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Allis Chalmers Discussion Forum
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WD-45 ignition: Resistance

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Guyzoo

05-01-2005 16:58:31




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Hi, I have a resistance on my WD-45 (a white piece with a kind of spring in it). What is this for? Is it to restrict electricity to the coil or what? There is a wire that goes from the instrument box to this resistance and from the resistance to the coil. My Grandfather updated the 6 volt system to 12 volts but it had a six volt coil that needs to be change. That's why I suppose that it restrict electricity to the coil.
Can I remove this resistance and put a 12 volt coil?
THANKS FOR ALL INFO

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Texas Denny

05-03-2005 08:25:46




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 Re: WD-45 ignition: Resistance in reply to Guyzoo, 05-01-2005 16:58:31  
Yes, you are entering into the Twilight Zone of ignition resistors. The auto stores sell 12 volt coils. But they often don't know if an exteranl resistor is required or not. I've had them say one isn't needed and later determined it was. The only method that I've found to determine if the resistor is needed is to measure the resistance of the coil between the two coil terminals. A 6 volt coil will have about 1.7 to 2.1 ohms which draws about 3.5 amps peak current when running (actually is less because points are open about half the time). A 12 volt coil that doesn't need a resistor will have about 4 ohms resistance. The trick is to get about 3 amps to flow through the coil and no more than 4 amps. At 6 amps, most coils burn up and so do your points! By the way, most volt ohm meters have trouble measuring low resistance. A known resistor of similar size is helpful as a standard.

Another way to check is to leave the resistor in the circuit. Measure the voltage across the resistor and across the the coil with ignition on and points closed. If the two voltages are equal, you need the resistor. If the coil voltage is twice that of the resistor, you don't need the resistor.

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Doug in OR

05-01-2005 20:05:07




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 Re: WD-45 ignition: Resistance in reply to Guyzoo, 05-01-2005 16:58:31  
All 6 volt and some 12 volt coils need an external resistor when you use them on 12 volts. If you damage this wire, you can get most any ignition ballast resistor from NAPA or other automotive place and substitute it.

You can also get a 12 volt coil with a built-in resistor. If you use this, take the resistor wire out of the circuit.



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Brian G. NY

05-01-2005 17:34:19




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 Re: WD-45 ignition: Resistance in reply to Guyzoo, 05-01-2005 16:58:31  
I'm no expert but most "12 volt coil" systems require a resistor. When cranking, the coil is fed 12 volts but when running, the power to the coil is run thru a resistor and the voltage is reduced to somewhere between 6 and 8 volts which, as I understand, reduces wear on the points. I have converted three WD/WD-45s; two of which I devised a system of providing 12V to the coil while cranking and the third is fed thru the resistor at all times. In all honesty, that one seems to start just as well as the others!

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