Thread: Correct ignition coil for 2.25 litre petrol motor

Well, GT40R the resistorthe "R" designates resistor, but you've probably already worked that out The resistor is there to reduce the voltage to the coil after startup. Running a R coil without the resistor will eventually burn it out. Maybe that's the problem you're having. I'll pop out to my SIII and have a look, because I don't remember. That will be in about 30 minutes or so, so hopefully someone who does remember will chime in first.

Hi,
My experience with coils and points is rusty, but.

If the car is has a ballast resistor, it will have about 9 volts??? at the coil with ignition on and battery voltage in the crank position.
If it doesn't have a ballast resistor it will have battery voltage at the coil at both the crank and run position of the key.

Is this coil the one you replaced in Gundagai?

If so why has it burnt out in one or two days, I have no answer, but hopefully someone will.

Best of luck with it.

Tony

Originally Posted by TonyC

Hi,
My experience with coils and points is rusty, but.

If the car is has a ballast resistor, it will have about 9 volts??? at the coil with ignition on and battery voltage in the crank position.
If it doesn't have a ballast resistor it will have battery voltage at the coil at both the crank and run position of the key.

Is this coil the one you replaced in Gundagai?

If so why has it burnt out in one or two days, I have no answer, but hopefully someone will.

Best of luck with it.

Tony

Some ( most? ) were 9, some were 7. If the coil is burning out regularly then I'd be pretty certain you have the wrong one. It doesn't matter which one you use, as long as you use or not use the resistor as the case may be, but the GT40R must have the resistor, and the GT40 will give weak spark, poor performance and hard starting if it has the R. Coils last decades. Hint though, while you're at it replace the condensor. I have not seen it, but I've heard of bad ones nuking coils. Personally I don't see why, because a bad condensor usually nukes the points.

OK, I've had a look at my 83. It does NOT have the resistor. However, if all you can buy is the GT40R and resistor it's a relatively simple matter to retrofit it. But the car needs to feed power to the coil on a seperate starting circuit to the "run" circuit. That's usually done at the key, but in your situation you could cobble something together. I'd go with the 12 volt coil myself.

A ballast resistor looks like this, and is usually mounted right next door to the coil. If you find one then it may be faulty. If you don't just get the GT40. Good luck.

Screenshot 2025-09-28 at 9.03.53?am.jpg

Originally Posted by TonyC

Hi,
My experience with coils and points is rusty, but.

If the car is has a ballast resistor, it will have about 9 volts??? at the coil with ignition on and battery voltage in the crank position.
If it doesn't have a ballast resistor it will have battery voltage at the coil at both the crank and run position of the key.

Is this coil the one you replaced in Gundagai?

If so why has it burnt out in one or two days, I have no answer, but hopefully someone will.

Best of luck with it.

Tony

Tony,

Yes, unfortunately it is the one fitted in Gundagai. It was starting well after that was fitted and the miss initially seemed cured with the new plug so I decided to take a run over to Tumut to see how it went and it was back to running rough so I pulled up and called the NRMA - bit of a mistake as the young guy didn’t have a clue on old electrics. I got it back to South Gundagai where one of the recommended garages was still open but about to shut.

The guy had a quick look at it and noted the new coil was very hot and not suited to the motor. In the process of testing it the coil cacked itself and that was that - immobile and now locked up at the garage until Monday. The advice was that a new coil, ballast resistor, points, condenser etc would have to be ordered from Wagga on Monday. My reading of the web, wiring diagram etc is that the truck never had a ballast resistor and didn’t need one now - just a new coil and whatever is adding to the miss. However, I want the correct coil to come over and that seems to be GT40 rather than GT40R. But as noted, across various forums you get either specified and one or the other can’t be correct for a vehicle with points.

I hadn’t tracked anyone down at the show who had a spare coil or points set. I sure wish I had.

Cheers,

Neil

A coil from anything that had points and condensor will do, as long as it isn't the resistor type. The "hot Coil" is imo a sign of it being the wrong one. They don't get hot. Warm, maybe.

Originally Posted by Tins

A coil from anything that had points and condensor will do, as long as it isn't the resistor type. The "hot Coil" is imo a sign of it being the wrong one. They don't get hot. Warm, maybe.

Yes, that and all of the previous advice is correct. It’s been years since I drove a vehicle with points including this truck which only came back on the road in February after probably 20 years off it. Somewhere along the line it ended up with the wrong coil (another story) which is the one that failed in Tumut. Repco only had a U1136 which Dr Google said was the correct one. Obviously it wasn’t as the replacement I fitted in Gundagai also got hot but died at the garage when it was being checked out.

Anyhoo, misadventure or not, I need to get the truck home to Brisbane sooner than later which means getting the correct parts sent from Wagga - if they are available from someone’s stock. I’m assuming it’s GT40 but if none of that are available it’s GT40R with a ballast resistor assuming that that’s also available in Wagga from local stock.

In the meantime if anyone at the Gundagai bash has a spare coil, points etc I’d sure like to meet you.

Cheers,

Neil

0419715234

PS. There are no wrecking yards in Gundagai.

Not a Series, but be aware of the following............

The stock wiring on my 74 RRC used a resistor wire in running mode and had a GT40R coil fitted, and NO ballast resistor fitted.

The resistor wire (brittle) is white with a pink (or vice versa) trace.

In starting mode the coil got the full 12 volts momentarily from the ignition switch (white wire, then white with purple trace to the coil), then switched to the resistor wire on the switch when you let the key go in running mode.

Both wires were joined at the coil.

That's how you end up having a resistor coil with no ballast.

DL