Ballast Resistor..is it still needed.

[chargerdon]

My 74 Challenger has a Stock Chrysler 5 pin ECU Electronic Ignition as do all stock systems from 1972-1978.   However, 5 pin ECU's are difficult to find, and so i have replaced it with a 4 pin ECU.   NOW...A 4 pin ECU does NOT use the ballast to ECU wire as that is the missing pin on a 4 wire ECU.   The 4 pin ecu supposedly passes current to that circuit internally, with internal resistance...hence 5th wire no longer needed.

Now, if you study a wiring diagram for a 5 pin system the dual ballast has two sides.   One side is 1.5 ohm resistance, and is the side that feeds the + on the coil, and the other side is the 5 ohm resistance side, that feeds the "5th pin" on a 5 wire ecu.  On a 4 pin ecu that side is not used!   Thus if you replace your 5 pin ECU with a 4 pin ECU then the 5 ohm side of the ballast resistor is not actually used.  In fact newer than 1978 systems only use a single side ballast resistor. 

Further studying the 5 pin factory wiring diagrams, or the 4 pin systems, you will see that during the "start" cycle, power goes to the ballast at the point where the + coil wire is located, thus, sending full battery current to the coil for "a hotter spark" during start.   This is true, but, unfortunately, the start wire does have to pass thru the ballast resistor to feed the rest of system including the ECU.  This is because during start the ignition key turns "OFF" the run wire.  Thus the ECU during start mode will get 1.5 ohm reduced voltage, while the coil is getting full power.   But during "run" just the opposite is true..ie. full voltage to the ECU and 1.5 ohm resisted voltage to the coil. 

Normally, this is fine, BUT,  if thru age you have a weak connector somewhere in the system, then the voltage at the Ballast during starting will be maybe 1/2 to a full volt lower, and when it then passes thru the ballast to get to the ECU the resultant voltage drop might be enough to keep the ECU from "firing" and you will get a no start condition.   I sometimes get this problem especially when the engine compartment is hot.   To get the car to start when this problem arises, i simply turn the ignition key to "run" and then use a small jumper wire at the starter relay to get it to crank, and it will then start.   

So, recognizing that the ECU gets full alternator voltage during run there is no need to reduce its voltage during start.   

I happen to have an MSD BLASTER II coil in my car.   I sent the question of is the ballast resistor needed on their coil during "run" to MSD technical assistance line.   I was worried that if i eliminated the ballast resistor completely, that the coil may have a much shorter life.  Their reply was: "The ballast resistor is mainly there for either the ignition module, points, or in our case that ECU. So, if you have swapped out your unit to one where it does not require the ballast resistor, then you can bypass that resistor and it should be perfectly fine."

So, i'm thinking of tying the "RUN" "START" and "+ coil" wires together at the ballast thus eliminating it entirely.   Thoughts?

[cuda hunter]

 

[7212Mopar]

At one point my car has the RevNnator ECU with a MSD blaster II coil. They still recommend to keep the ballast resistor so you might want to call them to find out why. I eliminated the ballast resistor when I switch to MSD 6AL with the blaster II coil.

[Rich G.]

I was with you up to the point when you said My 74 Challenger has a Stock Chrysler 5 pin ECU electronic ignition , then you lost me! So what exactly does the ECU do if a 4 or 5 pin ECU works?

[Bullitt-]

Think I follow what your saying & it may not hurt the MSD coil   
BUT I don't see how it would change anything when it comes to the staring issue.... ballast resistor is bypassed during startup.   
You have a wiring issue that needs addressing, probably on the wide connector at the steering column or bulkhead connector.   

[Chryco Psycho]

the circuit is designed ro you have 13 v to the coil starting & 8v running so the start circuit is on the same side as the coil feed bypassing the ballast for starting so in My opinion you still need a 2 pin ballast to protect the coil the 4 pin type to drop voltage to the ecu as well in not required though

[jimynick]

I read an article by Rick Ehrenberg about MSD coils not having the right impedance, which screwed something else up. I'll try to find it the article again.