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I wonder if somebody may have come across this problem before, and so know what is the likely cure.

A friend has an MGB,......... but the coil gets so hot once its running that you cannot put your hand on it, it still seems to run ok, but are concerned that it may be an issue.

A new coil has been fitted, just the same

New points and condenser fitted no change.

The dwell angle has been set at 60 degrees, still no change.

Thinking about this as I write, could it be the wrong coil for the system which ever it is balasted or unbalasted? would this cause the extra heat?

We are now running out of ideas and I wondered if somebody here may be able help sort this problem, or at least point me in the right direction for further things to checkout.

I'm sure my TR coil doesn't run that hot, I can usually put my hand on it without burning it, I know it can get hot if the engine isn't running though.

Many thanks in advance John

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Could be either a damaged coil (shorted turn inside), or a 6V coil as would be fitted to a car with a ballast resistor.

 

Normally a coil will get warm, but never too hot to touch.

Can you measure the resistance of the coil ? 12V should read approx 3 ohms, a 6V will be roughly half that ie 1.5ohms

 

Bob.

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Hi John,

coils will get warm/hottish but if it is really hot then there is a problem.

If the car should have a ballast system and you are running a 6V coil with 12V then that would be the problem.

 

Your friend needs to establish what coil he has and how it is being fed - workshop manual or Haynes manual should give a clue.

 

Roger

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I had a lot of trouble with Lucas pattern coils amd that included wrong part numbers so that the one I was using was drawing 7 Amps instead a bit under 3 Amps. It was 6 Volt labelled 12. I suggest that you put an ammeter in series with the supply and check how much current it's drawing.

 

The most reliable coils I've used are Accuspark's general purpose ones on EBay for £13.95, but Bosch VW Beatle ones are excellent too.

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Chrome bumper 4-cylinder cars had a 12v coil with a direct ignition feed (white). Rubber bumper cars and all V8s had a 6v coil connected to the 12v ignition feed via a ballast resistance. This resistance is not an identifiable component but a length of resistance wire contained within the harness. The resistance wire itself is usually pink with a white tracer, but has a white or white/brown tail at the supply end, and a white/light-green on a 4-cylinder or white/light-blue on a factory V8 tail at the coil end. This is how the cars came out the factory, but if replacing the coil it is important to know if a PO has bypassed the ballast resistance or a rubber bumper or V8 for some reason, or even added one to a chrome bumper 4-cylinder car. Using a 6v coil in a 12v system i.e. with no ballast resistance will result in overheating of the coil and burning of the points. Using a 12v coil in a 6v system will result in reduced HT spark. You can't go by the colour of the wiring, there are some unfeeling butchers out there, you have to do a simple electrical test. Remove the wires from the coil on the points-side, usually black/white. Connect a voltmeter on its 12v scale to the other coil terminal and turn on the ignition. On all cars you should see battery voltage i.e. 12v.

 

may help

Rod

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Sorry Rod, but if you disconnect the points side of the coil, then it will not be drawing any current, & so you will read 12V even if the connection is through a ballast resistor. You need to do the reverse, & ground the points side of the coil, then with IGN on, measure the volts on the IGN side of coil.

I no ballast it will read 12V (or there abouts) if ballast is fitted, then you will read 6V (or there abouts).

 

Bob. ;)

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I have had various types of coils on mine, I have a direct feed (bypassing the ballast resistor).

Mine gets really hot and has done from day one :wacko:

Done Le Mans twice now with not a beat missed.

Current coil is a flame thrower as recommended by the Dizzy DR ;)

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Hi Roger

I moved mine off the block when i first got the car back in 09,it is layer on its side also just incase they have not put enough oil in it ( It happens )

every coil i have had gets very warm.

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As was done on the Works' TR4 Rally cars, mount 2 coils on the left inner wing, the second as a spare.

Mount them in the normal, upright position (all connections at the top) and make the leads of sufficient length to be able to effect a rapid transfer should the first coil fail.

Run a separate earth return so as to ensure that the coil gets the maximum voltage.

Having a large surface area and not being subject to engine heat, the wing is comparatively cool and provides a mounting largely free of engine vibration.

Ian Cornish

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Ian. Coils do not make any internal connection to the metal body, so earthing is not required.

The primary current flows from ignition wire through to the contact breaker,

& secondary current flows from Ignition wire to earth via the distributer, & spark plug

 

Bob.

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Bob - you are correct about the coil not requiring an earth connection. I had forgotten that the earth required for my electronic ignition module (situated close by the coil) is, for convenience, taken from the 'foot' (mounting bracket) of one of the coils, which has a direct, wired connect back to earth. This ensures that the ignition module gets maximum battery volts at all times.

Ian Cornish

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Quite so and I'm such a pessimist that I carry a spare coil, and a fuel pump, and a complete distributor with cap and leads, and...

 

...and a nail.

Edited by peejay4A
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I had a coil fail (went open circuit) on my TR2 on a Friday afternoon in 1965, just as I swung left onto the A45 when driving home from Stafford. I coasted onto the grass verge and left the car there, with hood erect and sidescreens fitted, and returned the next day with a new coil.

Nowadays, one wouldn't dare do that!

Ian Cornish

P.S. diagnosing on open circuit winding was quite easy, using just a sidelamp bulb and length of wire.

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