Ignition resistor wire

From my limited understanding, the pink wire on the ignition switch on the steering wheel column is a resistor wire, designed to decrease voltage to the coil after start.

So, twelve volts to the coil to start the car, and then less voltage once the key is in the “run” position.

This was to protect the points. What would the voltage be at the coil in the run position? Thx

In a 12 volt battery system the battery is at about 12 volts when the car is off. The resistor is bypassed while cranking as the starter draw lowers voltage in the entire system to about 11 - ish volts.

After the engine starts the alternator powers the entire system at between 13.7 - 14.2 volts in order to charge the battery.

The resistor is placed inline with the coil while the car is running. Voltage at the coil therefore is maintained, again around 11 volts. If the stock coil is subjected to 14 volts it will overheat and start leaking the cooling oil which eventually leads to failure.

The points do not care about voltage. They will overheat and bend if the current is too high. This can easily happen if the key is left on for long periods of time (say a couple minutes) with the engine off but the points closed. If the points happen to be open and the key is on, nothing happens.

Nice description, Royce. I agree with you 200%; some folks say 9V at the coil, but I’ve always measured 10.5-11V.

You might measure 9 volts with the key on and the engine not running. This is not how it works and the 9 volts is not what it gets with the alternator charging.

Thx. The Mallory ignition module calls for around six volts at the coil after start. So it would need a $12 ballast resistor

Pertronix wants all 12 volts after start. So with that module, you’d need to bypass the pink resistor wire or buy their power relay for $30.

I have Mallory now but am considering a switch over to Pertronix coil and distributor.

I disagree. The Ford ignition system of the 60’s was to drop the voltage to the points to 7 volts to the coil to prolong point life. The power to the points during in the “start” phase is from the back (aft) position on the solenoid. When the car is in the start phase the aft post on the solenoid provides 12 volts to the coil and points until the key is released to the run position. At that time the power to the coil and points is through the resister wire dropping the voltage to 7 volts regardless of the engine running and the alternator charging.

A Mallory system will run on the 7 volts in my experience. The Pertronix system says it requires 12 volts, but I have installed them on the resister wire and they have run fine. The main concern for a Pertronix system is to run their matching coil. If you do not many times they Pertronix module will over heat and shut down or burn out. You must run their matching coil based on which version of the Pertronix system you purchase.


Use of a VOM will prove you are wrong about the stock system. Instead of repeating what you heard at the trailer park, why not see what it actually is?

The Pertronix unit wants 12 volts to the switching unit. You can easily wire it that way per the Pertronix instructions. It also works fine on anything above 6 volts. So for example in my GT-E it has been working since 1994 perfectly running off the resistor wire. The distributor wire from the Pertronix unit fires the MSD 6AL in my car. So the coil (an Accel oil filled one) gets 35+ volts at about 3 amps. None of this voltage goes through the Pertronix unit. Literally ignition you could weld with. My tach runs using the “Current type” MSD tach adapter wired per the instructions.

The Mallory Unilite module is quite fragile. I would put a Chrysler 1 1/2 ohm resistor(low side of the dual resistor) in line with the stock wire. Volts at the Mallory Unilite will be about 7 - 8 volts with the engine running.

If you have not purchased the Mallory Unilite yet, don’t. A Ford Duraspark or MSD distributor is far more durable and accurate. Add an MSD box and it is a very powerful and reliable system.

Thx. I replaced the Mallory 6100m module with another just to get it running, out of the shop and back in my garage.

I measure about 7.4 volts at the coil with the car running. It sounds like this is fine for the Mallory module?

I plan on replacing the coil and distributor with Pertronix parts. As noted, Pertronix says their coil needs twelve volts all the time, start and run.

If I don’t bypass the pink resistor wire, I will still have 7.4 volts in “run”. Royce - are you saying this is fine for a Pertronix coil and distributor with their ignition module? Thx

Measuring the voltage at the coil is not really easy. It is not surprising that people will have differing ideas about what is happening.

To begin with the formula for voltage drop across a resistor is Voltage drop = Current X Resistance. This gives a clue about why people read 12 volts at the coil even though there is a resistor in the line. If the points are open, or the Pertronix is open, that is they are not grounding and thereby completing the circuit, then no current is flowing through the circuit. So zero current times any resistance equals no drop of voltage. So you would read the 12 volts potential of the battery. Now when the points close the coil becomes a load and current flows through the circuit. Now you will see the voltage drop caused by the inline resistor.

