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Coil wiring

hawkwind

Freshman Member
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I'm trying to figure out my electrical. The wiring diagram in the Haynes manual (fig 10.28) hasn't been as helpful as I would have hoped. On my coil's low tension terminals, I have the white/green and the white/black, which I've seen clearly documented.

There is also a black wire on the negative post and a broken terminal lug on the positive post that I believe went to a red wire. The aforementioned red and black wires go to a black heat sink assembly with a power transistor on top. There is also a purple wire coming from the heat sink assembly which seems to have been going nowhere. Based on the sloppy terminal crimps and tie wraps, I think it's safe to say that a PO was doing some home electrical work.

Anyone know what that heat sink assembly might be? If it's located on the wiring diagrams I've looked at, then I'm not seeing it. I've been an electronic technician type most of my life so I can handle tech talk and read schematics.

Edit: This is for a 1976 Midget.
 
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hawkwind

hawkwind

Freshman Member
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Just remembered that the heat sink assembly also has a three wire connector going to the distributor so I guess I've partially answered my own question. It must somehow transmit the timing pulse. Still not sure how it needs to be wired up though. Is there an easy way to test this module? Is it just a switch circuit? And what about that purple wire? /ubbthreads/images/graemlins/tongue.gif
 

Dave Russell

Yoda - R.I.P
Gold
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Hi Hawk,
I don't know if the MGB & the Midget used the same ignition or not. Nothing seems to quite match what you describe.

Go here:
https://www.mgb-stuff.org.uk/s_ign.htm
See if you can find anything.
D
 

sparkydave

Jedi Knight
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Hi Hawk, it sounds like somebody replaced the stock ignition system on your car. The one from the factory had the guts contained in a box attached to the side of the distributor (Lucas OPUS system), but was a notoriously unreliable system. Lucas eventually had a better system to replace that one with (CEI), but I can't tell if that's what you've got from the description. Apparently it's some kind of aftermarket electronic ignition. Since mine was dead when I got the car, I went with a Pertronix system.
-Dave
 

als

Senior Member
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The heat sink is for a ballast resistor which is contained within it. If your system is still using the ballast resistor then it is not likely an electronic ignition.

My 76 had a replacement dizzy in it with the ballast resistor.

I have also replaced the points/condenser with a Pertronix. It made a huge difference in the performance of my car. The ballast resistor had to be taken out as it dropped the voltage on the coil too low for the Pertronix.

Hope this helps
Al /ubbthreads/images/graemlins/canpatriot.GIF
 

sparkydave

Jedi Knight
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Sorry Al, but I'm going to have to disagree with that. He did say it has a power transistor attached to the heatsink, and 3 wires going to the distributor. The heatsink is to cool the transistor, not the ballast resistor. The power transistor and 3 wires would be consistent with an electronic ignition, and electronic ignitions can still use ballast resistors. I had to keep mine since I used the stock coil, otherwise either the coil or the Pertronix might not have lasted long. A friend of mine found this out the hard way. I did have to run the one lead from the Pertronix to 12 volts, because as you said, the voltage was too low with the ballast resistor.
-Dave
 
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hawkwind

hawkwind

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I think it must be an aftermarket ignition, which explains why I don't see it on the wiring diagrams. I see a high watt resistor physically located in that area and I assume that is the ballast resistor. My distributor has the sensor type setup inside. Is that what they call electronic ignition? Seems like just a small step over breaker points to me. OK, I guess I can assume that this ignition module should connect to the lo tension posts on the coil. Any tips on what voltages or resistances I should look for? /ubbthreads/images/graemlins/smile.gif
 
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hawkwind

hawkwind

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Just got home and took a quick peek before it got dark. The heat sink module has a sticker on it that says Allison. I'm pretty well convinced that it is an aftermarket electronic module. The coil and ballast resistor are mounted on the pedal box cover.
 

sparkydave

Jedi Knight
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Hi Hawk, glad you've got the make of the aftermarket system figured out. I took a quick look under the hood of my car and in the service manual, and unless somebody made other changes, the white and green wire should be a resistance wire that makes up the ballast resistor, and it would go to the positive side of the coil primary. I'm not sure if the power resistor you refer to is part of the Allison system or not. There was a power resistor originally mounted under the coil bracket which was part of the OPUS system, but it wasn't a ballast resistor. This had a white wire going to +12 volts, and a blue and white wire going to the OPUS module.

-Dave
 
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hawkwind

hawkwind

Freshman Member
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Thanks for the link Dave. That looks like the inside of my distributor, though mine is not quite so new and shiney. The module though is much smaller than the one in those pictures. Obviously, mine would be an older model. I couldn't get the pdf to download though. /ubbthreads/images/graemlins/frown.gif It said the link wsn't there.

Thanks for all the other suggestions too. I have an idea of what I'm dealing with now. /ubbthreads/images/graemlins/grin.gif
 
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