Burning out coils

Post by **vibrodolly** » Wed Jul 13, 2005 11:30 am

Every 150-200 miles I am burning out a coil. Driving distance or local does not seem to have any bearing.
It starts with a small hesitation on accellerating - I know I have about 60 miles left. Gradually it hesitates more, with the last 1-5 miles be chronic.

Points, condensor, rotor arm, plugs, cap, ht + lt leads have all been replaced, some several times. Also replared - distributor (was badly worn and needed it)

I've now blown 5 new coils and running out of AA call outs (I now carry a spare coil) I've used lucas and intermotor coils - each the same prolem.

The engine is recon - professionally and trusted suplier - about 2000 miles ago.

I did have an ignition light stuck on and repolaced the charging box with a new one (spade connector) can the problem be there?

Please help. I love to use my moggie every day for work (30 miles) but it's too unreliable for this at the moment.
Thanks Nick

Post by **vibrodolly** » Wed Jul 13, 2005 11:31 am

Coil is wired correctly, by the way!

Post by **woo** » Wed Jul 13, 2005 11:36 am

I don't suppose you could be using coils designed for Ballast Resistors in a 12 volt car could you?

Post by **Cam** » Wed Jul 13, 2005 11:53 am

Are you positive or negative earth? If you have had the car converted to negative earth and fit the 'nomal' coil the 'original' way round then you'll shorten it's life but it will still work for a while. The same is true if you fit a 'normal' negative earth coil to a postive earth car.

It's possible that you have either been buying the wrong coils, or have the LT terminals swapped over...

Post by **Alec** » Wed Jul 13, 2005 12:22 pm

Hello Nick,
it must be a case of over voltage, either as suggested you are using a coil which should have a ballast resistor or your charging system is over charging (you mention the ignition light, this can come on if the system under or over charges) You need to get the running voltage checked ASAP, as overcharging will shorten your battery life also. I don't agree with Cam about the coil polarity, that does not affect the current drawn by the coil, and is designed so that the spark plug centre electrode is 'live'.

Alec

Post by **simmitc** » Wed Jul 13, 2005 12:49 pm

Do you have a radio wired via the ignition switch? If yes and you leave the ignition switched on to listen to the radio whilst not running the egine then that will burn out the coil. When the engine is running the points open & close, constantly switching the current off and on in the coil. If the ignition is left on then the current is continuous (assuming that engine stops with points closed of course). Same problem would arise if poor wiring is supplying power to coil some other way, but then the engine wouldn't stop when you turn off the key! Most likely the wrong coil, must be one designed for 12 volt operation. There have been some problems with new coils - see if you can get one off an old mini in a scrap yard.

Post by **rayofleamington** » Wed Jul 13, 2005 12:52 pm

see if you can get one off an old mini in a scrap yard.

or get one from someone scrapping a Minor.

Post by **Cam** » Wed Jul 13, 2005 12:56 pm

Alec wrote:I don't agree with Cam about the coil polarity, that does not affect the current drawn by the coil, and is designed so that the spark plug centre electrode is 'live'.

I'd say that the chances are that the wrong coil is being used (9v ballast type as you say).

Just as a point of interest, Alec. Why do we have to change the coil polarity over when changing from postive to negative earth?

Post by **Alec** » Wed Jul 13, 2005 1:13 pm

Hello Cam,

I didn't fully explain, it is to do with the ability of the spark to jump the gap. The centre electrode being hot 'excites' the electrons more and you get a better spark if it jumps from the electrode to earth rather than vice versa, it will work but the spark is weaker.
The old trick to check polarity was to put the point of a lead pencil between the H.T. lead and the plug terminal, with the engine runing, and watch the spark, if it 'feathered' towards the plug the polarity is correct.

Alec

Post by **vibrodolly** » Wed Jul 13, 2005 1:16 pm

woo wrote:I don't suppose you could be using coils designed for Ballast Resistors in a 12 volt car could you?

All coils are standard. Thanks Nick

Post by **vibrodolly** » Wed Jul 13, 2005 1:18 pm

Cam wrote:Are you positive or negative earth? If you have had the car converted to negative earth and fit the 'nomal' coil the 'original' way round then you'll shorten it's life but it will still work for a while. The same is true if you fit a 'normal' negative earth coil to a postive earth car.

