12V coil cooking

Hello and thanks for the help in advance.

49' 8N Front Mount.
Converted to 12V w/ 3 wire alternator & diode.

I keep burning up 12V coils. How is this possible? 2nd one died today after a few hours. I only measure 3.8A of primary current.

Circuit is:

Battery to fuse
fuse to ignition switch
ignition sw to original wire wound resistor
resistor to coil.

Measurements:

Tractor not running. Coil off distributor. Ignition switch on. Ammeter (DVM set to amps) from spring thingy on coil to chassis. 3.8Amps

Run tractor for a while. Tractor dies
Repeat test > 10 amps (meter limit)and resistor gets too hot to touch in ~10-20 seconds.

When I did the conversion I measured 12.6V at the top of the top of the coil points open and about 10 odd volts when running and points opening & closing. Did not repeat this test with second coil

Any clues? I'd have believed one crappy coil, 2 seems a bit unlikley.

Phil

(quoted from post at 20:59:51 07/18/09) Hello and thanks for the help in advance.

49' 8N Front Mount.
Converted to 12V w/ 3 wire alternator & diode.

I keep burning up 12V coils. How is this possible? 2nd one died today after a few hours. I only measure 3.8A of primary current.

Circuit is:

Battery to fuse
fuse to ignition switch
ignition sw to original wire wound resistor
resistor to coil.

Measurements:

Tractor not running. Coil off distributor. Ignition switch on. Ammeter (DVM set to amps) from spring thingy on coil to chassis. 3.8Amps

Run tractor for a while. Tractor dies
Repeat test > 10 amps (meter limit)and resistor gets too hot to touch in ~10-20 seconds.

When I did the conversion I measured 12.6V at the top of the top of the coil points open and about 10 odd volts when running and points opening & closing. Did not repeat this test with second coil

Any clues? I'd have believed one crappy coil, 2 seems a bit unlikley.

Phil

Click to expand...

Soooooo....

That's why I run a 6v, positive ground, Chevy alternator.

Sadly that wouldn't run the 12V sprayer...

Of cource you could say the same thing about the current situation. :- (

p

Phil,

Are you sure you are using 12 volt coils? 6 volt coils will burn out with just the original ballast resistor in line in a 12 volt system. Where are you getting the coils? You can measure the resistance to determine whether it is a 12 volt or 6 volt. I don't remember the omhs off hand. Someone else will post the numbers.

Good luck,
Danny

[i:654c4848f0]you could say the same thing about the current situation[/i:654c4848f0]

I don't think it's current. I think it's voltage.
wink

Since these old ignition systems were limited to a maximum of 4 amps primary current, you need about 3.2 ohms reisitance in the primary ignition circuit. I'm not all that familiar with the details of N Series Ford' ignition circuit but you still need to keep the resistance at near 3.2 ohm to keep from overheating your coil. That generally means that the primary winding in a 12V coil is ~3.2 ohms and for a 6V coil it's around ~1.6 ohms. SO check your coil resistance you maybe running a 6V coil on a 12 V system, something that doesn't work for long.

Front mount distributor 12 volt coils just are not what they say they are. Check the archives and you will see it covered more then once. You need to have a ballast resister in the system or yep they will smoke test on you

(quoted from post at 21:18:35 07/18/09) [i:009fd3152c]you could say the same thing about the current situation[/i:009fd3152c]

I don't think it's current. I think it's voltage.
wink

Click to expand...

No, it's current.

Volts have nothing to do with it, other than multiplying.

I found this diagram for the 9/2N conversion using a 12 volt coil. If this is different from a 8N conversion someone will advise. David............

3.8A should live, particularly if you don't let it bake with key on & engine stalled.
Whatever resistance it takes to limit to about 3 to 4 amps stalled, regardless of whether a 6v or 12v coil on the 12v system.
I guess a good question might be, why do you think it is burned up?
Is it that you start out with 3.8amps and after it stops running that same measurement is 10amps?
If yes, then I agree, the wire insulation of the windings is shorted.

My sprayer has its own 12v battery.Sprayer runs a long time between charges.

If you are sure you are burning up coils go to HOBO's web site and see how to gut out one of your burnt up front mount coils. You can then rig it up to use a real 12 volt round can coil. Get one that has written on the outside " no external resister needed". NAPA has them. This way you will not need any type of resister. I did this to my 1949 front mounted didstributor when I converted it to 12 volts and it works great.

(quoted from post at 07:05:36 07/19/09) If you are sure you are burning up coils go to HOBO's web site and see how to gut out one of your burnt up front mount coils. You can then rig it up to use a real 12 volt round can coil. Get one that has written on the outside " no external resister needed". NAPA has them. This way you will not need any type of resister. I did this to my 1949 front mounted didstributor when I converted it to 12 volts and it works great.

Click to expand...

hat really is the best plan when running 12v. I've done two and never have to mess with them again.
Coil is NAPA IC14SB
http://theviperr.com/hobo_dnn/Default.aspx?tabid=69

Phil.......my money is on miss-labled 6-volt squarecan ignition coil. They ain't built to run on 12-volts. Its the LAW, Ohms Law. You need 2-resistors when using the 6-volt squarecan ignition coil, the mandatory "infamous ballast resistor" ...and... a 12-to-6v converting resistor. (2.5-ohms @ 50-watts)

Iff'n yer running the new modern 12-volt squarecan ignition coil, it still needs the "infamous ballast resistor", but NOT the 12-to-6v converting resistor. Understand?

Unfortunately, neither the original 6-volt squarecan or modern 12v squarecan coil are labled. But amazingly enuff, they can figger out how to label country of manufacturing.

The modern squarecan coil resistance should be about 2.5-ohms and the original 6-volt squarecan coil will read about 1-ohm. (measured from top terminal to springy-thingy) Springy-thingy to hi-volt tab is about 4000-ohms.

You should and can make these reading on yer "burned-out" coils too. Its hard to short the "infamous ballast resistor" so it will glow red hot, but 10-amps sez it was shorted before the coil connection. Otherwize the internal resistance of the coil would reduce the total current. Its the LAW, Kirchhoff's Law.

While digital multi-meters are great things, they don't work well measuring coil amps or coil volts while running. Its a technical thing. Old fashioned needle meters (analog) will measure pulsing coil current. You should read about 9-volts while running. ......Dell, retired calibration lab engr