Ballast Resistor question

[Jims68]

OK, so this is NOT for a Sweptline, but it IS for a 69 383 Plymouth Roadrunner. Now... my car runs fine, and it has the normal DUAL Points and coil; and most important, I'm not having a problem (knock wood). I just wanted to buy a "spare" ballast resistor for the glove box. Of course, I can also put this in my 68 Sweptline as a spare too.

From what I see there are different Ballast Resistors with different ohms. So... what is the deal? I take it that the Ballast Resistor is either GOOD or BAD and no in between. I can check the one on my car (unplugged) with a multi meter and get a 1 OHM (TONE or Continuity) with my meter. Do I need to worry about the "other" OHM readings when they are being sold as a 1.5 OR 0.8?

This one says 1.5 OHM
https://www.summitracing.com/parts/upd-s1210/overview/

This one says 0.8 OHM I think this one may be for a MSD coil??
https://www.summitracing.com/parts/msd-8214/overview/

What's the difference?? OH... and don't bother saying for me to change to electronic, or MSD or whatever, it runs FINE the way it is and I'm happy with the way it runs now !

[my5thmopar]

Correct no good or bad....usually. If the wire is marginal, it can heat up a go open but, this is very rare. Go with the 1.5 ohm. The ones I like are from Autozone Duralast AL795. I haven't had one go bad, so my spare has been used for engine starting, testing etc. It also is the most original looking to me and comes with screws and nuts for the older Mopars. Craig

[Jims68]

Thanks Craig! Just the answer I was looking for. and one of our local Autozone stores has them in stock too.

[Kaegi]

that is the one I use as well. I didnt know about the different ohms.

[Jims68]

that is the one I use as well. I didnt know about the different ohms.

You can google it... but none of what I read (or found) made any sense to me and that is why I posted my question here .

[Jims68]

Just noticed... it says on Autozone Ebay Page:

Resistance (Ω) 0.633

[PwrWgnDrvr]

Purpose is to reduce voltage at coil and points, while running, to extend life of points. Lower ohms will be lower reduction of voltage, thus a hotter spark and shorter point life. All will work, but higher ohm unit will extend life of points.

[Jims68]

Purpose is to reduce voltage at coil and points, while running, to extend life of points. Lower ohms will be lower reduction of voltage, thus a hotter spark and shorter point life. All will work, but higher ohm unit will extend life of points.

NICE explanation!! Thanks! I did know that the Ballast Resistor was to reduce the VOLTS to the coil which extend the points life, BUT... I DIDN'T know about the difference between HIGH and LOW OHM readings. I wasn't sure if one company was showing a "cold" reading as opposed to a "hot" reading. OR if that even matters?

[Conrad]

FWIW, first thing I did on my truck was switch to the 'digital' regulator and updated voltage controller, and tossed the dang ballast resistor. Super happy with the more modern parts.

I know there are some folks who need to keep the originality of it. If I had a '69 Roadrunner I might hollow it out to bypass 'behind the scenes', but I just tossed mine in a box to gift to a fellow MoPar enthusiast at some point.

[HellBelly]

Correct no good or bad....usually. If the wire is marginal, it can heat up a go open but, this is very rare. Go with the 1.5 ohm. The ones I like are from Autozone Duralast AL795. I haven't had one go bad, so my spare has been used for engine starting, testing etc. It also is the most original looking to me and comes with screws and nuts for the older Mopars. Craig

Thank you for this. I was trying to figure out which one to buy as well for my 68

[earlymopar]

I have a new digital voltage regulator that I'm not using that is made to be stock appearing. PM me if interested.

- EM

[swptln]

The difference in resistance of the ballast resistors is for your coil not your points. Your ballast resistor has to match your coil. All coils have a primary resistance value and your ballast resistor is supposed to be matched to it. Otherwise, as mentioned by Terry, you'll fry your coil and points if the resistor has to low a value, if the resistance value is to high, you'll get a weak spark on the secondary side.
This is one of the biggest problems guy's have burning up coils, points and ECU's on electronic ignitions as well. Nobody matches the ballast resistor to the system ie coil and ECU's. The only reason it shortens point life is because your putting to much voltage to the coil and current across the points. So, if your burning up points, your frying your coil too.