It's possible that you have either been buying the wrong coils, or have the LT terminals swapped over...

Thanks - It's positive earth all standard and definitely wired the correct way. Nick

Post by **vibrodolly** » Wed Jul 13, 2005 1:20 pm

Alec wrote:Hello Nick,
it must be a case of over voltage, either as suggested you are using a coil which should have a ballast resistor or your charging system is over charging (you mention the ignition light, this can come on if the system under or over charges) You need to get the running voltage checked ASAP, as overcharging will shorten your battery life also. I don't agree with Cam about the coil polarity, that does not affect the current drawn by the coil, and is designed so that the spark plug centre electrode is 'live'.

Alec

Thanks Alec - its definitelt a standard coil, not BR. The ignition light problem was solved and happened before I started butning out coils by fitting a new charging box.
Nick

Post by **vibrodolly** » Wed Jul 13, 2005 1:22 pm

simmitc wrote:Do you have a radio wired via the ignition switch? If yes and you leave the ignition switched on to listen to the radio whilst not running the egine then that will burn out the coil. When the engine is running the points open & close, constantly switching the current off and on in the coil. If the ignition is left on then the current is continuous (assuming that engine stops with points closed of course). Same problem would arise if poor wiring is supplying power to coil some other way, but then the engine wouldn't stop when you turn off the key! Most likely the wrong coil, must be one designed for 12 volt operation. There have been some problems with new coils - see if you can get one off an old mini in a scrap yard.

I've just fitted an old lucas coil. This may solve the probelm. I have no Radio
Thanks
Nick

Post by **vibrodolly** » Wed Jul 13, 2005 1:29 pm

Is it possible the charging box could be the problem? I changed it as the old one made the ignition light stay on. It was fitted by an autoelectrician, but maybe a mistake could have been made?
Nick

Post by **Cam** » Wed Jul 13, 2005 1:59 pm

Nick, check the battery voltage when revving the engine to about 2000 rpm. It's possible that it's overcharging due to a duff/poorly adjusted regulator.

Alec, thanks for the explanation, but the direction of the electron flow only has a minimal effect on the spark strength and is more to do with the longevity of the plugs (terminal erosion).

What I was getting at was that coils have a connection between the LT and HT side. This is the CB connection and for the coil to run optimally it needs to be switched to the vehicle's earth and not the SW connection.

If you are positive earth, then the CB connection needs to be switched to +, if you are negative earth then the CB connection needs to be switched to -. If this is reversed, then the coil will act inefficiently as you will be earthing the coil through the primary and secondary coils in series instead of in parallel.

See:

The result will be a poor spark (30-50% of the power that it should be) and the coil will get warmer and probably draw a bit more current due to the windings opposing each other and there being a constant 12v at the centre CB 'tap' of the coil.

It's a bit awkward to explain, but that's the gist of it anyway.

Post by **Alec** » Wed Jul 13, 2005 2:01 pm

Hello Nick,
I take it that you are running a dynamo then, however, I would get the running voltage checked if possible. Normal voltage will be around the 14 volt mark. I can't think what else may be the problem without also giving severe running difficulties.
I take it that the coil is fitted in the normal position, (i.e., not in an extremely hot or vibrating position), and the clamp is a good fit and not squeezing the coil body.

Alec

Post by **vibrodolly** » Wed Jul 13, 2005 4:58 pm

I'll check the voltage at idle and about 2000rpm and get back to you.(I take it thats the voltage across the terminals of the battery) Nick

Post by **vibrodolly** » Wed Jul 13, 2005 4:59 pm

Yes the coil is on the normal place on the dynamo - Yes I'm running a dynamo
Nick

Post by **Alec** » Wed Jul 13, 2005 5:27 pm

Hello Cam,
I'm sorry, but the polarity of the coil does not alter its configuration, i.e. series or parallel, nor is the current more if it is reversed. From your drawing they are clearly parallel. By the way what is th component after the capacitor?
The temperature of the centre electrode is significant. It is exactly how a thermionic valve is able to operate.

Alec

Post by **Alec** » Wed Jul 13, 2005 5:29 pm

Hello Nick,

yes, across the battery is as good as anywhere.

Alec

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Burning out coils

Burning out coils

RE: Burning out coils