RE: IML: 56 Ballist Resistor

You understand the circuit exactly correctly. The higher resistance will cause the coil to operate below the design primary voltage; however it is probably not going to show up in the car’s performance except under unusual circumstances.

The ballast resistor is out of the circuit during start-up, so it won’t affect the engine starting.

The symptom of a bad (open circuit) ballast resistor is that the engine will start normally, but stall as soon as you go back to the “run” position on the starter switch. If this is what is happening, you might have a bad ballast resistor, but a more likely explanation is in the wiring or the switch itself, or possibly even the starter solenoid.

A ballast resistor is the simplest of electrical parts – if you have a VOM, just measure the resistance. If it reads very low (under an OHM), it is OK, look elsewhere for your problem. If you don’t have a VOM, use your test light to see if the bulb lights when you touch the coil end of the ballast resistor with the key in the run position. If it lights, even dimly, the resistor is probably OK.

Dick Benjamin (retired (since 1979!) Electrical Engineer, still with a few remaining functioning brain cells)

From: mailing-list-owner@xxxxxxxxxxxxxxxx [mailto:mailing-list-owner@xxxxxxxxxxxxxxxx] On Behalf Of JCantor791@xxxxxxx
Sent: Wednesday, June 01, 2005 4:29 PM
To: mailing-list@xxxxxxxxxxxxxxxx
Subject: RE: IML: 56 Ballist Resistor

Teresa,

Thanks for the suggestion. My local NAPA store has this one in stock and for much less than the one that they had been able to find listed with the correct resistance range which must be made of unobtainium given its several month lead time.

But this brings up the question of resistance range. The 56 FSM that I have indicates that the ballast resistor should be between 0.66 and 0.735 ohms. The 74 P/U unit you suggested is 1.2 ohms. Being that I'm a mechanical engineer not an electrical engineer, I'm not sure if this really matters. It is my understanding that the purpose of the ballast resistor is to cut the voltage going to the coil so as to protect it from the full 12V (or greater) of the system. In this case, I would assume that a resistor that was larger would knock the voltage down TOO far and risk preventing the coil from producing a hot enough spark while too small a resistor would risk frying the coil with too high a voltage. Of course I'm not sure what happens when the resistor has gone to an open circuit as I believe the current one has.

So, the question to all you electrical guru's is: should I keep searching for the right resistance range or is this close enough? Or have I completely miss understood the circuit and perhaps miss measured the resistance's and my current resistor and coil are just fine?

Thanks in advance,

Jeff
'56 Sedan
Trenton, NJ

From: "Chad & Teresa Smith" <hemi_powered@xxxxxxx>
Subject: RE: IML: 56 Balist Resistor
Date: Wed, 01 Jun 2005 06:37:17 -0700
Reply-To: mailing-list@xxxxxxxxxxxxxxxx
Jeff,

The ballast resistor was junk in my 56 as well...I will tell you what I did.
I walked across the driveway, opened the hood on my husbands 1974 Dodge
pickup, examined it..huh, sure enough...same darn thing....stuck it in my
car, and away I went... You can buy a new one for a 1974 dodge pickup
without problem. Got any friends with 1970s dodge pickups with a 440? I
have been driving the car for two years on the ballast resistor...no
problems.

Teresa Smith

>From: JCantor791@xxxxxxx
>Reply-To: mailing-list@xxxxxxxxxxxxxxxx
>To: mailing-list@xxxxxxxxxxxxxxxx
>Subject: IML: 56 Balist Resistor
>Date: Wed, 1 Jun 2005 06:55:40 EDT
>
>I have all of the needed parts to do a tune up on my 56 except the ballast
>resistor which does appear to have shorted out. Bernbaum is not able to
>get
>these currently so I'm looking for suggestions for other sources. I've
>tried
>the local sources including NAPA with no success.
>
>Thanks
>Jeff
>56 Sedan
>Trenton, NJ