I looked all over the shop and coulda swore I'd seen one. I know they're around 1 ohm 100 watt. Do I really need one just to run it for about 20 minutes? Maybe I could put an ice pack on the coil if it gets too hot.

 

More than likely you'll be fine, but if it were me I would just go buy a ballast resistor to be on the safe side, you're going to need one anyways.

 

Isn't the resistance built into the wiring harness on these years?

 

According to this diagram it looks like it was mounted to the firewall. I looked at the wiring diagram in the manual and don't see one. Possibly it's built into they grey box the call 'the brain'. I'm using a HEI module from GM.

 

Find out the resistance of the coil.

Find out the resistance of the HEI module.

If the coil is slightly higher than, or equal to, the module, you don't need a resistor.

If the coil is less than the module, that'll be the amount of extra resistance needed.

Too much resistance in the supply wire might rob the engine of power.

Too little resistance could fry the module.

Yes, the resistance could indeed be built into a wire, but we don't know the vehicle details in order to confirm.

 

Pretty sure most late 60's and 70's Fords had a resistor wire built in to the harness right after the ignition switch.
Bronco are a pink wire and I think the trucks are too.
On my '66 Bronco, if I measure the battery voltage to be 12 -14 volt range and measure the voltage at the coil it will be in the 7-8 volt range in "Run" .
During "Start" the coil will get the full voltage to aid in firing off.

You might want to determine if you already have a resistor before you add another one.

I don't think running for a few minutes without one will hurt you though.

I've seen those porcelain ballast resistors get so hot that it burns the painted surfaces that it is mounted to.

 

Your top picture is one of MoPar type.....Our rigs have the ballast resistor wired into the harness from the ignition switch out.

BUT! You're running an HEI module which doesn't need ~ 8 VDC to the coil to run as the vehicles with the ballast resistor wire do.

Unless I'm mistaken, HEI type modules, et al, require full 12 VDC and a coil - if you're going that route - that will handle 12 VDC.

 

Great, someone just brought me some 0.33 10watt resistors I was going to wire in series/parallel to get the ~1 ohm 100 watts. Great info, thanks.

 

You're welcome.

Just dump the ballast resistor wire and wire in switched 12 VDC to your (+) coil terminal and you'll be good to go.

Well, one never knows when a few resistors might come in handy.....

 

A new ballast resistor wire will smoke pretty good the first time. Don't be alarmed.

 

I read 0.8 ohms across the coil (terminals off)
10K ohms across w & g of HEI module
infinite ohms across B & c of module

 

0.8 ohms sounds about right at the coil.

Haha, I didn't see the module ohm readings coming.

They don't sound right. I'd expect something between 0.4 ohms to 1.50 ohms, but I could be wrong.

I fitted an HEI dizzy recently, but still haven't got around to checking its specs.

I'll try and measure the module resistance soon.

 

I'm talking out of my ar$e and wasting your time. Sorry !

I just measured my ICM and got similar results to yours.

I was initially thinking that where the Pertronix Ignitors specify safe resistance figures, the HEI setup would be the same. LOL, very wrong !

Clearly I have homework to do.

 

Ok, I see where I went wrong. Idiot !

I haven't finished my HEI homework yet, so please get a second (or more) opinion on the following logic.

Why did Ford fit resistor wires in the first place ? To safeguard the points in the dizzy.

Using that same principle, the Pertronix Ignitor1 ICM requires a minimum resistance of 1.5 ohms, but the coils were in the region of 0.8 ohms, and thus a ballast resistor or resistor wire of at least 0.7 ohms was required.

I've just checked my HEI coil resistance, and it is 0.6 ohms, and that is paired already with an HEI ICM.

So, if the ICM is protected with only 0.6 ohms, your 0.8 ohms will be good to go, with no resistor wire or ballast resistor.

Assuming that's correct, a quick check should show +/-14v at the coil + terminal with the engine running.
(Basically very similar to whatever the + battery terminal reads.)

 

Posts #7 and #9. Too confusing?