Farm Equipment - coil external resistor

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coil external resistor

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Topic: coil external resistor

Posted By: Nautyboss
Subject: coil external resistor
Date Posted: 24 Feb 2012 at 8:26pm

I have a 1952 WD that I'm changing over to 12V. The coil I have says use with external resistor. What size resistor do I use, and do I just put it in series with the coil?

Replies:

Posted By: Steve in NJ
Date Posted: 24 Feb 2012 at 8:35pm

Actually, you really don't want to use an external resistor. Mother Nature plays havoc with them. They didn't work under the hood of a sixties automobile, they sure as heck don't work well hanging out in the elements on a Tractor. If you are switching to 12V, with a 4 cylinder engine, you need to have 2.7-3.3 ohms of resistance in the Ignition circuit. Do yourself a favor and buy a new 3.0 ohm internal resisted coil to replace the 6V coil. Watch the "universal" 12V coils. They're only rated at 1.5 ohms. That's fine for a 6 cylinder or 8 cylinder engine. Not a 4 banger. If you have a local Napa store, take a stroll there and see if they have a 3.0 ohm coil. We have em' in stock also... HTH
Steve@B&B

Posted By: Coke-in-MN
Date Posted: 24 Feb 2012 at 9:18pm

Be sure to keep the polarity of the coil the same as the tractor or you will have points burning out quickly if reversed.

I have had the external resistor on my IH 460 since I bought it in 1972 with no problems at all with it . Replaced the coil once but resistor still good.

-------------
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Posted By: DougS
Date Posted: 24 Feb 2012 at 9:31pm

60s and 70s MOPAR vehicles used this series resistor. It is about 1.7 ohms. As Steve said, the elements will trash it unless you protect it. In use these resistors get HOT. If it gets hot and then a little rain hits it, the thing may very well crack. I agree with the above. Get a "true" 12 volt coil. It should measure approximately 3 ohms across the terminals.

Posted By: Gerald J.
Date Posted: 25 Feb 2012 at 1:16am

Reversed polarity on a coil primary won't burn up the coil or the points. Just the spark isn't as effective in igniting the mixture. My Ford truck has half the plugs each polarity but isn't point ignition and it has 4 coils for 8 plugs. Each plug is fired once per revolution. And the replacement plugs have platinum on both the center and the ground electrode.

I've had an external resistor on my gas 4020, one of the ceramic blocks with screw terminals, for at least 12 years. It was on there when I bought it and has given no troubles. Originally they used a resistor in the ignition switch and have an auxiliary contact on the starter solenoid that shorts the resistor while cranking. I did have troubles once with it firing while cranking but not continuing to run. So I finished up hay making that day with it hot wired bypassing the ignition switch and wiring. When I dug in I found a new wire spliced in with a 3M Scotch tap that had gone open. I replaced it with a Y made of spades, male and female and its been reliable ever since. I no longer accept Scotch taps as electrical connections.

Finding a true 12 volt coil is trivially simple at NAPA and other car parts places.

Vintage Chrysler hot rod products used a low voltage primary coil and a series resistor because that combination lets the current rise faster and so needs less dwell time for high speed ignition. The low voltage coil has fewer primary turns and so lower inductance and the rise time is a function of circuit resistance divided by circuit inductance. A 12 volt coil can have more primary turns or the same primary turns and a built in resistor, either way works for a slow speed tractor engine.

Gerald J.