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Only 6 volts at the neg. coil terminal...

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Old 04-10-2005, 06:01 PM   #1
Doug Brandon
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Default Only 6 volts at the neg. coil terminal...

Motor quit on me after running great for a little thrill ride with my visiting younger brother. The only way I could get it started was to bypass my MSD box, and use the 12 volt source off the back of my alternator. With key in 'run', I only get 6 volts at the negative coil terminal.

Also, when I went to disconnect my battery for the evening, I noticed my positive terminal of the battery was very, very warm. All my wiring is original. Any suggestions on trouble shooting? I have very limited knowledge of electrical systems.

Btw, still only have 100 or so miles on my new combo, but 3rd gear with over 500 ft lbs of torque is now quite a rush!
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Old 04-10-2005, 06:08 PM   #2
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Hmmm. I'm not sure that I understand, but if your coil is hooked up correctly with either a ballast resistor or a resistor wire, you should only have around 7.4 volts at the coil except when cranking. When cranking, the wire coming off the starter solonoid will supply the full 12 volts to the coil.
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Old 04-10-2005, 07:04 PM   #3
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The resistor wire is no longer a factor when a MSD box is hooked up. That wire normally supplys power to the coil, but the MSD has it's own power supply. Hense, the factory coil wire becomes a "signal" wire that tells the box when to fire the coil. Also, the fabric covered wire that goes to the coil from the starter is no longer needed.

I would go to MSD's website & download the instructions & double check the wire routing. Did you buy the MSD box new or used?
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Old 04-10-2005, 08:53 PM   #4
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I bought the MSD new and will go find the instructions.

Also, I just rechecked the voltage at the coil, with the key in the 'run' position. Now, a day later, I get 18 volts on the positive outer post of the coil and 17 volts on the negative outer post of the coil. Yesterday when the engine was hot and had quit running, I was getting less than 6?? Any ideas?
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Old 04-10-2005, 09:04 PM   #5
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i wonder what happens when a coil starts to crap out...and i wonder how are you getting the extra voltage???
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Old 04-11-2005, 12:39 PM   #6
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This may help.

My '64 with a Mallory Hyfire box and high voltage chrome coil were working fine and Lars and I went for a test ride so we could compare the difference after his tune-up (he drove). Lars was impressed with the performance. Lars then did a tune-up on the engine and when we took it out again, the engine would die every time he'd hit the throttle and open the secondaries.

After examining his work, we figured it was electrical and I by-passed the Mallory box letting the coil fire off the points through the ballast resistor. (Mallories are sensitive to such things as jump starts and I have the system set up where I can do this in seconds with no tools.)The car than ran fine (and Lars got it sideways short-shifting into 2nd. )

Sooo, I figured that with the additional cylinder pressure from the tune-up, the Mallory box couldn't keep up with the increased cylinder pressure. I examined the fuse on the Mallory and noted that it had sulfate/corrosion. This creates a path of increased resistance reducing the available current. I cleaned it with some steel wool, replaced it, and the problem was gone.

Perhaps, you are having the same problem either with a fuse, or even the battery terminal. A high resistance path can also generate the additional heat that you noted.

Last edited by toddalin; 04-11-2005 at 12:41 PM.
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Old 04-11-2005, 01:04 PM   #7
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Quote:
Originally Posted by Doug Brandon
Motor quit on me after running great for a little thrill ride with my visiting younger brother. The only way I could get it started was to bypass my MSD box, and use the 12 volt source off the back of my alternator. With key in 'run', I only get 6 volts at the negative coil terminal.

Also, when I went to disconnect my battery for the evening, I noticed my positive terminal of the battery was very, very warm. All my wiring is original. Any suggestions on trouble shooting? I have very limited knowledge of electrical systems.

Btw, still only have 100 or so miles on my new combo, but 3rd gear with over 500 ft lbs of torque is now quite a rush!

Trick question!!

The NEGATIVE side of the coil has various loads at various times (coil, points, condenser, etc.), therefore, voltage will vary. In a point system, the voltage will be different when the points are closed vs. when they are open. You didn't say if you have points (and a condenser) or not.

Where are you getting the power for the MSD?
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Old 04-11-2005, 01:15 PM   #8
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Double post!
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Old 04-11-2005, 11:17 PM   #9
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I have points. I didn't realize it would vary like that.

I have been doing some reading on the MSD website. One thread mentioned a loose connection on the battery as a cause of issues with an MSD box. My negative cable was only finger tight. I will try tightening and drive it tomorrow to see if that makes a difference.
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