Fine tuning my electronic ignition. When I check the voltage on the coil side of the ballast resistor I have 6.25 volts. Same 6.26 Volts at the coil. That's with key on but not running.That's after it's been running and is hot. This seems too low to me. Has a stock 1.2 ohm ballast.

How much Voltage on the coil with the engine running?

In my case I found with the engine running gave a more reallife situation as to what the coil sees during operation.

Battery = 12.5
Alt charge = 14.4
Measured 1.9 volts more at the coil, than with engine OFF.

I had a bunch of resistors laying around and chose one that gave me around 9V at the coil.
Also, I usually measure the coils temp after some running time with a lasertempgun to check if it doesn't overheat.

Which ECU ? Orange/Chrome/Parts Store ?

I just measured 10.75 volts at the coil when running fully warmed up on a 75 degree day
orange Mopar ecu.
Running nice.
Earlier today I swapped ballast resistors. The previous brand new one gave me 6.25 volts at the coil key on not running. Then I swapped an old extra and got 7.25 volts key on, not running. Both measured within .1 ohm of 1.2. Then I started it up and it ran noticebly better

Dunno what car you are working on.

Here are the "good words" from my '72 FSM (I drive a "worthless" '72 Mopar) :

Ballast Resistor Resistance @70-80 degrees F = 0.5 to 0.6 ohms

Current Draw Engine Stopped 3.0 amperes
Current Draw Engine Idling 1.9 amperes

The clerks at the parts stores typically will sell you the 1.2 ohm ballast resistors.

In my opinion, "it depends". I use a .25 ohm resistor with a chrome box. Your mileage may vary.

After another good test drive, it is working very nice. The ballast resistor made a huge difference.

The voltage is a simple matter to figure out.

The voltage is dependent on the resistance of the coil and the ballast. If the ballast resistance is 1.2 ohms and the coil resistance is also 1.2 ohms then they will both drop half the voltage. So if the voltage at the input of the ballast is 14v, then you will see 7v at the coil + terminal.

If the ballast is 1.2 ohms and the coil is .6 ohms with input voltage at 14V you will see 4.7V at the coil with the rest being dropped by the ballast resistor.

http://www.allaboutcircuits.com/vol_1/chpt_5/2.html

What other people see is not germane to your setup unless they are using the same ballast, same coil and have the same input voltage.

Which ballast resistor to use depends upon which distributor, coil, and ignition box you are running..

The primary purpose of the ballast resistor is to limit current not voltage to the coil..
Certain later aftermarket coils can take significantly, higher current (and voltage) that the earlier coils.. As well as certain boxes may be triggered by an analog or a digital signal which directly affects the amount of frequency/time the coil is being fired..

Disclose what distributor, ignition box and coil you are running then we can post more specific relevant comments..

Just my $0.02...

Working with my Brother in law, we were trying to diagnose a problem in a truck that wouldn't start due to a lack of spark. We tested a few other Mopars to get a group comparison. My Duster showed 9.1 volts on the output side of the coil and it runs great.

You have to (should know) remember that a ignition coil is a step up transformer, 9.0 volts in and 25,000 volts out the coil wire at wide open throttle revved above 5000 RPM I seemed to remember one of the factory Mopar race ignition guru engineers saying at one of the old Mopar drag seminars that the current model(back then) MSD 7C race boxes had around 200 volts D.C. on the input side of the coil( I've never checked that though )so they could have 45000+ volts out of the special race coils made for MSD and Chrysler 7 sereis race boxes I do know if you get hit by one of them it will knock you into silly land, they flat hurt Stay clear of them

My take is on this is, on a racecar, coil-input is 'fairly' irrelevant (in case of too much voltage) as a coil hardly gets time to fully warmup and maybe overheat in the 5-6 combined minutes the engine runs during a run. The hotter the spark the better.

But in a (daily) driver the coil better not get too hot during prolonged use otherwise it could short internally and fail prematurely.
(Hence I usually check the coils temperature after some running time when I switched resistors.)

The real point is that most people use crummy coils, crummy ballast resistors, when if you do the math, one can come up with a good setup (like mine) 1/4 ohm resistor with a chrome box.

I don't get yer math ; Ohm's law is still V=IR. So , scuff.

As was stated whats important is the current flow (amps) in the primary ign circuit. Voltage will vary depending on the ballast and coil. Most standard electronic ign systems will see from about 5.5 to 7.5 volts at the coil pos side with the key on eng not running and ballast hot. Remember the ballast will increase ohms as it gets hotter. If you check the volts at the coil pos side with the eng running it is not an accurate way to check the volts because with the eng running everytime the ECU fires the coil it opens the primary coil circuit so the magnetic field will collaspe and induce high voltage in the secondary windings. But everytime the primary circuit opens the volts will read battery volts (12 to 14.5) at the coil pos side because then it is reading an open circuit volts which is not an accurate voltage reading. So when you read the volts at the coil pos side with the eng running you are just reading the average of the volts with the primary circuit closed and current flowing and with the coil primary circuit open and no current flow. The only way to check the volts correct is with the key on eng not running which is the same way you can troubleshoot the primary circuit when you do voltage drop test readings. Bottom line is most of the standard Mopar ECU ign systems will read about 6 to 7 volts at key on with the eng not running on the coil pos side. Thats for a basicsally stock system with a standard ballast. As long as it puts out a good strong spark then that is fine. I use a .75 ohm ballast on my 63 with the orange MP box and an Acell coil and I have turned it to 6500 running 10's and its never missed a beat. Less ohm's in the ballast will give higher volts and more current flow but may damage the electronics if not used right. And of course the MSD systems will shoot 200 to 300 volts on the coil primary circuit and they do work good to keep plugs clean with the multi sparks. Ron