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Old 04-27-2011, 10:37 PM
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Coil and 12V upgrade

Hey guys I'm working on a 1951 Plymouth Cranbrook and the guy did a mickey mouse job on the electrical. I can't find an electrical diagram anywhere on the net for this car. Any ideas where I can grab one?(diagram)

1. The coil positive wire is just hanging loose...does it need a switched 12v source?
2. 12V upgrade alternator....its got two extra wires hanging loose (stator maybe?)

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Old 04-28-2011, 05:56 AM
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wiring

The coil will always need a sw'd wire... are u using the stock dizzy that has the points in it ???? do u have a volt mtr to ck for voltage from the sw to the ballast resis, out of ballast to +side of coil the -side of coil to points...
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Old 04-28-2011, 08:02 AM
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oh boy

yeah I have a volt meter, so I can hook it up the + to any 12v switch? As for the coil the - wire is going into distributor somewhere. and as far as I know he's using the stock distributor

Last edited by 67MercCat; 04-28-2011 at 08:24 AM.
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Old 04-28-2011, 08:06 AM
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Quote:
Originally Posted by 67MercCat
The coil positive wire is just hanging loose...does it need a switched 12v source?)

You actually will need two wires to do this properly one supplying the "hot" side of the coil will be full voltage in "START" only and electrically "cold" when the starter is released, that is it will have full power on it ONLY while the engine is cranking. This wire is usually taken from the starter solenoid so it will be "hot" only during cranking and can be run directly to the coil. You will then have another wire from the ignition switch that will have a resistor to cut the voltage to about 8 or 9 volts by using either a ballast resistor or a resistor wire, this wire will be hot anytime the switch is in the "run" position. The coil should not see a full 12 volts while the engine is running if a standard coil is used. Standard factory type 12V coils are actually designed to operate on about 8V and that is why the resistor is REQUIRED, this is done to compensate for the system voltage drop caused by the starter load during cranking which will drop the entire system voltage down to about 8V.

By using the resistor circuit bypass wire from the solenoid full voltage will be applied to the coil during cranking, this voltage will be about 8V or so due to the starter load dragging down the system voltage. When the starter load is released the voltage will rise back up to the system full voltage of 12V+ and this will then be reduced to the same level as it was during cranking by using the ballast resistor. This circuit is necessary to prevent either over powering the coil during "run" or under powering it during "start", if a coil requiring 12V was used then it would have a weak spark during cranking due to the starter load voltage drop. OTH if an 8V coil was used to provide a hot spark during "start" then it would be over powered when the switch was released from "start" and the system voltage would again rise to a full 12V+. This is going to be required by both a points type system and most solid state factory type ignitions (except GM HEI systems). The resistor circuit is required for most factory type coils due to this voltage drop from the starter load and it has nothing to do with whether points are used or not but if an after market system is used then of course you want to follow the manufacturers instructions for wiring.
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Old 04-28-2011, 08:31 AM
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hmmm

so I can run a wire from the starter to the coil, and another to the run position wire (hot/on/run) with a resistor. I have some 560 ohm ballast resistors are those good enough to reduce the voltage? I guess I'd have to check with my voltmeter.



also what about the alternator? Is there a ground that goes to the alternator (back) right now theres only one wire going to the starter solenoid for charge.
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Old 04-28-2011, 08:50 AM
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Quote:
Originally Posted by 67MercCat
I have some 560 ohm ballast resistors are those good enough to reduce the voltage?


Not sure what kind of resistor you are talking about but DON'T GUESS about this, go the parts store and get a standard ceramic ballast resistor designed for ignition systems, they are quite cheap and easy to find. I think the last one I bought was less than $4.
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Old 04-28-2011, 10:02 AM
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560 ohms ???

wow thats alot... don't know if your measuring just the resist or what.. but it will be in the 1 1/2- 5 ohms area if the olde brain serves me right.. u use the open small lug on the st sol for the 12v to coil starting. but if your engine starts very easy u don't need it... i don't use one cause i just look at my motor and it'll start up...
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Old 04-28-2011, 10:32 AM
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Quote:
Originally Posted by delawarebill
but if your engine starts very easy u don't need it....



That can create major problems on a cold day especially if the battery is weak, or ANY time the battery is low for that matter! I agree that most of the time if everything is "up-to-snuff" and the battery is fully charged then it is not likely to be a problem but omitting that circuit is just asking for trouble!
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Old 04-28-2011, 01:17 PM
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Quote:
Originally Posted by 67MercCat
Hey guys I'm working on a 1951 Plymouth Cranbrook...Any ideas where I can grab one?(diagram)
Here's a diagram for your Plymouth:


Looks like there were originally three wires coming off the ignition coil positive post: Power from the ignition switch, power to the horn relay coil, and power to the turn signal flasher. It's an unusual way to power the flashers and horn, but the factories did a lot of experimentation back then. Also there could be an additional wire on the coil negative terminal if the car has overdrive or the electric semi-automatic transmission that some of these old Mopars had back then.

Hope this helps.....

Joe G.
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Old 04-29-2011, 09:42 AM
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That wire coming from the positive on the coil and going to the horn relay and flasher is a good example of how modifying one thing (in this case changing from 6V to 12 V) usually leads to changing something else. Because the new ignition circuit will require the power going to the coil to use reduced voltage during run these two items (horn relay and flasher) will need that wire removed from the coil (if it is indeed still there) and powered from another 12V circuit. OPf course these items have most likely been changed anyway along with lights etc, or should be, due to the voltage change.
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Old 05-01-2011, 06:49 PM
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I am an electrical moron so I can't help with your questions but I can say to sign up over at the P15-D24 Site . There is a ton of knowledge on the old Mopars on the site and the guys are always willing to help.
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