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MG MGB GT V8 Factory Originals Technical - Ballast resistor

I suspect that this might be a big question but I rely on your patience (and expertise).
I am putting a 3.9 1995 Rangerover motor with EFI into a 1975 B. I am using a SD1 starter motor and I (by chance) have a coil from a 1986 Rangerover. Do I need a Ballast resistor and how can I wire it into the circuit, or alternatively, short it? Ballast resistors & what they do are a new (& worrying) concept to me but from the archives I've been able to gather that systems with them have a six volt coil and during normal running the resistor (which I haven't been able to identify although I believe they were present on 1975 Californian cars) reduces the voltage to the coil from 12 to six volts. During cranking a terminal on the starter (the lower "small type wire" terminal on the SD1 starter?) causes this resistor to be bypassed. I'm not too sure of its colour but on the MG is this the "other-not the ignition wire"?

To try and break this down a bit-

1-Is my coil a 6V or 12 V? How can I tell? Which one should I be using with the above '95 EFI set up?

2-If I should be using a 6V coil and the '86 Rover coil is a 12V, how to I bypass the elusive Ballast resistor?

3-If infact I don't have a Ballast resistor (and should have for the '95 EFI) how do I wire it in? I'm guessing that a resistor should be placed between the positive terminal on the coil and the Battery. How much resistance is required, which wire?

I hope this isn't all too much to ask but I would imagine that it I try to run a 12V coil with a Ballast resistor I'll get a very poor spark and if I try a 6V coil without a resistor somthing (probably the coil)is going to get red hot.


Whether a coil needs an external ballast resistor or not is entirely due to what coil and ignition system you are using, not the engine.

You can generally tell whether a coil is 6v or 12v by measuring the primary resistance - MGB factory coils measure about 3.2 ohms for the 12v and about 1.5 ohms for the 6v. Lucas 'Sport' 12v coils can measure about 2.4 ohms. Specialist coils used as part of an electronic ignition setup can measure as low as 0.1 ohms, these should only ever be used with the appropriate ignition system.

Rubber bumper MGBs for all markets had 6v coils with external ballast, in these cars the ballast is a length of resistance wire contained within the main wiring harness.

If you have the later solenoid with the boost terminal on it then using a 6v coil with ballast is preferable as it gives a boost to the spark during cranking, rather than the degradation which occurs with 12v coils, and can make the difference between starting and not starting under adverse conditions.

The ballast resistance is uaually about equal to the coil resistance, some 3rd-party systems comprise a 6v coil and such a resistor, usually a hefty ceramic item so as to cope with the 10W or so of dissipation. They are usually placed in the white wire between the ignition switch or relay and the coil +ve terminal - assuming a negative ground system. Another wire is taken from the coil +ve - typically white/light-green or white/light-blue on the MGB and V8 - to the boost terminal on the solenoid and it is this which supplies the full battery voltage to the coil during cranking. You can use a relay to do the same job if you have the early solenoid but want to use a 6v coil with ballast and still want the boost. You can use a 6v coil and ballast without the boost, but then you get the degraded spark during cranking.

If you want to use a 12v coil on a ballasted system you take a white from the white or white/brown at the fusebox to the coil +ve. If you disconnect the ballasted wire at the coil you should also locate and disconnect the other end of that wire at the fusebox or ignition relay and solenoid and tape-up all the loose ends to stop live wires flapping about and potentially (ho ho! (seasonal joke)) shorting out.

If you run a 12v coil on a ballasted system you will get a weak spark as you say, the other way round is likely to burn-out the points or trigger as well as overheat the coil.
Paul Hunt

Thanks for the great response. My '75 loom seems to be an odd mix of '74 American and '75 English, with the added complication of the Left to Right conversion..
I've located the ballast resistor wire, pink and white, it runs across the front of the car and back again. As well as the ignition & coil this wire is also conectioned to a small black (1 by 1 by 4 cm)'thing' with heat radiating fins (3 way conection; coil, Ballast resistor and Thing. The Thing isn't in series between coil and ignition. This thing has a male connector on each end and was located on the radiator bulkhead.. I had thought this might be the resistor figuring heat radiating capacity but it isn't connected to anything else, any idea what this is meant to do?.
You mention a relay from the boost terminal (the one which runs the starter?). I was thinking of running a wire direct from this to the coil, connecting the green and white wire to it. This would then mean a green and white wire direct from the boost terminal to the + coil and a green and white wire from the ignition through the pink and white Ballast resistor wire to the + coil. Is a relay necessary, will dire things happen without it?

Hmmm, the 'thing' is a bit of mystery. If is screwed to the body work and has one other connection that goes to the junction of the ballast wire and the coil wire then it *could* be a radio interference suppressor, but that wouldn't need fins.

If you have the solenoid with the boost terminal you connect up as you have described i.e. the white/light-green from the end of the ballast to the coil to the solenoid and no relay. The relay would only be beneficial (but not essential) if you had the later 6v coil and ballast but the earlier starter which doesn't have the boost terminal.
Paul Hunt

Peter, it sounds like a resister & may be obsolete from a previous application & never been removed.I get the impression only one end is connected ? This might be used as a convenient junction point for the 3 wires mentioned. Barrie E
Barrie Egerton

I hooked it up(as per above) last night and (with the ignition on) and the loom wire ballast resister overheated to the point of melting its' pink and white plastic coat. Could be the 'thing' is a Ballast resister? added on after the orrigional overheated some time in the past (earthing somewhere?). It (the thing) has a resistance of 10 ohms.
I also identified the ceramic type ballast resistor on my old SUBARU (next to the coil) and it has a resistance of 2 ohms so on second thoughts the Thing can't be that. Are Ballast resistors usually about the 2 ohm mark?
Maybe the Subaru is going to become an organ donar.

Ballast resistors are typically about the same resistance as a 6v coil in order to reduce the voltage at the coil by about half, i.e. about 1.5 ohms in the case of the MG coil, which is close enough to your Subaru ballast.

The ballast wire will get hot - about as hot as the coil since they are both dissipating the same power (about 24 watts) - even hotter if the ignition is left on with the points closed and engine not running for any length of time.

North American 45DE4 distributors did have an additional resistor as an identifiable component which fed the electronics, but this was supplied with the full 12v off the white/brown i.e. before the coil ballast. I don't know what the value of this resistance was.

If the 'thing' measures 10 ohms and is connected between the junction of the loom ballast and the coil on one side, and ground on the other, then not only will it be drawing more current through the ballast wire than normal and hence making it even hotter, but it will also be reducing the voltage to the coil.

It's unlikely to have been added because the ballast wire was grounding, that fault would result in zero volts at the coil end of the wire i.e. where the 'thing' is connected so would have made no difference to a dead engine.
Paul Hunt

Many thanks for the advice/knowledge. All is clear now and I shall go and buy a new resistor and wire it in as they are certainly cheap enough. The "Thing" is a goner and destined for the rubbish bin. I suspect that you are right and it was for the distributor, which on my car have been changed for a Bosh dizzy

Just for laughs..
Umm .In hindsight it's not such a good idea to directly connect the starter motor to the ignition, you definitly need a relay, otherwise the obvious happens.

This thread was discussed between 27/12/2002 and 09/01/2003

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