What makes a coil overheat?

[Onne]

Hello all,

I had a coil fail on me, as you do.

What can makes things stop working? I can really only think of heat.
Thankfully I was on my way again with a spare one that has been sloshing around in the boot for two years...

When I eventually arrived at my destination, the replacement coil was only just cool enough to touch.

Set up is:
1098 bog standard
Electronic ignition (no points/condenser)

Any thoughts greatly appreciated.

[bmcecosse]

Probably the wrong coil for the electronic ignition ? What make is it - and what Ohms coil do they ask for? And of course - what Ohms is your coil!

[Onne]

This was a points coil, as per instructions of the electronic ignition.

In other words a bog standard Morris Minor coil, be it a modern replacement one. She now has a 60s Lucas one on.

[bmcecosse]

I would ask the maker what Ohms coil it needs ! Then put a meter on the coil. The Lucas coil will probably be ~ 3.2 ohms. The failed coil may possibly have been for a system designed to run on 9 volts via a dropper resistance wire - with a bypass boost to full volts when starting.

[Onne]

It worked for nearly 11 months though with this set-up.

But to get back to my actual question, what goes wrong with the coil when it breaks?

[bmcecosse]

A wire or connection burns out! If the coil loses it's oil - it will soon overheat and burn out.

[Alec]

Hello Onne,

it's not so easy to answer with certainty but broadly speaking, too much current. (Loss of oil, which helps cool the windings, is less common unless you have the coil installed upside down, some have a screw in the H.T. pillar so oil could leak out)
Coils work on a duty cycle basically, which is given as a dwell angle. I'm not sure what the electronic dwell angle is compared to the points. Also, as mentioned earlier, ballast resistance coils have a lower impedance and so draw excessive current if fed from 12 volts instead of the 8 or so they are designed for. The other thing not do do is just leave your ignition on if the engine is not running.

Alec

[southerly95]

I have had this problem but solved it by trial and error. Coils were failing every 4 to 6 months with monotonous regularity and gnashing of teeth over a period of two years. Ignore the wibbling on about 9v coils and how good originals are, if you order a MM one then you will get the correct one. Personally I use the gold Lucas one from Classic Mini Spares and I have it located just behind where the water bottle was, i.e. in a vertical position and above the hole in the engine bay tie plate and it keeps cool there. Fingers crossed no trouble for over a year - and just in case we keep a spare in the boot - and have trained wifey to change it herself!

[rayofleamington]

if the ignition is left on, they can overheat as they are not intended for 100% duty.
The overheating causes the insulation on the wire to degrade. This causes the coil to start to short circuit and this reduces the resistance... which increases the current and creates more heat.
When the windings start to degrade, it's only a matter of time.

[bmcecosse]

Southerly - I object to your " Ignore the wibbling on about 9v coils " comment. If you had coils failing every 4 to 6 months - there was something very wrong with your set up! It's very possible that a 9volt coil could be supplied from the likes of Hellfrauds or Autoparts place - they could well assume it is same as a Mini coil - and they have all been 9 volt type for many many years. It IS the most likely explanation for repeated coil failure - I can think of no other - unless you are in the (bad) habit of leaving IGN on with engine not running.
I have never had a coil failure, on ANY of my A series cars (and there have been many!) - the one on my Traveller engine is likely to be the original - going on 45 years old, and working well.