Ballast resistor question

-
str12-340

str12-340

FABO Gold Member
FABO Gold Member
Joined
Dec 26, 2012
Messages
4,400
Reaction score
4,054
Location
Vashon, WA
I'm working on a 1970 Dart. All the wiring is new M&H stuff. I've had the car running for a month or so and have put a few hundred miles on it and everything runs fine. In working to check various connections to be sure they are secure, I put my finger on the ballast resistor and burned the **** out of my finger. This is a 2 terminal original type that has a metal band around it that carries the bolt that attaches the ballast resistor to the firewall. It is the band that is hot. Is it supposed to be that way??? It seems to work fine.
 
The ballast resister gets hot!

The ceramic is getting hot and making the mount metal band hot.

Ceramics might not feel as hot to the touch as metal.


Basically the ballast is burning off a bunch of power so the coil does not get cooked.


I goofed and mis read the origional post.

You have a single resister ballast resister.

The resistance should be in the 0.5 to 0.6 ohm

Screenshot_20230327-062937.png
 
Last edited:
str12-340 said:
I'm working on a 1970 Dart. All the wiring is new M&H stuff. I've had the car running for a month or so and have put a few hundred miles on it and everything runs fine. In working to check various connections to be sure they are secure, I put my finger on the ballast resistor and burned the **** out of my finger. This is a 2 terminal original type that has a metal band around it that carries the bolt that attaches the ballast resistor to the firewall. It is the band that is hot. Is it supposed to be that way??? It seems to work fine.
Click to expand...

Yep, normal.
 
Absolutely normal, absolutely guaranteed.

One thing you might do is get the car warmed to "normal" and get the battery "normal" and then monitor battery voltage, at idle, simulated low cruise, and simulated high cruise. Battery voltage should "run" about 13.8---14.2, and not much over that. If it's 15 or more, post back and look into it
 
The correct BR for that model is 0.5 ohm. That is for a factory ign coil. If neither part is original, I would check that the correct value BR is being used with the correct coil.
 
Thank you, thank you, thank you - how I managed to work on these cars for 50 years and never happened to experience that I don't know. It might be because of the metal band - most of the ones that I have used are all ceramic on the face.
 
I imagine you could cook a lil smokie on it. They get that hot.
 
Bewy said:
The correct BR for that model is 0.5 ohm. That is for a factory ign coil. If neither part is original, I would check that the correct value BR is being used with the correct coil
Click to expand...
I was surprised to see 0.5 but the parts book confirmed it.

(My old brain had remembered 1.5 for some reason. Also I misread the OP's post and have updated my post to reflect 1 resister BR. )

I would test it out of the cir ( disconnect one wire from the BR) and check that you are getting around that 0.5 ohm number.
 
That 0.5-.6 is hot also, around 80. Just FYI that is right for the stock points ignition, electronic are different.
 
Part of the reason they MUST run so hot is, they are a ballast rather than just a common resistor. They are designed to average current through them due to changes in current and temperature. The change in operating temp of the resistive material, as well as it's alloy, is what gives the ballast it's regulating characteristics.

Common resistors can be calculated for operating conditions and wattage, and sized up in wattage to operate somewhat cooler
 
Are the 2 terminal ballast resistors different between points and electronic ignition? This is a 70 car (originally points) with an MP electronic conversion done many, many moons ago. The current ballast resistor has been in place for 20+ years and I can't remember (a shakey proposition) changing it when I installed the electronic ignition. Looking for info...
 
The only specs I know were the OEM ones in the service manuals, which you can download some of, at MyMopar.com. Diffferent brands of coils were different resistance as well
 
Is there a year in which the factory made an electronic ignition car with a 2 terminal Ballast resistor (as opposed to 4 terminal). It's hard to look up a 70 Dart with electronic ignition.
 
You must log in or register to reply here.
-

Back
Top