Subject: Re: [Mgs] Coil type for a 1980 MGB

From: PaulHunt73 via Mgs <[email-masked]>
Date: Mon, 31 Aug 2015 09:09:42 +0100

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 
coils.

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.

PaulH.

----- 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. 
> All
> of the wiring diagrams show two green and white wires going to the 
> positive
> terminal on the coil.  Based on what I read, I would expect about 6 volts 
> at
> the positive coil terminal.  My car has a single white wire on the 
> positive
> 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 
> on,
> I get about 11 volts at the positive coil terminal.

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From: Charley & Peggy Robinson via Mgs <[email-masked]>
Date: Mon, 31 Aug 2015 14:49:48 -0500

You could measure the coil resistance when the coil is both cold & hot, 
see if there's any difference.  Otherwise Paul's advice on determining 
if there's a ballast or not is fine.

I carry a spare coil in the boot.

CR

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