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Oct 25, 2006
Hi Guys, new to site. I have a question concerning wiring I think one of you may be able to help me with.
Is it possible to convert a commando to negative earth and if so what are the steps?
Reason being is that I have an extra thunder heart wiring system and was thinking of using it on the Commando project I have going at the moment, if I can convert it to neg earth.
I run one of these in my shovelhead harley and it really cleans up the wiring.
Any advice :?:
Cheers Jughead

Shouldn't be too much of a problem to convert to negative ground, just a matter of swapping wires to the correct terminals on battery & rectifier. Biggest problem would be the two voltage regulating zener diodes, that are bolted to the alloy bracket behind the gearbox. These would need to be replaced with units having the cathode on the stud side; otherwise use one of the aftermarket regulator modules.

It isn't too difficult to convert to Neg Earth/ground, Parker mentions just swapping the battery and rectifier wires although it's not quite that simple, at least if you have an original rectifier? As its fitting stud is its ground terminal. If one of the modern small encapsulated rectifiers is used then there's no problem, and the same applies if a modern voltage control box is used instead of the rectifier/Zener setup.

Parker mentioned two diodes, although two are normally only found on the 850 Mk III models that use the 180W RM 23 alternator (or three phase upgrade kits), and I'm not sure if reversing the polarity would affect the electric starter rotation on a Mk III, -possibly, but this may not be a MkIII model anyway?

The earlier models (as standard) use only one diode which will need to be changed to a Neg type, and it would probably be easier to use one of the control boxes available (Boyer Power Box/Podtronic/Tympanium) in place of the rectifier and diode/s.

If you have an electronic ignition fitted then that will also need to be wired according to polarity. And with the original points ignition the coil connections would need to be reversed, also the same for the 2MC capacitor (if fitted) and possibly the flasher relay as well?
I am starting with a clean slate. No wiring at all apart from that which comes with the kit. The controller unit in the thunder heart system controls all functions such as circuit breakers, flasher unit & high/low beam etc in one small electronic gizmo. I do have one of those solid state rectifiers and an accel duel coil and have boyer electronic ignition. Dont have a zenor diode or MC2.
If any of you electronic wizzes can help with a rough wiring diagram showing connections etc, I would be forever grateful.
Personally I've no experience of this Thunder Heart system or how it would need to be integrated into the Commando wiring.
As far as the harness is concerned then there's no difference between wiring up for either Neg. and Pos. Earth (the wire colours would normally be different), so the original wiring diagram can be followed and the polarity changes made to the necessary items.

If the rectifier that you have is the type about 1.1/4 inches square then that is just a basic rectifier, and some sort of voltage regulation will be required in the form of a Zener diode, although already mentioned, it could be better to use a modern type control box as this would perform both regulator and rectifier functions (you have not mentioned so far, which Commando model/year or what alternator version is fitted, as the setup for the MkIII is different to the earlier types, and it is possible that a three phase unit could have been fitted which would require a different setup again.

Charge circuit info:

Boyer Negative Earth/Ground info and simplified wiring diagram info:

I am not sure what this Thunder Heart system would do for you. Do you already have a solid state voltage regulator or are you using the original recitfier and zener diode? Personally, I would ditch the recitifier and diode and install a solid state voltage regulator (either single-phase or three-phase depending on your alternator as indicated by LAB). You don't need a separate control box for flasher and high/low beams do you? Why not just buy or make a new wiring harness and leave everything positive ground? Weird wiring is just one of the many charms of British iron isn't it? :wink:
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