Coil - resistor - explain plz

[sasilverbullet]

I need some old school schooling!

I have a 70 Lincoln Mk III with a 460. It currently has the original style coil/dist setup. I have replaced the points with a Pertronix II unit. My understanding is that the wire that's connected to the plus side of the coil is a "resistor" wire that's connected to the ignition switch.

Please explain the old style coil/resistor wire setup/theory and troubleshooting.

Specifically:
1. How do I check the resistor wire with the meter?
2. How do I check the coil with the meter?
3. Why are some coils 1.4 ohms, 2.5 ohms, etc. why the diff?
4. How do I check a plug wire/coil wire with the meter?
5. Can I replace the resistor wire with a 12v switched supply that has an external resistor inline?
6. Is it possible for the coil to work fine but when it gets hot it doesn't work right?

Thanks for the help guys!

 

[The_Jeff]

You check for about 6 volts at the + terminal of the coil when running. There's a bypass for starting so the coil gets "full power" which is about 10-11 volts with the starter draw. It would overheat if it had full power all the time.

You don't want or need the resistor after you dump the stock points ignition.

Coils do overheat. Stay away from Wells junk with tin or aluminum connections (goes without saying, really)

You could check the coil with a scope (fancy) but another good test is one of those adjustable spark gap things; should get a blue spark over at least 1/2 an inch in air to represent the spark plug gap under compression. If you suspect your coil is dying from heat you can just pull over, yank a plug wire, and hook the tester up.

Plug wires have a spec for ohms per foot; IDK if a cheap DVOM can read it or if you need a "megger".

 

[AandPDan]

The resistor is used to limit current so that the points don't burn out as fast. Its bypassed during startup, that extra connection on your starter solenoid,so that during cranking, you get the full battery voltage to the coil.

1. Isolate the wire and using your ohmmeter, connect across each side.
2. Using your ohmmeter, measure across the two smaller posts, that's the primary coil reading, and measure between the center post and -side smaller post for the secondary (high tension)side.
3. The more turns of wire the more resistance.
4. Using the ohmmeter, measure from one end of the wire to another. Wiggle the wire a bit as you do this. They can break internally.
5. Yes, you can use an external resistor. You can pick one up at a parts store. If they don't know of one, use one from an old Dodge. They are a ceramic style resistor that mounts to the firewall.
6. Yes, it's very common. When the coil gets hot it expands and the circuit opens = no spark.

As for testing the wires, if they are more than a few years old or look questionable, put in a new set.

 

[meep]

note- the resistance of plug wires is high--- like ~2kohms per foot or something like that. I was able to test with every meter I've owned, and all have been under $100.... one was a $15 ratshack special. High resistance is OK since the plug system relies on voltage, and minuscule current.

If there's a break, and you're getting the .1 or .L or whatever your meter shows as infinite resistance, the wire is arcing inside and will soon be arcing thru the insulation to the vehicle ground somewhere.

 

[expat]

As eljefino says.
You are talking about the resistor on the + side of the coil. not the capacitor under the Distributor cap (where the points go)?
On the stock set up you use a 6-7v coil and a resistor drops the cars 12v down to 6-7v EXCEPT when starting.

I think petronix want you to use a 12v coil and NO resistor.

 

[Chris142]

Originally Posted By: sasilverbullet
I have replaced the points with a Pertronix II unit.
I'd rip that junk out and put the points back in.

I've had 2 Pertronix ignitions fail on 2 different trucks. The 2nd one caught on fire, burned my distributer,Cap, Rotor, plug wires and also burned the paint off my hood!

I went back to points. Much more reliable and simple to fix.

 

[sasilverbullet]

Originally Posted By: Chris142
Originally Posted By: sasilverbullet
I have replaced the points with a Pertronix II unit.
I'd rip that junk out and put the points back in.

I've had 2 Pertronix ignitions fail on 2 different trucks. The 2nd one caught on fire, burned my distributer,Cap, Rotor, plug wires and also burned the paint off my hood!

I went back to points. Much more reliable and simple to fix.


wow! really! I've never heard anything bad about them. Will keep that in mind. I still have the old points.

 

[SS1970chrysler]

I've recently installed an Ignitor 1 system on my 70 newport. I ran 12v from the in-side of the resistor to the Pertronix unit and to the new 12v coil I installed. 2 separate wires from same point.
My car uses a separate resistor instead of an internal one.
The only issue I've seen with these systems yet is that the module can melt/catch fire if the key is left in the "on" position too long without the engine running.

 

[Chris142]

Mine caught on fire just driving down the road. Engine just quit then I saw smoke then bubbling paint on my hood.

 

[sasilverbullet]

I just got off the phone with Pertronix. I have the Pertonix II which has automatic cutoff circuitry if left on. The guy on the phone said they had problems with the first generation.

Is that what you guys are running?

 

[OVERKILL]

No, I was running a similar setup to the one I posted earlier with the stand-alone ignition box.

Running an MSD 6A on my Town Car.

 

[mechtech2]

I don't think you want or need any resistance to the coil with your new system. You want full power.

But check with your Pertronix guy.

 

[tom slick]

Originally Posted By: mechtech2
I don't think you want or need any resistance to the coil with your new system. You want full power.

But check with your Pertronix guy.


no need for the resistor with no points.

 

[sasilverbullet]

yep, talked to them again last night.

Yesterday I bought a MTD coil with .6 ohm resistance and 45Kv output. I'm going to hook that straight to switched +12v with no resistor.

I am also going to put in a new distributor with the new coil.

Might get to it tonight, or maybe tomorrow. I'll let you guys know how it turns out.

 

[mechtech2]

You may want to open up your stock plug gap .005" or so, to take full advantage of the nice fat spark you now have.

 

[sasilverbullet]

I put the rebuilt OEM distributor in along with a new MSD coil.

Car runs puuuuuuuurrrrrrrrrrrrfect!

Thanks guys for all the hints!