Ballast Resistor Question

I'm a beeg dummy with the electrical stuff. Did a search and found no real answer to my question and I'm too dumb to develop an accurate conclusion from what I've read so.........a friend is trying to get his V6 jeep running. No spark condition. Has firewall mounted ballast resistor. I suggested testing coil, replacing points and condenser and resistor.

He tested the coil and he determined it was bad. He replaced it with a correct coil (not internally resisted). Still no spark.

He replaced the points and condenser. Still no spark.

Today he went and got a resistor. Plugged it in and got spark but the resistor began to smoke. He removed it, cleaned up the terminals on the old one (they were rusty he says), reinstalled it and got spark.

So my question........what would make the new resistor smoke?

ed. Just got off the phone with him and the jeep is now running with the old resistor in place. ?????? My question is still valid.

 

Wrong resistor or bad out of the box? I get a lot of things bad out of the box anymore.

 

I was thinking wrong part but, they are either 6V or 12V right? What other difference could there be in them? I figured a 12V resistor is a 12V resistor. I could be, and probably am, wrong about this though.

 

Burning the "new" off the new resistor - oil, contaminates?

 

Well, yea but, how? From where? Current flows from ign switch through resistor to coil, right? What would/could be the source of the overload?
Overcharging alternator?

 

Oh. OK. Likely from the ignition switch I presume?

 

bad resistor !!!???? if it was a short both would have smoked .

 

That's true Chris but, I'm really just asking what would cause ANY ballast resistor to smoke. In case I ever run into this again and, hoping to gain some knowledge on how the system actually works along the way. Been googling and watching youtube videos.

 

He released the magic smoke!! there is a prize for that. I go for bad resistor. It needs to drop the voltage to 9.8 volts? right? It should just work. I suppose you could hook it up backwards, but it shouldn't matter. It should not smoke unless it it overloaded. Is the voltage regulator charging at 13.8~14 volts?

 

Hmmm. OK. So, are there in fact different resistors, in terms of how much resistance they provide? In other words....is it NOT true that all automotive ceramic 12V ballast resistors are the same?

Chris- I have no hands on time with this JEEPs current issue (pun intended..hahaha). I don't know what it's charging at, how the coil was tested, etc, etc. I've done what I could to help by phone.

 

Yes - it is NOT true... There are various ohm values available.

 

What PeteL sez. The resistance between different ballast resistor is not very much but the coil resistance and ballast resistance are engineered. The smoke is from contaminates on the resistor wire and the ballast should still be good unless it is way out of spec with the coil resistance. I would measure the resistance of both ballasts to find the diff. Would like to know the part number of the new ballast for my own info.

 

I'll see if he still has it so I can get you a number.

 

NAPA numbers are different for a Delco or Prestolite distributor. Delco- ICR13 1.82 ohms Prestolite-ICR23 1.2 ohms
The FSM only lists a Delco distributor and lists the ballast resistor at 1.8 ohms.

 

It'll be early next week before I can get hands on this. I'll get distributor info, P/N from new resistor, and attempt ohms readings from both parts.

Thanks Doug, for the posted numbers, and to all who are helping.

 

Skeptical... I recall from my parts store days that we sold the basic Dodge ballast resistor for almost any application. Like this one - More Information for AIRTEX / WELLS 1L2 - Not a lot of variation from application to application, at least back in the days of points ignition.

All the resistor does is limit current when the points are closed. 12 volts across it at 1 ohm is 12 amps, or 144 watts. However, it's always in series with the coil primary winding, which I would expect to have about the same resistance (online sources say 0.4 to 2 ohms). Plus, when running, the duty cycle is roughly 50%, so again halve the power dissipation. So you'd expect something like 36 watts dissipation for both the coil and ballast resistor at most - and likely much less than that due to the reactive nature of the circuit.

Also, this is why you don't leave the key on while the engine is not running. If the points are closed, power is constantly sent to ground through the resistor, coil and points. It's ok for say 30 seconds, but you shouldn't go away and leave the key on.

I suspect something fishy, like you had the first ballast resistor connected to the (-) post of the coil. That bypasses the coil and goes direct to ground when the points are closed.

 

MOPAR 2 terminal resistors were 1.2 ohms