I have an unknown ignition coil for a car, how can I tell if it is 12v or 6v. If I put quick 6v across a 12v coil primaries would it fire?<?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>

 

Any model numbers, serial number or anything else marking wise.... Most to almost all coils are 6v, that's the point of the ballast resistor - to drop the voltage down. All that said, if you run 12v juice without a ballast resistor on a 6v coil it will get really hot very fast (and you have shortened the life by a year probably). Run 12v all the time and you are driving around with a grenade - tend to pop the top and frag out.

The few "12v" ballast resistors that do not require a ballast resistor have an internal resistor so internally they are 6v anyhow. You can put 6v to em, and then the output is going to be very weak - dunno if you could even get it to start.

My full explanation typed up earlier:
https://www.ford-trucks.com/forums/1...l#post10892062

 

I thought that the ballast resistor was only for starting. Devices like radio would cut out while starting due to the drop. Also some coils claimed to have internal ballast resistors. Wonder how you know which is which.

 

Also, some cars like mopars have an external ballast resistor on the firewall so I guess you would have to get a mopar coil since some have the internal resistor, no universal coil.

 

I didn't know offhand what the resistance figures should be so I googled:

Ignition coil function and testing

HOW TO TEST IT:

For preliminary test, use an ohm meter to check the resistance of the windings of the coil. For a non-ballasted ignition system like the MGA, coil primary resistance should be about 3.2 ohms. A high energy coil may have slightly lower primary resistance. A coil for use with a ballasted type ignition system (such as late model MGB) will have about half as much primary resistance, around 1.6 ohms. Resistance for the secondary winding will be very high, something like 10,000 ohms. Check that circuit resistance between the HT output and one of the primary input terminals.

I can't tell you if that site is correct, but 2*X and X are figures I expected.

 

Ballast resistor are always in the ignition circuit EXCEPT for starting with a 4-wire Ford solenoid. With a stock solenoid they are ALWAYS in the ignition circuit.

Measure the resistance of the primary coil and compare to the values in your Service Manual. If it is close, it will work without a ballast. Most 12v external ballast coils want 7.5v at the primary terminals, that is after a ballast resistor. If it a 12v with an internal ballast, you will find 2x the resistance of a 6v coil. Those are not very common.

(edit -- Bryan was typing faster, I think we're saying the same things)

BTW the reason radios cut off on starting is because they are on an Accessory circuit that is dead while cranking.

 

I bought a 6v coil at autozone but the specs are way off. The secondary is 10K ohms. I think the factory book says 5k. Could it be that all of these chinese coils are the same 12v and 6v and you just run a ballast resistor depending on what you have.

 

I am going "yes" - see below...

There are 3 styles of coils - but you will probably only find 2 and more commonly just 1 around town.
#1) True 6v coils - they like to live off 4.5v
#2) modern 12 coils - they like to live off 7.5v (and need a ballast resistor)
#3) 12v "no ballast resistor needed" coils - they live off the full 13.8v of a "12v" system.

#1 is rare - probably only found New Old Stock (NOS)
#2 is all over the place, common (also sold as 6v to put in 6v systems w/o ballast) - you use a ballast resistor with these in 12v applications
#3 is rare - 90% of the coils are #2, only about 10% are really full 13.8v

Ross is certainly more knowledgeable on this one as he learned it before me - I researched to death when I installed my supposedly #3 style which turned out to be #2 style (and learned first hand that they get really hot if ran without a ballast resistor when they should be)

edit note - I typo'd 7.8 above when I meant to put 7.5, changed.

 

Chuck at Chuck Trucks claims to have new #1's to spec for $30. I am weiry of that though. Its all Chinese junk and if it were make in the USA it would not be $30.

 

John, does your truck even have a ballast resistor?

It is the PRIMARY resistance you need to check, not the secondary. They vary the output of the coil (secondary voltage) by increasing the # of windings. The points only care what the primary resistance is. While you're shopping, stock up on 6v condensers, you'll need them.

 

Try a Napa IC7 (really Echlin). It is listed as being for 49-55 trucks, so it's a native 6V unit. You'd be able to buy locally and actually hold it in your hand before you pay your money.

 

Yeah, back in the day, we used to have to wind our own coils from old doorbell transformers, coat them with tar, and put them in old soup cans..... You young guys have it so easy!

 

I guess I should say he educated me not "he's as old as the dirt" eh

 

Older than dirt and twice as dusty!!
Back in the day, if we wanted a drink of water, we had to combine the hydrogen and the oxygen!!

 

If you had a soup can, you were the bee's knees. Some of us had to make do with an old wooden box.