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Tractor Talk Discussion Forum

Distributer Coils

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Bob-IN

01-30-2005 16:40:32




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I have a number of coils where the 6v or 12v stamping has disappeared because of time, use, etc. Is there any way to distinguish between the 6v and 12v coils?




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Bob

01-30-2005 17:08:18




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 Re: Distributer Coils in reply to Bob-IN, 01-30-2005 16:40:32  
A 6 Volt coil will have 1.5 to 2 Ohms resistance measured across the primary terminals, while a 12 Volt coil will measure about 3 to 3.5 Ohms across the primary terminals.

Be aware that it takes a good ohmmeter to measure resistance this low accurately. (Be sure the meter is working by shorting the two meter leads together, and verifying that the meter reads "zero" Ohms.)



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jdemaris

01-31-2005 18:45:38




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 Re: Distributer Coils in reply to Bob, 01-30-2005 17:08:18  
That's not universally true. I've got several types of brand new 12 volt coils sitting here on the shelf that will test out (and are rated) at 1.8 ohms across the primary terminals - for example, the Echlin IC27, 12 volt coil made for use with an external resistor (i.e. there is not an internal resistor) and listed for early 70s Dodge trucks. I've also got many 6 volt coils that will read anywhere from .8 to 1.6 ohms.

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buickanddeere

01-31-2005 23:21:39




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 Re: Distributer Coils in reply to jdemaris, 01-31-2005 18:45:38  
It varies, should the ignition system be electronic and vary the dwell with rpm. A points system "12V coil" used on a "bypass" start system. Really is a 6V 1.5 or so, ohm coil. The 12V coil with no ballast on my vehicle is 0.6 ohms. The electronics limit current application time (dwell) to prevent overheating at low rpms.



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jdemaris

02-01-2005 06:02:49




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 Re: Distributor Coils in reply to buickanddeere, 01-31-2005 23:21:39  
That's my point, it does vary. Since the guy is looking for a way to identify unmarked coils, I assert there is no absolute primary resistance value using Peter Ohm's law that will identify what the coil was intended for. When these coils first began to be used, instead of vibrator coils, i.e. "high tension a.k.a. jump spark ignition" along with "synchronous ignition", many only had an output of 8,000 - 10,000 volts. Voltage requirements increased along with compression ratios. And, even some early coils intended for 6 volt systems had internal primary resistors which compounds the issue. Secondary values vary widely also. You can find two old, unidentified coils, they can both give similar resistance readings, but one might be rugged enough to endure full-time 12 volt use, whereas the other might not. And, some are polarity sensitive, and some are not. I find it frustrating at times since I've got hundreds of them laying around - and with many -have no idea what they were originally used on.

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RAB

01-30-2005 22:56:30




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 Re: Distributer Coils in reply to Bob, 01-30-2005 17:08:18  
If your meter is not up to measuring low vlues of resistance that accurately, measure volts and current and calculate resistance from Ohm"s Law.
Regards, RAB



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txblu

01-30-2005 20:04:41




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 Re: Distributer Coils in reply to Bob, 01-30-2005 17:08:18  
I happen to catch the design current for ignition points one day in a MF Shop manual. It said the points were rated at 4 to 5 amperes. That makes Bob's numbers right on. That's why the infamous "ballast" resistor is necessary when using a 6v coil in a 12v circuit.

Mark



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