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Old 05-06-2011, 11:52 AM
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Mallory coil "start" wiring Q

78 Ford Ranchero 351W. Installed a Unilite dizzy and Promaster 30440 coil. Per instructions I wired key on (run) B+ through a ballast resistor to the coil. These Fords have a separate "start" ignition circuit, however.
Question is: Should I run the start (full B+) voltage direct to the coil, bypassing the ballast?
Instructions don't mention this and a call to Prestolite Tech Support was not very confidence building. I don't want to burn up the dizzy and/or coil.

Anyone?

thanks..

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Old 05-06-2011, 02:16 PM
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Seems this has come up in one form or another several times lately.

If the instructions are to use the ballast resistor then it is necessary to protect the coil so do use it. Then attached to the same terminal as the resistor wire you need a wire from the solenoid (by passing the resistor) that will be hot ONLY during cranking, this will be the small terminal beside the one marked "st". However on that 78 it is possible that your solenoid may not have that second small terminal and if so it is simple and inexpensive to just replace the solenoid with the older type that has both terminals. This is important because if you just leave off this wire and use only the resistor circuit you will get very low voltage and a weak spark while cranking due to the starter motor load dragging down system voltage. If OTOH you just run a wire bypassing the resistor circuit that is connected to a source that is hot anytime the key is in run you will defeat the resistor circuit that protects the coil. It is important that the by pass wire be connected to, and ONLY to, a source that is hot only when the engine is cranking and goes cold when the starter is released.


Was the ballast resistor added along with the new coil? If so was the factory hot wire to the coil used? What I am asking is was the ballast resistor possibly added to a circuit that already has a resistance. This is easy to check and just as easy to remedy if it is a problem. With the ballast resistor in place and the key "on" (but not cranking the engine) check the voltage on that wire at the hot terminal on the coil (wire disconnected from the coil), it should read about 8 to 9 volts. If it is lower than 8, say about 6 volts or so, then you may already have the resistance needed and you simply need to leave off the ballast resistor and check again. The coil should never have more than about 9 volts when the key is in run but it does need that temporary by pass during start.
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Old 05-06-2011, 03:50 PM
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That's a very good explanation by Oldred.

The 78 Ranchero came stock with the Duraspark II ignition,
which had a resistor wire (located under the dash) for the
run portion of the coil's power. The 'start' power came from
another wire off of the DS ignition box. Since the DS box is
now gone, follow Oldred's suggestion for the start voltage
wiring (from starter solenoid/relay). I believe there's indeed
a terminal there for it on your 78.
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Old 05-06-2011, 04:40 PM
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Thanks for the info. To clarify a bit, I had already installed a new "key on" relay that supplies the coil in "run" as well as other items needing it (electric choke, etc.). The solenoid is the single terminal one, but in my parts collection I found a new two-terminal one. So, I will run a new wire directly from this to the coil. Actually, I guess it will be easier to just run it to the "low" side of the ballast that goes to the coil.
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Old 05-06-2011, 05:00 PM
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To sum it up then, you used a relay to supply power in "run" and this circuit goes to the ballast resistor and then to the positive side of the coil? Using the new solenoid to simplify this is a good idea, so you will then run a wire from the "ign" terminal on the solenoid to the same positive terminal on the coil by passing the resistor? Does that sound like what you have in mind? If so that should work just fine.
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Old 05-06-2011, 05:06 PM
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Quote:
Originally Posted by oldred
To sum it up then, you used a relay to supply power in "run" and this circuit goes to the ballast resistor and then to the positive side of the coil? Using the new solenoid to simplify this is a good idea, so you will then run a wire from the "ign" terminal on the solenoid to the same positive terminal on the coil by passing the resistor? Does that sound like what you have in mind? If so that should work just fine.
Exactly. Just took a peek at that new (Motorcraft D2AF-11450-AA) solenoid and, sure enough, the small terminals are marked "S" and "I" so all should be good to go.

thanks again...
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