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To coil or not to coil #2779790
05/30/20 03:45 PM
05/30/20 03:45 PM
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Posts: 192
TC, Minnesota
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Gabmando Offline OP
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Gabmando  Offline OP
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TC, Minnesota
Hello folks,

I bench tested the primary resistance of two Mopar coils i have on hand. F.Y.I. I used a Fluke 88V Automotive Multimeter. Crossing the leads, I got 0.3 Ohm reading.
these coils have two part numbers on them.
Mopar coil 4176009 / 4289390 primary resistance 1.5 Ohms.
Mopar coil 2495531 / 2444242 primary resistance 1.8 Ohms.
My understanding for a long time has been that the 1.2 Ohm ballast resistor was to limit current to Mopar low primary resistance coils (0.75 to 0.81 Ohms per MyMopar.com).

Just as a sanity check, I also checked the primary resistance of an ACCEL Super Stock 8140 and got 1.5 Ohms, which is per spec.

So here is my question: Are these Mopar coils above not meant to be used with 73 (and some 72) and up models with 5 pin ECUs and 4 prong ballast resistors?

Great forum, great expertise and great people, so thank you all.

Re: To coil or not to coil [Re: Gabmando] #2779794
05/30/20 03:54 PM
05/30/20 03:54 PM
Joined: Aug 2003
Posts: 42,992
Bend,OR USA
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Cab_Burge Offline
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Ballast resistors where used to reduce the voltage from 12V to 8 volts to help stop the points from pitting when the car makers switch to 12V from 6 volts back a long time ago, 1953 to 1956.
Todays ECU act as the switch to the coil like the points use to do work shruggy
I think you can use either one on your deal with the standard single ballast resistor for your ECU twocents


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: To coil or not to coil [Re: Cab_Burge] #2779806
05/30/20 04:40 PM
05/30/20 04:40 PM
Joined: Apr 2004
Posts: 192
TC, Minnesota
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Gabmando Offline OP
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Gabmando  Offline OP
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The 5 Ohm portion of the 4 prong ballast resistor limited voltage to the ECU. The 1.2 Ohm ohm portion limited the voltage to the coil.
Assuming I'm reading the wiring diagram correctly, switching from 5 pin to 4 pin ECU eliminated the need for the 5 Ohm portion of the 4 prong ballast resistor. Am I right?

Re: To coil or not to coil [Re: Gabmando] #2779813
05/30/20 05:05 PM
05/30/20 05:05 PM
Joined: May 2019
Posts: 6,136
nowhere
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Sniper Offline
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You are correct

Re: To coil or not to coil [Re: Cab_Burge] #2779814
05/30/20 05:05 PM
05/30/20 05:05 PM
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Auburn WA
Dave_J Offline
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What you want at the final (Coil and Ballast) is about 2.7 Ohm's +/- 0.25 Ohm.

The more volts will give a hotter spark. But the ECU has a transistor that provides a timed shunt to ground.
If the voltage to the coil is too much that transistor will over heat.
If the voltage to the coil is too low the spark is weak.

I have run a 1 Ohm Ballast with a 1.5 Ohm coil on an Orange Box ECU for years.
My son tried a 0.5 Ohm Ballast with a 1.5 Coil and burnt out a few ECU's before we finally put a 1.2 Ohm Ballast on, no more failures.
A Pertronic's Flamethrower 0.3 Ohm coil and no ballast gave one very hot spark but the ECU lasted 1 week.

If this is a street/strip driver, ether of those coils are good.


But in a race car, a 1.5 Ohm coil is a 1.5 Ohm coil is NOT true. There are many things to take into account. Turns ratio is a big one. More turns will give a bit hotter spark, IE, A MSD Blaster is 120 turns and an OEM is about 90-100 turns. But a lot of newer 'E' coils will be even hotter.

Picture stolen from B Bodies Only

Ballast Resistor.PNG

Retired, US ARMY 1973-1994
ASE mechanic, Electrical 1994-1997
Retired GTE/VERIZON/FRONTIER 1997-2015


Posting cheap tech help (CRAP) here since Nov 97, 1000's of posts, some may be good.

03 Suzuki Burgman 650(Burger King) Scooter
65 Formula S Cuda
78 Little Red Express Truck
98 Buick Regal (wifes car)
Re: To coil or not to coil [Re: Dave_J] #2779834
05/30/20 06:43 PM
05/30/20 06:43 PM
Joined: Apr 2004
Posts: 192
TC, Minnesota
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Gabmando Offline OP
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Gabmando  Offline OP
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Thank you Dave.
Bottom line is, the ballast resistor is needed to "protect" the coil and the ECU even if a 4 pin ECU is used instead of 5 pin.
This is bone stock 1973 Charger Rallye 340CID 4 SPD. The factory system is just fine.

Re: To coil or not to coil [Re: Gabmando] #2782848
06/08/20 03:24 PM
06/08/20 03:24 PM
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Balt. Md
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383man Offline
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Balt. Md
I have ben running the Accel super coil and the MP orange ECU for at least 5 years with no ballast and its worked fine. The coil never gets hot and I have been using that same orange box since 2001. But I bypassed and eliminated the ballast about 5 years ago to see how it did and its worked great. I don't remember what the primary winding ohms reading was though on the super coil. I just changed to MSD and their coil since I got the MSD at a good price but that orange ECU worked fine with the super coil and no ballast. And I pushed the orange ECU to 6500 rpm with no misfire at all. Ron

Re: To coil or not to coil [Re: Gabmando] #2782856
06/08/20 03:29 PM
06/08/20 03:29 PM
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Posts: 27,421
Balt. Md
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383man Offline
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Originally Posted by Gabmando
Thank you Dave.
Bottom line is, the ballast resistor is needed to "protect" the coil and the ECU even if a 4 pin ECU is used instead of 5 pin.
This is bone stock 1973 Charger Rallye 340CID 4 SPD. The factory system is just fine.



Not when using the Accel super coil. The resistance of the primary winding in the super coil must be ok to work without a ballast using the MP orange ECU. I have been running that setup for 5 or more years and never a problem. I actually don't remember the ohm reading on the super coil but I have one out in my garage I will check later. And that coil never got hot either as I checked it a lot. The factory system may have to have it as you stated though. Ron

Last edited by 383man; 06/08/20 03:30 PM.
Re: To coil or not to coil [Re: 383man] #2782987
06/08/20 07:17 PM
06/08/20 07:17 PM
Joined: Jan 2003
Posts: 10,468
Freeport IL USA
poorboy Offline
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Freeport IL USA
Years back I ran an Accel Super coil for 2-3 years with no ballast and had no issues. Then one day, the car failed to start. I ended up replacing the super coil with another Supercoil. That one lasted about a month, the last one died at a major car show. The guy at the Accel booth told me some of the Super coils survived without the ballast, and some didn't.
I added the ballast along with Supercoil #3 and it ran the 4-5 years I owned the car.

It seems like it might not be a very hard thing to try, add the ballast with the new coil and see. If it runs OK with the ballast, run it. Gene







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