Miata Coil Pack Fried During A Compression Test

I destroyed a coil pack while doing a compression test last week. This happened due to me disconnecting all spark plug wires from the coil pack and then performing compression tests on all four cylinders. The car was cranking strangely but I persisted.

Disconnecting spark plug wires but not the wire harness seemed like the wrong thing to do, but that is how I interpreted some Internet wisdom and I wanted to try it. Result: one dead coil pack.

When I was doing compression tests before I had 3 of the 4 spark plug wires attached to spark plugs in the engine. From what I have read recently the spark plugs may be grounded through the threads, therefore simply inserting a spark plug into the wire when the spark plug is resting on top of the engine may be no different from not having a spark plug attached to the wire. Need to do more research on this.

The ideal way of doing compression testing is to disconnect wiring harness from the coil pack. This is a bear to do on 99-00 Miatas at least because of very limited maneuvering room behind the coil pack. A flat blade screwdriver helps a lot with pushing on the connector clips.

Coil packs are fairly affordable at RockAuto - $70 for a Beck/Arnley one. Standard Motor Products coil pack is $180. Mazda Motorsports sells the coil packs for around $250 which means the retail on them at a Mazda dealer is probably around $400.