Low coil voltage

[71340RT]

I have been fighting some cold start up issues on my stock other than added electronic ignition on my 1971 RT 340 automatic that I have owned for 31 years. It all started when my ECU box over the winter all the sealer on the back side ran down my firewall. I got another box and it never started the same. I had a ignition switch go bad a couple of years ago and after 2 after market junk switched I bought a NOS made in USA on and no more problems with the switch. My problem started last year with the hard cold engine start up and have traced it to low coil voltage. After looking at the wiring diagram I checked power at several location and the only low reading I found was at the coil. With the ignition on in the run position the coil only has 5.4 volts on the positive side. I changed out the reproduction voltage regulator and now have 6.6 volts but still not the 7 to 9 volts that I think I should have. I had already replaced the coil which I didn't help. I'm fine at the switch and the firewall plug for voltage and wonder what else I might be over looking. Anyone with ideas let me know. With the increase to the 6.6 volts at the coil it started right up today.

[dodj]

Should be battery voltage at start-up, minus small voltage drops over connectors. 


How do you see with all those stickers on your windshield?

[Chryco Psycho]

Ballast resistor controls the voltage at the coil , also the coil load can drop the voltage , I would start with a lower resistance ballast .

[dodj]

Ballast resistor controls the voltage at the coil

At initial start up?

[Bullitt-]

  I would trace the power back at each connection & see where the voltage drop occurs
  Voltage should be close to your batteries voltage at the ballast resistor but the "start" power comes from a different wire and bypasses the ballast. You should get close to battery power at the coil on the Brown wire with the ignition switch turned to the "start" position.
  Once the ignition switch is released to the "run" position power comes through the Blue wire & is redirected through the ballast resistor which drops the voltage to the coil.
  A good ground on the ECU is also necessary
    Start is the Brown wire Run is the Blue wire, both originate with power at the ignition switch 

[71340RT]

Should be battery voltage at start-up, minus small voltage drops over connectors. 


How do you see with all those stickers on your windshield?

t
The Mopar National stickers have been on there so long I don't have the heart to take them off.

[71340RT]

  I would trace the power back at each connection & see where the voltage drop occurs
  Voltage should be close to your batteries voltage at the ballast resistor but the "start" power comes from a different wire and bypasses the ballast. You should get close to battery power at the coil on the Brown wire with the ignition switch turned to the "start" position.
  Once the ignition switch is released to the "run" position power comes through the Blue wire & is redirected through the ballast resistor which drops the voltage to the coil.
  A good ground on the ECU is also necessary
    Start is the Brown wire Run is the Blue wire, both originate with power at the ignition switch 

I have a good ground at the ECU and it is brand new.  I will start checking power and different locations and see what I find. Thanks to all for the good information.

[chargerdon]

Interesting that your problems started when you put on a new ECU!!!!!!  You might want to do this to check things:

Pull the ground wire off of the starter relay so that there will be no starter motor.   Next, lets check the voltage going to the ECU using multimeter check the voltage at the blue wire connector on ballast resistor when the key is in ON/Run position.   It should read at LEAST 9-11 volts.  Next, with multimeter on same blue wire connector on ballast resistor while the key is turned to start.  See it if stays above 9 volt !!   It will be lower voltage when on start because the power to it goes thru the ballast resistor. 

Put the ground wire back on starter relay and again check voltage at voltage at the blue wire connector on ballast resistor when the key is turned to start.   

ECU's need i believe at least 8-9 volts in order to read the distributor and tell the coil to fire !!!   Less than that and you will not get the coil to fire.

It is a "quirk" in Mopar Electronic ignition systems, that during "start" the path to the coil bypasses the ballast resistor for higher voltage to the coil, BUT goes thru the ballast resistor to the ECU.   If the Battery voltage (not amperage) is low then the voltage to the ECU can be too low for it to operate during starting.   Or if the connections thru the bulk head connector is weak, you can get a voltage drop to the ballast resistor.   STUPID DESIGN by Mopar to disable the Run wire during starting operations.     Some ECU's are more sensitive to the voltage than others!   

PS, you can also validate if this is the problem, by simply putting a timing light on the system during start attempts to see if it is "firing". 

[Chryco Psycho]

Ballast only affect running not starting , could be a bad coil drawing down the voltage tho