Hey folks.

I bought my car with a pertronix ignition installed, which eventually failed. I converted it back to points and have been running it that way all winter long. Anyway, the ballast resistor was removed from the car since its not needed in electronic ignition setup. My car still does not have one.. Is this going to ruin the coil? If so, how would I hook one up. I'm not sure how its supposed to be wired.

The coil design determines the need for a ballst resistor. If you are using a stock coil you need the ballast. Start by look at the factory service manual for the wiring diagram. If you don't have the manual it is available for free at XenonS30.

  beermanpete said:

The coil design determines the need for a ballst resistor. If you are using a stock coil you need the ballast. Start by look at the factory service manual for the wiring diagram. If you don't have the manual it is available for free at XenonS30.

That makes sense. Since it was bypassed for use with the pertronix, is it safe to say that its correct as is? I mean I've been driving it for months. Since I don't know what all was done with the install since it was the PO, I just pulled of the ignitor and popped in the points and condenser.

  ninjazombiemaster said:

That makes sense. Since it was bypassed for use with the pertronix, is it safe to say that its correct as is?

No, as not everyone does it correctly, and there are more than one correct way to do it.

Step 1 - You need to identify the coil for certain. Not just the brand, but also the resistance. Only then can it be determined for sure whether you should have a resistor or not.

  Arne said:

No, as not everyone does it correctly, and there are more than one correct way to do it.

Step 1 - You need to identify the coil for certain. Not just the brand, but also the resistance. Only then can it be determined for sure whether you should have a resistor or not.

Reasonable enough. I'll take a look, and if I can't identify it I'll post a picture. Is 3 Ohms is the correct resistance? I'm not experienced at all with car electronics.

The factory coil is 1.5 to 1.7 ohms on the primary winding.

  beermanpete said:

The factory coil is 1.5 to 1.7 ohms on the primary winding.

So it needs 1.5 ohms additional resistance from the ballast, if I understand correctly? If it is the stock coil, anyway.

I'm working now so I cant go look, but if I remember right, my coil is blue. I don't know if that helps anyone determine if it is or is not stock.

Okay, I looked and everything seems to be appropriate. I was wrong, my coil is black, but honestly if it was the wrong resistance it probably would've melted months ago.

I may upgrade to a 12V coil eventually, after I get my alternator upgraded.

Thanks for the help guys.

Your coil will be fine. Your points, however, may not be so lucky...

What's missing here is an understanding of the purpose of a ballast resistor. Allow me to explain.

When the vehicle is being cranked, the battery voltage may drop down as low as 9.6 volts. At that voltage the coil needs every bit of available voltage to make a "good enough" spark to start the engine.

So typically the coil is wired directly to the starter, so when the starter is engaged, the coil gets full battery voltage.

Scenario #2: You're motoring down the highway and the alternator is putting out 13.8V or so.

The same coil that BARELY puts out a spark at 9.6V is now putting out 60,000V or more with an input of 13.8 V. Perhaps even higher, 100,000V.

That's way more than your plugs NEED, and it's way more than the insulation on the plug wires, cap, etc. are designed for.

So when you're NOT cranking, simply running, instead of running the coil directly off the battery, the juice for it comes THROUGH a ballast resistor,

which will knock it down to 10V or so. This comes through the ignition switch, or any ignition-hot circuit, into the ballast resistor, and on to the coil.

So you typically have TWO wires going to the coil, one from the starter circuit, and the other from the ballast resistor.

So I had a coil fail on a Subaru, I went and bought some ginormous yellow mega-sparky coil, wired it in without a ballast resistor and what happened? It ran great. For awhile.

I pulled the plugs out some 3000 miles later and the electrodes were nearly gone. Evaporated. MIA. The 100-200K volts that super coil was putting out simply

obliterated them. My guess is if I'd put my hand anywhere NEAR any of the spark plug wires I'd have gotten a good jolt as well.

So I went to the auto parts store and asked them for a generic (Ford) ballast resistor, and wired it in. No more evaporating electrodes!

SOME coils have dual inputs, with an internal ballast resistor on one side. Or so I'm told, as I haven't run into one yet. Don't confuse + and - inputs with "dual +" inputs as

are most common.

The points in an older car switch the 12v ignition voltage to the coil on and off. Without a ballast resistor they'll be switching a lot more current, and wear out a lot faster ---

basically plating one point over onto the other.

Electronic ignition simply replaces points with a switching transistor. It won't last as long either if you're switching full voltage to a coil. Most electronic ignitions use a different coil,

and, because they're electronic, they can automatically adjust the dwell (the % of time the coil is powered up) to produce a better spark at lower voltages and a less intense one at higher voltages.

So you see coils, like the one in my 280ZX? 4-Runner?? , that is stamped "For electronic ignition"

Hope this helps you understand what's going on and how to wire things correctly.

Edited March 13, 201312 yr by Wade Nelson

From reading on the site, the ballast resistor was put in place to lower the voltage thru the points on the running engine to prolong the life of the points. The start circuit sends full voltage to get the engine running but shifts (ignition switch "run") thru the resistor after start to prevent premature wear of the points. Assuming stock coil output, of course.