So, do I need a ballast resistor or not?

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Dec 22, 2013
El Paso, TX
Hello, checked and read a bunch of threads and still not sure.

For the fun of it, I converted my '78 to the non-US points distributor (19100-61080). The stock coil is 90919-02083 and says 12V-R, but I believe it also says use with resistor. The primary resistance is 1.8 ohms. There has never been a visible external resistor but the wiring diagram shows an inline resistor.

There are two B/Y wires (one fat, one thin) that connected to the coil/igniter combo. Connected the points distributor to the coil (-) and the coil (+) to the fat B/Y wire (which is how it connected originally).

Running the engine:
  • battery voltage is 14V
  • 12V on the two B/Y wires
  • 12V on the coil (+)
  • 9V on the coil (-)

My question: Do I actually need an external ballast resistor for this coil or can I run it as is?

I measured my son's '71 points setup, which does not have a ballast resistor, and his values are:

  • Primary resistance: 3 ohms
  • When running, full battery voltage to coil (+)
  • When running, 7V to coil (-)

Oh, and question #2/#3.

My starter has the B/W ballast resistor bypass wire but where does it end up near the coil? Is it the same B/Y wire that attaches to coil (+) and receives 14V only when cranking?
I’m not sure exactly how you need to wire this, but FYI, my ‘78 has the Yazuki (sp?) 8ga pink resistor wire spliced into the ‘Fat’ BY wire.

The thin BY wire carries the full 12v.

When I replaced my oem dissy, with a DUI, I needed the full 12v to the DUI. I also had to eliminate the coil and igniter.

To send the full 12v to the DUI, I taped up the fat BY and used the thin BY to trigger a relay.

The relay pulls a full 12v, from the battery and sends it to the DUI.

Hope this helps... good luck!
The coil will run with full 12+ volts but will burn out pre-maturely. You will want to run the coil at 9.6+ volts. The 78 will come with a built in resistor like pngunme said. The large BY wire should read 9.6+ volts in the run position if there is a built in resistor. If you live in a warm climate you don't have to worry about the resistor bypass wire.
It is very simple: If you have a ballast resistor type coil, you need a ballast resistor. If you run it without the resistor, it will overheat and may fail.
Do live in a warm climate so understand the no need for the bypass wire.

Around where would I find the built in resister, close to the plug; close to the firewall; midway? I am not getting 9.6V on the B/Y wire so I will have to go into the wiring.

As I am understanding, the 9.6 V has to be going into the coil (+), right?

Pin_Head, you have stated before that an "external resistor type will have a primary resistance of about 1 ohm while the regular 12V coil will be more like 2 ohms". I measure 1.8 ohms so that is where I also am confused. Still not sure if my stock coil is ballast type or not. LOL.
the resistor wire is about 4' long and goes from driver side to just through the firewall into the engine bay, No need to hack up your wiring harness.

Yes the coil + should get 9.6+ volts if the coil has no resistor built in

If your getting 12+ volts at the BY wire then you can use an internal ballast coil or get a ballast resistor.

Got any pics?

Here is a thread i started when i was having issues, there are some pics of the resistor wire.

Replaced Starter now there is no spark
Ahhhh, o.k. Thanks for the info, I would have gone crazy looking for that "built in resistor". Pics of what exactly?

Also, this is the original '78 coil (217,000 miles on it) and I would assume it has always been receiving 12V at the large BY wire as I have not changed anything.

In another post, I did read that the small BY wire that goes into the igniter receives 12V at start/run and the larger BY going to the coil (+) would receive reduced voltage during run. This further confused me as I did not measure less than 12V on the (+) when I had my igniter connected.

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