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Old 1st August 2020, 13:12
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Mister Towed Mister Towed is offline
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So, electrical woes now hopefully sorted.

I'm really keen to have all the electrics fully sorted before my first attempt at starting the engine as I have fitted a reground cam. The instructions for the cam say to run the engine for 20 mins at 2,000rpm on first start without lots of slow cranking beforehand, as this will fuse the bearing surfaces of the cam and followers together, scrapping both, so it's important the engine fires quickly.

Now, the wiring diagram and colours/combination of actual wires in my loom indicate I have a built in ballast resistor wire on the ignition circuit, so I need a 1.5 ohm ballast coil rather than a standard 3 ohm coil.

As luck would have it, my Midget runs a separate ballast resistor on the ignition circuit and I have a spare ballast coil in the boot, hurrah!

For those unfamiliar with such wizardry, the ballast resistor or wire reduces the voltage from 12v down to 6v, which powers the 6v ballast coil in normal running.

The ballast resistor or wire is then overridden by another wire carrying 12v directly from the output side of the starter solenoid on engine start to make sure there's still a strong spark when the starter motor is pulling all the current out of the battery. The coil won't fail during start up because it doesn't have long enough to overheat while it's being 'overpowered'.

With this in mind I connected the battery and checked the voltage at the coil positive terminal, which should be 6v. Hmm, 12 volts present.

I checked the resistance from the ignition switch to the coil positive terminal and it's 2 ohms. The manual says 1.5 ohms, which is supposed to drop the voltage to around 6, but I have 12. I also checked the resistance through the secondary coil positive circuit from the starter solenoid and that's virtually zero ohms, which is right.

I then checked the voltage at the Midget's positive coil terminal and it's 8 volts as it should be.

As if that wasn't confusing enough, I then checked the resistance across the coil that's happily running in the Midget, the new spare coil, the coil that was in the Midget when the electronic ignition module failed and a new Lucas DLB105 Sports coil I have on the shelf, and they're all the same at 3.5 ohms.

After much head scratching (electrics are a bit of a mystery to me) I decided to run a zero resistance 12v to the coil positive terminal, use the Lucas sports coil which is the only one I know for sure isn't a ballast coil and ditch the ballast wire.

Everything seems to be fine now, I have 12v where I expect it and the right coil fitted to handle it. Hopefully...

I'm waiting on a few parts for the cooling system at the moment and once they arrive there's nothing to stop me firing up the engine. I do hope it all works.

Last edited by Mister Towed; 1st August 2020 at 13:15..
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