Mopar Electronic Ignition Question For The Experts

Why does the Mopar electronic ignition still use the resistor. Couldn't they have eliminated it with the electronic smart box. I have two cars with it and have always wondered about that

 

Short answer: easier starting.

What year are your cars? Chrysler's first electronic ignition debuted in 1971 and was used nearly unchanged until the mid eighties. It's a good reliable unit but the basic design dates to the sixties. The latest computer controlled units are far more sophisticated.

 

The box is the trigger on the negative side of the coil.
The resistor is before the positive side of the coil.
So the coil determines if it's used or not.
As Rusty says, being able to bypass the resistor during cranking to provide full voltage, then revert back to lower voltage in the Run position allows easier starting and continued use of the standard coil design already in use.

 

The ballast resistor also changes its value with RPM and decreases current flow at lower RPM. At higher RPM the resistance will decrease and help keep the spark up to the demand. One may need to refer to a textbook to get a fuller explanation. I used to teach this stuff in my previous life as a college instructor.

 

I think they use the resistor because the power transistors available at the time couldn't handle the current and the circuit wasn't designed to limit the current like the later GM HEI was. I think you could use a coil with a built in resistor and do without the external ballast but when I asked the MOPAR tech line if this was OK to do they said the ballast was required. Probably reading from their script.

 

So that you can mount it on the firewall like we all did in the early sixties when running points. Yes this is a Chrysler ign. system. Every hotrod that I ever remember back then had one mounted on the firewall. To me it is a classic sign of an old hotrod.



I used a ballast resister from a 57 Ford that was mounted on the engine and protected with a perforated steel box. That's it in the back under the gas linkage. I'm a detail freak and I'm pretty sure you wont see another 57 Ford ballast resistor on a streetrod. This is a Chrysler system also. I'm just weird.

 

Thanks guys, I think the technology at the time answered my question

 

Tech of today is different, and better in some ways but if you add say an MSD, it is susceptible to other issues. They require dedicated wiring and are touchy about voltage noise.
It was built so that it had extra power during starting, which is a good thing with slow cranking speeds and cold conditions.

 

I have a ballast on my 341 Desoto hemi powered Model A. I'm also using a 53 Desoto dash panel and wanted to use the original ignition switch. That switch doesn't have an extra terminal on it like later switches. That terminal puts full voltage to the coil when the key is turned to the start position. When the engine starts and you release the key another terminal is hooked to the ballast resistor. Part of the 12v is dropped across the ballast. I had concerns about not having the extra terminal to put full 12v on the coil during start. I can tell you it has never been an issue. One of the easiest starting carbureted engines I ever had. If everything is right and it remains that way not putting 12v to the coil will never be a problem. I have never had a problem in cold weather either.

 

jbtine what ignition are you using. I'm still contemplating my ignition for my Dodge 270 hemi rpu project. I do like the Mopar electronic but I'm open to suggestions

 

Absolutely true. The Mopar electronic ignition system is designed for use with the ballast resistor. No script required. Just truth.

 

Wiring diagram for the system.

 

From the wiring diagram it looks like the resistor will only change the voltage seen at the coil. In points systems, the ballast resistor was used to provide lower the voltage in the coil and points circuit so the points last longer when the charging system is raising the voltage to 13.6. The resistor is bypassed for higher starting voltage to counteract the voltage drop from starter draw. Electronic systems don't need to limit the voltage to their trigger system.

 

Also by using the resistor the primary wattage is lowered by dividing the voltage to the ignition coil . Helps prevent the coil from getting too hot....

 

The 341 Desoto actually has a Mallory Unilite in it. Completely self contained. No brain box, nothing to ugly up the firewall. I have a chrome Mallory coil hidden on the valley pan under the intake. It does have the ballast resistor and the instructions with the distributor said to use the ballast resistor. The Mallory system is probably the reason it starts so well without putting 12v to the coil during startup. The distributor is for a 318.
My 39 Plymouth truck has a 318 in it with a completely stock Mopar ignition system. Works well but I had to make a cover for a recessed area in the firewall to hide the brain box.

 

Here is a pic of the 341 Desoto. If you look between cylinders 3&5 you can barely see the coil.

 

Just so your clear on this, on a Mopar electronic ignition system, the resistor is there to protect the coil. Gene

 

Transistors have a voltage range a car charging system may see a high of 17 volts and cranking voltage may drop below 10 volts so they use a resistor to cut off the top. And bybass the resister when cranking.

 

Thanks again guys, all this has been very helpful. I'm 74 years old and my lessons just never stop. By the time I get to 100 I'll be pretty smart

 

I am running a coil with a built in resistor and a mopar electronic ignition with no ballast resistor, no problems yet.