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Old 11-30-2011, 09:49 AM
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Wiring Diagram Question

In the diagram below I have a question about the wiring associated with the alternator and the ignition coil. On the #1 wire out of the alternator is shows a loop and a curvy line associated with it. It is labeled "resistance wire".

Exactly, what is it and how is it supposed to be connected? That sort of connection wasn't part of any of the standard wiring diagrams that I learned in my electronics courses. It almost looks like it was added "after the fact".

Thanks!



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Old 11-30-2011, 02:35 PM
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I think I may have answered my own question. That notation means that the yellow wire off of the #1 terminal is the resistor wire. Is this a correct assesment?

In a 1988 Grand Wagoneer with stock ignition, what is the value of the resistance?

There has been some rumors floating around that the resistor isn't necessary with an aftermarket coil such as the MSD High Vibration Blaster Coil. Is this true? The MSD instructions say otherwise.
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Old 11-30-2011, 02:56 PM
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You got it.
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Old 11-30-2011, 03:08 PM
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I'm no electrical guy but the way I read it is that the yellow wire is a resistance wire and the goofy line with the circle just points to it. A normal draftsman would have just used an arrow pointing to the wire but electrical guys love their own symbols.
On the after market coil I generally assume the people that designed built the thing would know if it'll handle 12v or needs to be dropped down. I would go with what the manufacturer says. If in doubt call them and ask them how much of a resister is needed to get the voltage down to what they want cause you can bet if you fry it you eat it.
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Old 11-30-2011, 03:32 PM
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In some older FSJ wire diagrams, the resistor wire is shown with the inverted horseshoe. I can't remember the ohm value it is supposed to have. This wire is only used in vehicles without the warning light.
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Old 11-30-2011, 04:06 PM
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Quote:
Originally Posted by CommanderChaos
In a 1988 Grand Wagoneer with stock ignition, what is the value of the resistance?

15 Ohmz!

However, if you choose to go with a 12V coil, be advised that resistive wire also goes to the alternator pin 1. So best to run a new wire between the ignition switch pin 11 (where the coil yellow wire begins) and the new coil. Leave the resistive wire in place in case the future wants it back.
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Last edited by Rich88 : 11-30-2011 at 04:19 PM.
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Old 11-30-2011, 04:28 PM
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15 ohms?!!! Are you sure? MSD's instructions call for a 0.8 ohm resistor.

I'm getting somewhere between 4 and 5 volts at the coil when running.
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Old 11-30-2011, 08:33 PM
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Quote:
Originally Posted by CommanderChaos
15 ohms?!!! Are you sure? MSD's instructions call for a 0.8 ohm resistor.

I'm getting somewhere between 4 and 5 volts at the coil when running.

I'm sure MSD is calling for a .8 ohms off of a 12V source, which is the requirement for their coil.

The requirement for an OEM coil is X ohms between the coil and a 12V source (ign sw), to give you 8-9V at the coil. You're probably getting only 4-5V because of aged connections at either or both ends adding to the 15 ohms of the resistive wire.

Is it 15 ohms? I got this from some reliable source I regret I cannot dig up at the moment. But I do know that when I measure from the ignition module to the coil + terminal I read 14.7 ohms. So its really only a clue that the resistive wire is in there.

Unfortunately the schematic is not very clear about the extent of the resistive wire. I don't know exactly where the splice points are but I do know that on my '88, I see a stranded wire at the ignition module, a solid core resistive wire at the alternator terminal#1, and stranded again at the coil.

To complicate things, we know that both the alternator and coil need resistance between 12V and the devices. But the schematic doesn't really show that.

Related subject: I had a very experience with this once. Replaced coil and ignition module only to find out the crimp on the wire at the coil was crudded/corroded. After stripping back and soldering a new spade connector I was golden.
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Last edited by Rich88 : 11-30-2011 at 08:40 PM.
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Old 11-30-2011, 09:08 PM
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Quote:
Originally Posted by CommanderChaos
15 ohms?!!! Are you sure? MSD's instructions call for a 0.8 ohm resistor.

I'm getting somewhere between 4 and 5 volts at the coil when running.


I found it. I WAS WRONG. Resistance between the ignition switch to the alternator is 15 ohms. But resistance between ignition switch and the coil is 1.35 ohms. Different years have different ways of wiring or how its shown on their respective schematics. I only ever looked at the '88, since that's what I have. But I checked out various years' drawings on Tom Collins site.

So this tell me that if you install a 12V coil you can pick up 12V by splicing into the green wire coming of the ignition module.
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Last edited by Rich88 : 11-30-2011 at 09:16 PM.
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