Voltage at coil (+)? Might be on to something...

The Roach is still running horribly...seems to only run on 7 most of the time. I checked the voltages at the ballast and coil today, I see 11.3v at the alternator post (battery's in the trunk, may be down a bit), and like 3.6v out the ballast! The ballast checks around 2.5 ohms...coulda sworn it was listed as a 1.0 ohm. The obvious (?): at that low voltage, would that cause it to hardly run? The less obvious: from 2.5 ---> 1.0 ohms, that big of a difference in voltage?

ya, thats a problem. i like to see around 7v at the coil

11.3V tells me the charging system, isn't.

When it's running 13.8V is more like what you should see. It will vary a bit, but should be in that ballpark. Low voltage tot eh ballast means low voltage out adn low voltage at the coil.

Fix that first, then see what's at the coil. If this is a dual ballast resistor you probably have it hooked up to the wrong side, 2.5 Ohms is way too much.

How voltage drop works is simple to figure. Measure from the ballast input to ground (no voltage applied). Subtract the ballast resistance, in this case 2.5 ohms. The difference is the coil resistance. Let's say both are 2.5 ohms, then the voltage at the coil + will be half the voltage at the input to the ballast, the equally drop the same voltage. If the coil resistance was 1.25 ohms and the ballast is 2.5 then the ballast would drop 2/3 of the voltage and the coil the remaining third, so at 12V into the ballast you would see 4 volts at the coil +. IOW, the percentage of resistance the coil has in the circuit is the same percentage of voltage dropped across the coil.

I'd check the alternator also. Voltage at the alternator should be the highest in the system normally 13-14.5 volts if its charging. Battery in the trunk should not matter. Was the car idling when you checked? Try speeding it up and check again. If its still low, your alternator isn't working.

Sorry, those weren't engine running.

Like I said, the battery looks a bit down, so it's pulling almost 15v right after starting. Still, I'm only seeing 4.75-4.9v at the coil (+).

Also just discovered the ceramic core of the ballast is broken, but not the wire. Needs to be replaced anyway.

fix the charging fault, and replace the ballast. If youa re using the single ballast, msd makes a really nice .8 ohm one

OK, I dug around the garage and found a ballast that reads .7 ohms. Might be the one that came with the MSD coil, I dunno. Still seeing less than 5 volts at the coil! Went over the wiring to the ballast and the box, seems OK. Jumpered the ballast out, I get 8.3v at the coil, engine off. Was tempted to start it just to see, but didn't want to fry my only chrome box.

BTW, are ballasts rated hot or cold? The .7 ohm one read more like 1.5+ after running the engine a bit.

Get a new ballast... and then if you still have a problem...
Check voltage from the ignition switch "on" position. Check for loose/bad wire terminals.

Start at the beginning(starter relay power) and go forward..check the fusable link volts at the firewall bulkhead and then the ignition switch power wires.


No MSD box?

No, no MSD box. Mopar chrome box, MSD Blaster 2 coil.

The 8.3v reading without the ballast tells me I have some resistance somewhere...I'm heading for the bulkhead connector next.

Thanks for the replies!

Engine needs to be running (Alt charging) before you measure any voltages.

Wow look what the cat dragged in... LOL

Sup JC???

Check it when running. Sounds like it may be a bad wire somewhere. They don't run well on 3 volts at the coil...

As a test, you might try running a hot lead right to the ballast, opposite side of coil lead, and see if that helps.

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OK, I dug around the garage and found a ballast that reads .7 ohms. Might be the one that came with the MSD coil, I dunno. Still seeing less than 5 volts at the coil! Went over the wiring to the ballast and the box, seems OK. Jumpered the ballast out, I get 8.3v at the coil, engine off. Was tempted to start it just to see, but didn't want to fry my only chrome box.

BTW, are ballasts rated hot or cold? The .7 ohm one read more like 1.5+ after running the engine a bit.

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With the ballast jumpered out you should see battery voltage at the coil. Since you are not you need to find out where you lost the voltage, probably at the ignition switch. I would measure the switched 12V at the ignition switch and see what that is, if it's low measure the voltage into the switch. If the switched 12V is OK at the ignition switch then the wire between the switch and the balast is the issue.