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New ignition coil question.


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She's giving me a hard time starting this year. I'm starting with the ignition coil and spark plug wires. My question is  on the side of the new ignition coil it says " 12V use with external resistor 907".

Is this something I need to worry about. I read some cars don't need the resistor?  Thanks on advance.muFsit5.jpg

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Our cars originally came with a resistor wire that drops the voltage. An 'external resistor coil' would normally be correct. But a lot happens in 45 years...

When starting the solenoid sends a full '12 volts' to the coil and that's okay. For one, it's not really twelve volts because the starter sucks up a lot of power.

The higher voltage helps with starting but lower voltage is needed for higher rpms.

 

With a multimeter check the voltage while cranking (should be around 9 to 12 volts) and while running. Running it will be much less than 12 volts. I forget the exact value bit if it is at or over 12 volts while running the resistor wire has been replaced.

 

To verify while cranking you might find it handy to remove the big coil wire to make sure it doesn't start.

 

Can you give more details on the symptoms?

'Mike'

73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

 

Pics of modifications included in: [button=http://www.7173mustangs.com/thread-1973-convertible--3335]My Garage[/button]

 

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Our cars originally came with a resistor wire that drops the voltage. An 'external resistor coil' would normally be correct.   But a lot happens in 45 years...

When starting the solenoid sends a full '12 volts' to the coil and that's okay.  For one, it's not really twelve volts because the starter sucks up a lot of power.

The higher voltage helps with starting but lower voltage is needed for higher rpms.

 

With a multimeter check the voltage while cranking (should be around 9 to 12 volts) and while running.  Running it will be much less than 12 volts.  I forget the exact value bit if it is at or over 12 volts while running the resistor wire has been replaced.

 

To verify while cranking you might find it handy to remove the big coil wire to make sure it doesn't start.

 

Can you give more details on the symptoms?

 

I went down to warm her up and she started and idled for about a minute.  After that I can't get it to go. https://youtu.be/8LR1EaVG4Eo. I'm starting with the coil and wires and going from there I guess...

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I don't think that is a coil or ignition issue.

 

With the engine not running turn the key to the 'on/run' position and measure the voltage at the coil. If it is not zero your coil is geting voltage.

Next, try spraying some starting fluid into the carb. Put the air clearner on and try starting it. It should run for a few seconds. If it does, and then dies, you have a fuel issue.

'Mike'

73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

 

Pics of modifications included in: [button=http://www.7173mustangs.com/thread-1973-convertible--3335]My Garage[/button]

 

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If you're going to use it with points in the distributor you need to have the external resistor, otherwise you'll quickly pit the contacts on the points.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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If you're going to use it with points in the distributor you need to have the external resistor, otherwise you'll quickly pit the contacts on the points.

 

Now why isn't this the case with the previous ignition coil?

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Sorry guys. I don't know if I'm being unclear or if Im just not getting it.

I bought this ignition coil to replace the one in my car. The only difference I see between the two is that it says to use an external resistor. Now does that suggest that this Ignition coil is different than the one I previously installed and that I must add an external resistor or is this just a warning because this ignition coil is widely used on different vehicles, and I already have an external resistor somewhere?

 

I know I'm being a pita, just learning as I go.

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Our cars came with an external resistor, it's a resistance wire that runs between the ignition switch and ignition coil. As long as nobody has removed or bypassed the resistor wire you still have the external resistor. All cars with a 12 volt electrical system and running points in the distributor require the voltage to be reduced to the coil to prevent the points from quickly deteriorating. The coils that came on our cars required an external resistor, it just didn't state it on them.

 

This is why Mike recommended measuring the voltage to the coil, to make sure you do have the resistor.

 

Aftermarket electronic ignitions, that replace the points, may not require the resistor and reduced voltage.

 

One caveat, there are ignition coils available that have a built in ballast resistor. So, if you​ are running a point ignition you need an external resistor, either the one that was originally in the car or an add-on ballast resistor.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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