Ignition coil question

[T.R.K.]

I have a box of old ignition coils. I need a 12v coil. What primary resistance am I looking for in a 12v. coil?

 

[old]

If I remember right 3-4 ohm for 12 volts

 

[John T]

Over many years (used tractor dealer & farmer) I have seen them in the 2.5 to near 4 ohm range, while a typical value would be around 3 ohms LV Primary resistance on a true 12 volt coil.

NOTE A Coil labeled 12 Volts for use with external ballast is in reality more like a 6 volt coil and indeed needs the series external voltage dropping (12 down to 6) ballast resistor or it will overheat. A coil labeled 12 volts NOT for use with ballast or no ballast required is like it says, a 12 volt with no ballast used.

NOTE to measure low resistance accurately takes a quality ohm meter

John T

 

[Geo-TH,In]

Ohmmeters are the least accurate meter you can buy.
Cheapie digital meters can't be adjusted to zero ohms like the old analog meters.
Ammeter are more accurate.
use a 12v battery and connect it to the coil.
Most coils require 4 amps.

Ohm's law
12v = 4 amps x 3 ohms..

So 4 amps means the internal resistance is 3 ohms.

My Jubilee was converted to 12v and it has a 6v coil.
I measured 4 amps and 6 volts used by coil and 6 volts used by ballast.




Try using an ammeter instead of a voltmeter.

A 6v coil connected to 12v will draw 8 amps..

Don't leave connected to 12v too long or it will damage coil..

 

[BarnyardEngineering]

Ammeters come with their own special considerations.

You need to connect the ammeter up correctly. It's not like a voltmeter where you touch the + and - and take a reading. You have to connect the ammeter IN SERIES with the load to read the current. That is + battery to + coil, - coil to + ammeter, - ammeter to - battery.

The ammeters built into your average multimeter, cheap or expensive, are limited to 10A loads for no more than 10 seconds. Beyond that you risk overheating the meter and causing damage.

 

[Geo-TH,In]

4 or 8 amps won't damage ammeter.
I do it all the time.

Best to measure amps and if necessary
increase or decrease the ballast resistor
to get 4 amps.

Points will last longer.

Digital ammeter is no brainier.
No need to worry about polarity like an
analog meter.

 

[T.R.K.]

Thanks for all the replies. I will look for a 3-4 ohm resistance coil.