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Just finished putting a supercharger on my wife's car (cause she's complaining that it's not fast enough , you know how it is ) and went to retard the base timing and struck an unsettling problem.
I put the timing light sensor around the lead to No 1 cylinder and got no light pulses from the timing light so I initially condemned my cheap ass timing light.
But then I tried the light on the lead to No. 2 and it worked fine. So did the light when connected to 3 & 4.
Then I swapped the lead going to No.1 cylinder around with the other one on the same coil and hey presto, the timing light works when connected to the No.1 lead.

So is it possible for just one half of a coil pack to start to die?

The car starts and runs on all 4 cylinders so there is spark there, but I'm guessing it's just not strong enough to trip the timing light. It will be interesting to see if it breaks down under the positive boost pressure though.

One thing I haven't done yet is to remove the leads and check the coil pack for corrosion or bad contacts.

Cheers,
Rob.

I wouldn't think only one half of a coil would fail unless, as you suggested, there's serious corrosion of a high tension connector. Auto electricians sell a conductive grease for it which works well. Can't remember what it's made of.

You can get plug-in replacement for the individual coils as an entire coil pack from Mazda is $400+, but yeah, if one lead on the same pack fires OK, then hopefully it's a just connection problem or just a really piss-weak no.1 lead.

If you are after leads, I would suggest (considering todays exchange rate) Accel or MSD.