Don't get hung up on ballasted and unballasted coils, those terms cause an
awful lot of confusion and they are completely irrelevant.
Coils are either 6v or 12v. Either, both or none may or may not contain
internal ballast. However which (6v or 12v) you use does depend on whether
the cars harness contains a ballast resistance or not. 12v coils generally
measure 2.5 to 3 ohms across the primary winding, whereas 6v coils measure
about 1.2 to 1.5 ohms, i.e. about half. This is because the ballast
resistance also measures about 1.5 ohms, so the running current is the same
in both cases, but the 6v coil only has half the voltage developed across it
compared to a 12v, which has a couple of benefits.
A 1980 (in fact all rubber bumper MGBs) would originally have had the
ballast resistance in the harness, and hence a 6v coil. Before that there
would be one white wire on the coil +ve or SW, after that it would have been
two white/light-green wires. If you only have a single white on the +ve
then your ignition system has been modified for some reason. If you remove
the ignition feed wire from the coil you will always see 12v on it,
regardless of whether there is a ballast in circuit or not, so that is no
guide as to which coil you need.
What voltage you see across the coil terminals depends on whether the engine
is stopped or running, and if stopped whether the points or electronic
trigger are closed or open. Many electronic triggers don't 'close' to pass
current through the coil until the engine is cranked. Unless the coil is
passing current you will always see 12v on the 'live' (i.e. +ve of a 1980)
end of a coil, regardless of whether it is a ballasted ignition system with
a 6v coil or an unballasted system with a 12v coil. It's also complicated
by people bypassing the ballast when fitting aftermarket ignition systems or
With the engine running you will see different voltages, which I won't go
into as it depends on whether you have the right combination of coil and
ballast and what type of ignition trigger.
The easiest way to check is first measure your coil primary resistance.
Then connect the wiring to the +ve terminal, turn on the ignition, connect
an earth to the -ve, and measure the voltage across the coil. If you see
12v then there is no ballast resistance in circuit, so you need a 12v coil.
If you see less than 12v, then you probably have ballast in circuit, and it
depends on which coil you are using as to what voltage you are seeing. If
your coil measures 1.5 ohms and you see 6v then that is correct. If your
coil measures about 3 ohms and you see 12v that is also correct. But if you
have a 12v coil i.e. 3 ohms in series with a ballast, then you will see
something between 6v and 12v. Ballast resistances can also go faulty, which
will also result in different voltages being seen across the coil.
If your coil has failed and you are trying to find out which one to buy,
then unless you can borrow another coil that is for the same type of
ignition system, about the only thing you can do is turn on the ignition and
measure the resistance between the wire that goes onto the coil +ve, and the
white or white/brown wires at the fusebox, looking for 1.5 ohms. Turning on
the ignition when you are only measuring resistance may seem strange, but
the ignition wiring can vary or have been mucked about with on late model
cars with ignition relays like yours. However ballast resistances like
coils can go faulty when hot, so you may see 1.5 ohms when cold, but it goes
higher when hot, affecting your HT.
----- Original Message -----
> Everything that I am reading says that an 1980 model should use an
> unballasted coil because the ballast resistor is in the wiring harness.
> of the wiring diagrams show two green and white wires going to the
> terminal on the coil. Based on what I read, I would expect about 6 volts
> the positive coil terminal. My car has a single white wire on the
> terminal. If I remove that wire and measure the voltage with the ignition
> on, I get 12 volts at the connector. With the wire connected, ignition
> I get about 11 volts at the positive coil terminal.