation, resulting in breaks and shorts.

Depending if your car has "direct ignition" coil-on-plug igntion coils for individual cylinders, or a distributor ignition system with a single remote-mounted ignition coil, a failing coil may manifest as individual cylinder misfires or a random/multiple misfire respectively. 

Dead ignition coils usually can't be detected from a simple visual inspection, so you'll have to do some supporting diagnostic work (e.g., scanning for repeat misfire codes on particular cylinders, checking the wire harness, changing spark plugs) to figure it out. 

A good test for coil-on-plug cars like P2s, P1s, P3s, and P5s: if you have a misfire code on one cylinder, swap the coil between that cylinder and the good cylinder, clear the codes, and see if the misfire comes back. If it's back and on the new cylinder, that coil is toast!

Luckily, ignition coils aren't particularly expensive nor difficult to replace, so this is one instance where loading the parts cannon isn't a bad idea if you've got misfire codes or a stumble under heavy load or high RPMs. If you've already done this diagnostic work and chased spark and wire issues as far as you can, check out our available ignition coils & related components as a next step.

Extraordinary failure: exploded coils

The particularly high voltages and thermal loads modern coil-on-plug ignition coil packs have to deal with can occasionally make them fail in dramatic fashion.

Direct ignition coil packs typically have a bit of oil, paste, or gel in the flat "cap" part of the coil, both for electrical insulation and to help carry away some heat. If this leaks out at all, then the coil windings inside overheat, insulation melts, and shorts occur, leaving you with inconsistent or missing spark and a dead coil.

In a worst case scenario, you can get a thermal runaway situation: electrical resistance produces heat and resistance itself increases as the wire gets hotter, so a coil winding short can cause a destructive feedback loop with the coil overheating until part of the wire finally melts down and the circuit breaks.

Temperatures that are hot enough to melt wires are also hot enough to burn plastic and rubber, so this kind of failure is...messy.

IPD's P3 XC60 T6 Overlander development car encountered this kind of failure recently, and the onset was sudden and without warning. No misfire codes or performance impact were noticed in the preceding days or hours, but on returning from a short drive, the car suddenly began misfiring and smoking from under the hood.

By some miracle, we managed to fish out the burning and smouldering fragments of ignition coil before any damage occurred to the ignition wire harness or other surrounding components. Check out the carnage in the photo gallery below!

Fixing the problem

As we already had the old ignition coils out of the car, we elected to replace the spark plugs at the same time, and in the process we discovered that the car had incorrect spark plugs installed.

Our best guess for the cause of this explosive coil pack failure is that these incorrect plugs had the wrong spark gap and internal resistance compared to the right plugs for a 3.2i or 3.0 T6. Thus, the coil packs had to work harder and draw more current to overcome that extra resistance to make a spark.

The Aventyr XC60 is now running healthier than ever with a fresh set of OES Denso plugs and HQR Hella ignition coils, but our experience (and the experience of others) definitely underscores the importance of regularly checking your ignition system components and using the right parts. 

If you want to do coil packs on your P3 6-cylinder, we offer OEM Genuine Volvoquality aftermarket, and High Quality Replacement options. If you want to know more about our OEM, OES, AMR, and HQR brand quality classifications, check out this article!