57 Ignition coil, external resistor.

[derbydrifter]

Hi guys, I'm just getting my rebuilt 390 back together and noticed that on top of my ignition coil there is what looks like an ignition condenser, which I thought was a suppressor to prevent radio static, that's why I didn't worry about the wire being snapped off. But checking the manual it's  actually an external resistor to limit current through the breaker points. More important than I thought!!

Trouble is. I can't get one. I'm in the UK and have searched the net with no luck apart from being able to find a new coil by 'Mallory' which because of it superior design apparently elliminates the need for an external resistor, but no separate resistors for sale anywhere. I might as well buy a new coil if needs be . . . darned car seems to decide what it wants anyway - I'm just its slave. But before I go ahead if anyone's got any thoughts on this I'd be interested to hear em.

Kurt.

[pdxmose]

You may want to double check what you read.  My understanding is that there's an external resistor or resistor wire to limit voltage (and current) in the primary side of the coil.  Then there may also be a suppression capacitor like you mentioned.  (Then there's the condenser in the distributor to protect the points.)

If the 'supressor' you mention only has a single wire in a can that attaches to ground, then I don't think it's a limiting resistor. A resistor would be between the + terminal of the coil and the 12V switched power source.

           ...mose

[derbydrifter]

I think what I'm looking at then is actually a supressor; like you say, a single wire from a can attached to ground. And checking the manual's wiring diagram, the coil resistor is way separated from the coil itself, attached to the body somewhere by a wire, I'll check that out. Thank you for drawing my attention to it. No wonder I couldn't find a resistor anywheres that looked like my little can suppressor!! 

After all this I think I'll renew my coil anyway. But if I find that ballast resistor I don't need a resistor type coil, right, just a straightforward coil?

[The Tassie Devil (Bruce Reynolds)]

There should be an external Resistor, on the firewall, and unless it has been removed, and the wiring changed to full 12 volts, you will need a "Resistor" Coil.

This Resistor is made of Ceramic, about 3 to 4 Inches in length, and attached to the firewall by a steel strap that wraps around the central part of the ceramic

Yes, the one on the coil is a Suppressor for the radio.

Bruce.

[derbydrifter]

Thanks Bruce, yes I see the ceramic resistor on the bulk head now, and yes it's attached to the coil by a wire as it should be. And that little can in a bracket on top of the coil is a radio suppressor like you guys say.

Just let me get this straight though. Even though I have an external resistor, if I replace my coil it does need to be the resistor type? I'm wondering if I order a coil for a 57 Deville, from somehwere like Rock Auto, it will automatically allow for the car's established electrical circuit. Or do I need to double check that I'm getting a coil with an internal resistor. I see certain coils for sale stating that they eliminate the necessity for and external resistor. I'm confused.  But It'd be good if I could just buy a coil over here in the UK instead of having to ship one.

[STDog]

Yeah if you order a standard part from RockAuto and the like it'll work correctly.
Or something that says "for points"

Can you post a manufacturer and part number of some examples of the "no resistor needed" types you've seen?


It really depends on the internal resistance of the coil. what you need is something with 1 to 2 Ohms in the primary.
If it's lower, you'll overheat the coil and/or burn the points.
If it's higher, you'll reduce the current and loose some upper RPM performance.


I've seen a lot of "performance" coils that have 0.6 Ohms or less. Those are likely to cause problems.
I've not seen any that are over 2 Ohms. What I think they mean with no resistor needed is that they are suited for use withe the OEM setup.
The low resistance, performance coils often need an extra resistor, at the coil to bring the total resistance up to be suitable for use with points.


I tried a few "performance" coils on my '70 and they didn't last. The stock type holds up much better and are much cheaper.

[The Tassie Devil (Bruce Reynolds)]

The Coils for the 8 Volt circuit don't have an internal resistor as such, they just require less voltage for them to work correctly.

In the case of the GM vehicles, like the one you have, there is an 8 Volt operating voltage from the Ignition Switch, but there is also a 12 Volt circuit which is only in use whilst the Starter Motor is cranking the engine over on start-up.

These coils will have stamped into the bottom of the case, "Use with Resistor".   Any coil without a marking like that, will require a constant 12 Volts to operate it.

Fords use an inbuilt "Resistance Wire" in their wiring loom, and therefore don't use an external resistor.

The Ford Resistance Wire, and any other resistance wire for that matter, has a different textured insulation around it, to allow for the heating up as it would destroy normal plastic insulation, and therefore cause a wiring fire.

Bruce.

[STDog]

There is no 8V circuit either. That a misunderstanding of how the circuit works, and the time averaged reading you see on a multimeter as opposed to the instantaneous voltage seen at terminals (which varies with time as the primary winding magnetic field builds/collapses).

The ballast resistor (or resistance wire) is there to limit current not voltage. When the points are open the voltage at the terminals is ~12V (battery or alternator voltage) When they are closed the - terminal drops to 0V and the + terminal voltage will drop as the field builds, to ~4V (depends on the primary resistance, the ballast resistance, and system voltage).

That happens fast (at least in human terms), 66 times a second at 1000 RPM (or ever 15 milliseconds)
So when you put a multimeter on the + terminal  with the engine running you see the average voltage, and that average depends on the dwell setting.

But, with the engine not running, you can open the points and see 12V, and close them and see ~4V.

[derbydrifter]

The main 'no external resistor needed' coils I came across were by Mallory. I guess they are American. But thanks to all the technical info supplied by you guys, I understand what I need now and will get a good quality stock one from Rock Auto. Plus another bonus is, I don't have to look at my ignition coil anymore thinking to myself 'I wonder what that thing does exactly?'

Thanks for your help. I'm in the very early stages of getting this rebuilt 390 up and running. Including the carb's total restoration everything is new but the inginition coil and condenser. And being as she runs very strong but certainly far from smoothly it is now going to be the process of elimination plus plenty of tuning.

[pdxmose]

Have you considered converting to a Pertronix electronic unit? Now's the time if you are buying a coil anyway.   It may smooth out the idle and eliminates points tuning and maintenance. The second gen Pertronix II with a high voltage coil is about $125.  I've used a few of them and have had great luck.  (The first Gen ones were prone to burning up and leaving you stranded, especially if you leave the key on.)

Search the forum for Pertronix if you're curious.

               ...mose

[derbydrifter]

Thanks Mose,

I'll check it out!

Kurt.

[EZ]

I changed mine over to Pertronix.  I was glad to get rid of the points!!!

[derbydrifter]

That's interesting to know.
 I'm going to go ahead and get the car running as smoothly as I can for now with it's contact breaker points set up. Especially as all the parts are new now. But if I have problems, now or in the future, I'll go for the Petronix option. I really like the sound of it.
Kurt.

[EZ]

Pertronix isn't anything new either.  They've been around for about 30 years I believe.  They make a good product.