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MGB 1977 MGB Ignition Coil and Wiring.

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YesImDoug

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Hey guys,

I recently purchased a 1977 MGB. Let me say, the wiring is a complete mess, I don't even know where to start.
I wish I could upload pictures right now, but I can't.. (Will edit some in later if needed)

I want some help, I don't exactly know what wires should be going to this Ignition Coil. Currently there's only one wire on it, and it's a ground.
Coil, plugs, and distributor are all new.

I was told the ignition coil is the only thing not causing the engine to start (It cranks, no spark ofc) Is there any way to by-pass this just to hear it started? Also, does anyone have pictures of how a properly wired Ignition Coil looks like? Maybe some good wiring diagrams, any advice would be appreciated!

Thanks,
Doug
 
Hi, Doug, and welcome to the BCF!

First, check here for your wiring diagram; scroll down to find your year model: https://www.advanceautowire.com/mgb.pdf

Once you've done that and hooked up the proper wires get back to us and let us know what happens.

Mickey
 
Hi Mickey,

Thank you for the welcome, also for the wiring diagram, that makes it so much easier. Do you happen to also have a picture of a Coil Properly wired? I like to have something visual I can follow the diagram with first then tackle mine!
I'll be doing this, this weekend.. Sucks working 8 am - 8 pm in Canada... dark when I leave, dark when I come home :(

Thanks,
Doug
 
Sorry - not near my cars at present. But if you Google MGB ignition coil images you'll get plenty of pictures. Hopefully, someone hasn't replaced or switched wire colors on you...

:cheers:
Mickey
 
The coil has two low-voltage terminals. One goes to the points on the distributor, the other needs +12V. The latter should be switched; i.e., turned on and off with the ignition switch.

For testing, you can just run a wire from the battery. This is called "hot wiring" the car.

I forget how the coil terminals are labeled. The coil does have a polarity (i.e., a particular terminal has to be connected to the distributor and the other to the +12V), but it's not all that important. The car will run even if you get it backwards.
 
I *think* your '77 has the same ignition setup as the '78....a ballasted ignition. If so, you should have two white/black wires on the negative post (one supplies the signal to the tach, the other goes to the distributor). The positive post has two white/green wires. One feeds power to the coil directly from the starter solenoid. The other feeds power to the coil as well, but through either a ballast resistor or a resistor wire.

If you do have the ballast resistor wiring, the wire direct from the starter solenoid energizes the coil when the ignition is in the "start" position. This supplies full battery voltage to the coil for a hotter spark. When the switch is returned to the "run" position, the coil is supplied via the resistor wire...reducing the voltage to the coil.

The coil for the ballasted system is a 1.5 ohm coil (measured across the + and - terminals). The one for the non-ballasted system is a 3 ohm coil. If you have only one white/green wire, your system is non-ballasted and should have a 3 ohm coil. If a 1.5 ohm coil was run continuously on a direct 12v supply it would kill the coil Lots of cars that were originally ballasted have been converted when converting the original ignition to some other type, and converted to a 3 ohm coil in the process.

Hope this helps get you started!
 
Joe Reed said:
I *think* your '77 has the same ignition setup as the '78....a ballasted ignition. If so, you should have two white/black wires on the negative post (one supplies the signal to the tach, the other goes to the distributor). The positive post has two white/green wires. One feeds power to the coil directly from the starter solenoid. The other feeds power to the coil as well, but through either a ballast resistor or a resistor wire.

If you do have the ballast resistor wiring, the wire direct from the starter solenoid energizes the coil when the ignition is in the "start" position. This supplies full battery voltage to the coil for a hotter spark. When the switch is returned to the "run" position, the coil is supplied via the resistor wire...reducing the voltage to the coil.

The coil for the ballasted system is a 1.5 ohm coil (measured across the + and - terminals). The one for the non-ballasted system is a 3 ohm coil. If you have only one white/green wire, your system is non-ballasted and should have a 3 ohm coil. If a 1.5 ohm coil was run continuously on a direct 12v supply it would kill the coil Lots of cars that were originally ballasted have been converted when converting the original ignition to some other type, and converted to a 3 ohm coil in the process.

Hope this helps get you started!
Click to expand...
Hi Joe, thanks for your explanation. I have a 1977 MGB with messed up wiring. The green white from the starter is intact. The "run" wire has been cobbled into a splice and is sending 12 volts to the coil. There is a hot, but cut green white white near the coil, but it is also testing at 12 volts. There is also a cut white wire at 12 volts. I have an original Opus set up, with an old 1.5 ohm coil. Originally, do you know how the resistance ignition wires ran? Was it all white? Or white into green white? Or green white into white?
I put an aftermarket resistor in front of the coil, but it made little difference. Thanks, Mark
 
I'll try to help, but I'm no expert - and my car hasn't had an Opus system since sometime in the last century. :smile: Someone smarter than I am will probably be along later to correct any of my errors.

I'll start with the easiest part. The negative side of the coil should have two white/black wires. One of these goes to the center position of the plug to the distributor. The other goes to the tachometer.

The positive side of the coil would have two white/light green wires. One comes directly from the starter solenoid. The other is where it gets a little more complicated. That white/light green wire at some point becomes the ballast wire (maybe a pink wire?) that goes forward in the harness, then makes a U-turn back toward the distributor. The length of that wire is what provides the resistance required.

That ballast wire (I believe) goes to a 4-way connector. There should be 3 white wires at that same connector.

One goes to position #5 on the fuse box (second from the bottom on the left side as you face the box).

One goes to the distributor resistor, where is exits as a white/blue wire that goes to the left side of the plug at the distributor (as you face the end of the plug from the top, looking toward the distributor).

The third white wire goes directly to the other side of that plug (right side).

This arrangement (of course) is assuming that a PO hasn't been creative elsewhere in the wiring.

I hope this at least leads you in the right direction.
 
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