ignition coil cutting out due to overheating

[Hans Efde]

Yesterday I fired up my Midas after at least half a year sitting in the garage. That went very well, so I went to the gasstation to fill the tank and took it for a spin. Within a mile it started spluttering. A bit of dirt or air in the fuel lines I thought so I kept on going. Until I was 5 miles away and the engine cut out completely. I found the ignition coil too hot to touch, so I waited until it cooled down and then it did start again and I made it home.

This is a problem I have on this car since I bought it, but never gave it me problems like this. When I bought it in 2004 it had an Aldon flame thrower (3 ohms resistance it says on the coil), but because it was hotter than a normal coil I replaced it with a blue Bosch I bought from a Mini specialist. But this became as hot. I suspected the under bonnet temperature due to the turbo, so I didn't pay attention to it anymore.

Question is what causes an overheating ignition coil. I have googled it and on many car forums it pops up. The main cause seems to be that on certain cars a resistor needs to be in front of the coil. This drops the voltage to the coil to 9v. But both coils I have work on 12v, so that doesn't seem to be the problem. Also I can't find any specific Metro wiring that is missing or faulty on my car. It used to run on ignition points, I replaced these with a Lumenition ignition a while ago. Leads seem to be standard items. Spark plugs are the long nose ones (cool type) needed for a turbo engine.
Yesterday evening I put the Aldon flamethrower in again. I'll be testing it the coming Friday (but will stay close to home!)

Does anyone have a clue why my ignition coil gets hot?
Cheers, Hans

[manifold]

Hans,

I wouldnt even bother working out the logic.

Could I say megajolt/squirt? .........(nearly) solid state. Rich, can you back my claim up?

I havent had a problem with my ignition in 4 years. Now that is saying something!

[Hans Efde]

Hans,

I wouldnt even bother working out the logic.

Could I say megajolt/squirt? .........(nearly) solid state. Rich, can you back my claim up?

I havent had a problem with my ignition in 4 years. Now that is saying something!

I have looked at megajolt, but that's still 350 pounds and I need it for other jobs. If I can get a decent powerband from the ex-turbo engine with the old fashioned stuff that's presently on I am happy. Maybe in the future I can snatch a cheap megajolt system from Ebay and improve things.

[mikeeskriett]

Hi Hans,

Normally this will be a ballast vs non ballast resistor problem you can also cook the wiring with this. The ballast resistor is not a separate item it's actually just a length of resistive cable in the loom if it's fitted at all. Similarly the coil is different for the factory electronic ignition. Now it all depends on the origin of your loom. Looks like the ballast resistor got dropped in 1984. will have more of a dig through the factory manual later on as I'll be late back from lunch if i'm not careful.

cheers,

Mike

[Hans Efde]

Thanks Mike. The loom is from a) the 1989 Metro donor or b) from GTM. I'll have a look through the bills that came with the car. Tonight I will measure the voltage on the coil with the engine off and running. Could it also be a high resistance somewhere?

[mikeeskriett]

off the top of my head the factory wire colour for the ballast cable is pink/white or white/pink. If it's a late style loom it should be 12v.

The manual does list the coil resistance for each system so I'll let you know in a while.

This is quite handy

http://www.minispares.com/Articles.aspx?ty=ad&aid=327

[mikeeskriett]

OK i've got the 1986 factory manual/data book and...

1985 > 1275 & 998cc Non electronic Ignition - Bosch 0-221-119-353or Ducellier 520068A Resistance at 20 degrees - 1.2 to 1.5 ohms
1980 >1985 1275, 998 & 850 AC Delco 9977230 - Resistance at 20 degrees - 1.2 to 1.5 ohms
1980 > 1985 998 Lucas 16C6 - Resistance at 20 degrees - 1.2 to 1.5 ohms
1985 > 1275 Electronic Bosch 0-221-122-360 or Ducellier 520067A - Resistance at 20 degrees - 0.82 ohms +- 5%

Cars with a ballast resistor in the loom have a pink and white wire to the postive side of the coil in addition to the white and yellow wire. The Turbo just has a white wire to the positive until 1985.

I've just confirmed the colours on the gold and bronze the gold being a 1986ish loom and the Bronze running a 1984 Metro loom.

Hope this helps a bit

Cheers,

Mike

[Geoff Butcher]

I second David's comments, although it sounds as if the price of MJ must have gone up since I fitted mine. I think it cost about £140 all in. Tempting fate, but all my ignition problems are a thing of the past, although a hot coil was not one of them.

[Hans Efde]

OK i've got the 1986 factory manual/data book and...

1985 > 1275 & 998cc Non electronic Ignition - Bosch 0-221-119-353or Ducellier 520068A Resistance at 20 degrees - 1.2 to 1.5 ohms
1980 >1985 1275, 998 & 850 AC Delco 9977230 - Resistance at 20 degrees - 1.2 to 1.5 ohms
1980 > 1985 998 Lucas 16C6 - Resistance at 20 degrees - 1.2 to 1.5 ohms
1985 > 1275 Electronic Bosch 0-221-122-360 or Ducellier 520067A - Resistance at 20 degrees - 0.82 ohms +- 5%

Cars with a ballast resistor in the loom have a pink and white wire to the postive side of the coil in addition to the white and yellow wire. The Turbo just has a white wire to the positive until 1985.

I've just confirmed the colours on the gold and bronze the gold being a 1986ish loom and the Bronze running a 1984 Metro loom.

Hope this helps a bit

Cheers,

Mike

Yes, I have just 2 white wires running towards the plus on my coil. I am not sure what you mean with the resistance at 20 degrees. Is that the dwell angle, making contact so you measure the coil to earth?

This evening I did a few checks with the multimeter.
Both the Bosch and Aldon coil have a 3 ohm resistance.
Engine on contact but not running: battery 11.8V, coil 10.7V That would indicate a resistance wire since it's a pretty big drop?
Engine running at idle+: battery 13.8V, coil 13.3V
That last one does not seem right to me. I found this thread:
http://www.allfordmustangs.com/forums/c ... t-why.html
They say there should be at least a 1 to 1.5V drop at the +side of the coil and the coil should get 12 to 12.5v maximum.
The weird thing is that there is a voltage drop with the engine not running, but it disappears when running. Possibly there is a wiring error in my car.
I would expect a ballast resistor to sit near the bulkhead near the resistor of the heater fan. I am trying to recall if my previous cars had 1 or 2 ballast resistors mounted. Maybe someone can clarify if they have a 2nd ballast resistor in their car?

[Rich]

I second David's comments, although it sounds as if the price of MJ must have gone up since I fitted mine. I think it cost about £140 all in. Tempting fate, but all my ignition problems are a thing of the past, although a hot coil was not one of them.

I think mine went on for less than that, having said that you can't buy the DIY kits any more.