How Can I test a coil?

One of the old ways was to check on how big a gap the spark could jump in clear air. I can't recall exactly what gap it should be able to jump, but I think it was somewhere about 8-10mm. A google might give you a more precise figure. Mike.

Check the resistance on the primary and secondary circuits.

Nedavno Mike P napisa:

Look at the thread in this group from few days ago "how do you test an ignition coil"

That method might work about 10% of the time if the HT winding has broken, but high voltage coils usually suffer from insulation breakdown when they fail and this doesn't affect any readings you can make with a normal DVM or analogue multimeter unfortunately. Even a fairly expensive insulation tester is unlikely to produce more than 1Kv so probably won't show up a failed coil as typically coils start to break down at a few Kv. Insulation breakdown can either stop a coil working completely or more likely restrict the voltage output to the level at which it breaks down, which will now be below what the spark plug needs to fire (about 12Kv minimum under compression).

HT coils can also be OK at first and fail as they warm up after a couple of minutes or so. So, substitution remains the easiest method of diagnosis although the 'how long is the spark' method (1cm = 30Kv in air) mentioned by Mike G has merit but could cause the HT to break down somewhere else if you were unlucky. Another method not mentioned that works if the coil is marginal is to reduce the plug gap to 2/3 normal, if the engine runs much better then you definitely have a problem in the HT circuit which could be the coil, HT leads or dizzy if there is one. If the engine runs, then heating the coil up with a hair drier would confirm the coil as being the culprit if misfiring gets worse (or better, but that's unlikely).

What about the weird one I had years ago? A Mini that cut out after going round right hand bends, after MANY hours I found it was the ignition coil, all the oil must have been running up to one end and letting the sparks leak internally, a few minutes on the level and it would go again!

Mrcheerful

But it is a quick and simple way to check the basics.

Funny you should say that... my daughter's Mini has a similar problem on left-handers - doesn't cut out as such, but loses power unless it's accelerating.

The coil was suggested to me, but I've tried swapping that and it made no difference.

Check the cables to the coil, rather than the coil.

They are certainly dodgy, and replacing them completely is on my to-do list. I'll move it to the top!