[hmaynord]

All coils have a primary resistance value and your ballast resistor is supposed to be matched to it.

disputing Mark D is a fool's errand, but here goes. (When he says, "be matched to it," I read that as "have the same value.")

his explanation is not consistent with the following specs from the '65 TSM for all motors:
1. coil primary resistance @ 70-80 deg F (there are 2 possible coils listed):
(a) prestolite = 1.65-1.79 ohms
(b) essex = 1.41-1.55 ohms (N/A to 361 ci).

2. ballast resistor resistance @ 70-80 deg F (there is only 1 possible ballast resistor listed):
0.5-0.6 ohms.

[PwrWgnDrvr]

Its just basic Ohms law.
E=IR
I=E/R
R=E/I
Total circuit resistance of resistors in series = R+R+R.....
Circuit I = E/R+R+R....

[swptln]

All coils have a primary resistance value and your ballast resistor is supposed to be matched to it.

disputing Mark D is a fool's errand, but here goes. (When he says, "be matched to it," I read that as "have the same value.")

his explanation is not consistent with the following specs from the '65 TSM for all motors:
1. coil primary resistance @ 70-80 deg F (there are 2 possible coils listed):
(a) prestolite = 1.65-1.79 ohms
(b) essex = 1.41-1.55 ohms (N/A to 361 ci).

2. ballast resistor resistance @ 70-80 deg F (there is only 1 possible ballast resistor listed):
0.5-0.6 ohms.

Yes, but like Terry just explained, there is no way to get a total amperage draw of 3Amps with a 1.79ohm coil and .6ohm resistor. Your going to be over 5Amps......
1.79 + .6 = 2.39ohms. 12V or E / 2.39ohms or R = 5Amps or I.
1.79 + 1.5 = 3.29ohms. 12V / 3.29ohms = 3.6Amps.
A good rule of thumb with coils and ballast resistors is if you have a 1.5ohm primary resistance, run a 1.5ohm resistor.

[PwrWgnDrvr]

And, if the engine is running and the alternator is putting out the needed 14V, the amperage thru the coil is going to be even higher.

[hmaynord]

Purpose is to reduce voltage at coil and points, while running, to extend life of points. Lower ohms will be lower reduction of voltage, thus a hotter spark and shorter point life. All will work, but higher ohm unit will extend life of points.

Otherwise, as mentioned by Terry, you'll fry your coil and points if the resistor has to low a value, if the resistance value is to high, you'll get a weak spark on the secondary side.

And, if the engine is running and the alternator is putting out the needed 14V, the amperage thru the coil is going to be even higher.

First, I thought hotter spark was usually a good thing. So why not start with the lower resistance? If that works and you later develop an ignition problem with the same setup, you've eliminated weak spark as a cause. If that burns the points or fries the coil, then you can go to the higher resistance. Starting with the higher resistance might be solving a problem you don't even have.

Second, the '65 TSM states that the engines are designed to run with BR at 0.5 - 0.6ohms. In other words, given that the engine must be running for the BR to be in the circuit, the '65 TSM is fine with 7.3A running continuously through the coil:
1.41 + 0.5 - 1.91ohms. 14V / 1.91ohms = 7.33A.

[hmaynord]

[double posted]

[Wildergarten]

, given that the engine must be running for the BR to be in the circuit, the '65 TSM is fine with 7.3A running continuously through the coil: 1.41 + 0.5 - 1.91ohms. 14V / 1.91ohms = 7.33A.

It isn't the DC that fries a coil but the current at peak voltage (~12KV if memory serves) when the input is a square wave made by the points. Watts are what melts insulation materials.

[hmaynord]

, given that the engine must be running for the BR to be in the circuit, the '65 TSM is fine with 7.3A running continuously through the coil: 1.41 + 0.5 - 1.91ohms. 14V / 1.91ohms = 7.33A.

It isn't the DC that fries a coil but the current at peak voltage (~12KV if memory serves) when the input is a square wave made by the points. Watts are what melts insulation materials.

OK. the main point (pun intended) was burning the points. So, here's a correction to my summary of the TSM:
'65 TSM is fine with 7.3A running continuously to the points (and the coil): 1.41 + 0.5 - 1.91ohms. 14V / 1.91ohms = 7.33A.

OP does not have burning points. he was asking about general rules for BR. It seems to me that rule 1 always is go with the TSM, which in this case is a lower resistance BR than what swptin's rule of thumb would give. If rule 1 burns points, then you need an exception to rule 1, such as swptin's rule of thumb.

Wildergarten: if input V is 14, coil primary resistance is 1.41, and other variables constant, what would the current at peak voltage be if BR is either 0.5ohms or 1.79ohms?