Current is found by dividing voltage by resistance. So if we have 12 volts and a 1.5 ohm coil (for a total of 3 ohms in the circuit) it would pull 4 amps of current through the circuit. Now substituting those values to get voltage drop we get 4 amps x about 1.5 ohms (from the resistance wire) we get a drop of 6 volts, so the voltage measured at the coil would be 6 volts. So when the engine is running and the alternator is charging properly you will see something like 14.4 volts. The increase in voltage will result in an increase in the voltage drop to about 7 volts, giving you about 7.4 volts at the coil. Now if the coil was 3 ohms by itself, (bringing circuit total resistance to 4.5 ohms) then the voltage drop would only be 4 volts across the 1.5 ohm resistor wire. So you would measure 8 volts at the coil in the 12 volt example and a little over 9 volts in a 14 volt example. There are actually other series resistances in the circuit so the exact numbers will vary from car to car.

To further complicate things modern digital volt meters average a reading over time, it is call the integration time and the idea is to show the average voltage under the curve if there is one. In the case of the ignition circuit the voltage is constantly bouncing. Different meters will read this slightly differently. In this case an oscilloscope is the best tool to see what is really there.

Would the spark plug gap be the same between the petronix and a points based system ?


I still use my old MATCO analog meter. It was actually made by Simpson for MATCO. It has a dwell meter and an amp meter shunt attachment that is great for measuring starter amps while cranking. I don’t know of a modern tool that would replace all that it does.

In my GT-E I use the Pertronix unit to replace the points in the stock Autolite C7OF 12127-F distributor. It controls an MSD 6AL which provides power to the ACCEL coil. None of the coil voltage goes through the Pertronix unit. All of the coil voltage comes from the MSD 6AL.

The system you describe is something I have zero experience with. I guess I would try wiring it the way Pertronix says to. If you have an XR-7 of course this means your tachometer won’t work.

Thx Royce. It’s obvious to me I don’t understand how ignition works.

My limited understanding was the battery/alternator supply voltage to the coil, which delivers voltage to the distributor which sends power to the plugs for spark via the rotor cap.

It sounds very different in your set up. I need to do some research on this MSD 6AL.

are you available for a phone call? Thx

Bill as usual is right on. Gosh Royce, My trailer park buddies must be smarter than yours. You started it big guy! No load 12 volts. Load will make it drop. Get your VOM out and check her out big guy! Guess I better log in every day to see comments. Time to change the tarp on my double wide!!! She be leaking!

The Mallory Unilite is far from fragile. I have run multiple Unilites on customer cars and not one has had a problem. Again running on the stock resister wire when loaded is around 7 volts. Coupled them to MSD 6ALs and such with no problems. I also have a Tach Adaptor on my car and the stock tach works just fine.

As far as plug cap, you can experiment opening the gap to .040 or .045. If your car has high compression and you use it on the track and/or spin it fast, stay at .035 even with a MSD box. The larger spark gap tends to get “extinguished” at high RPM in high compression engines.


Does any of this, including the quality of today’s fuel, change the stock spec from 1970 for timing at 10 degrees BTDC? Thx

This is a great chance to let Cougar members judge for themselves. I have posted these thoughts on many Ford sites. Todays gasoline is a very slow burning blend. The octane numbers (87,89, &91) do not represent a direct coalition to the octane numbers of the late 1960’s. Where premium was 99 and Chevron white pump custom supreme was rated at 101 octane. The slow burning gas blends of today react very closely to the high lead gas blends of the 1960’s in a high compression engine. I have ran a 12 to 1 extreme build solid roller cammed Weber inducted 289 on todays slow burn gases at 38 degrees of total advance. No pinging and great power. Todays slow burn gasses rated at 91 or 92 octane run great with old school high compression engines and anyone who tells you to reduce compression will help you build a dog motor. Anything lower than 10 to 1 will produce very poor power.

To address the initial timing question, I run as much as 12 to 16 degrees initial timing on all my Ford builds. Now this must be coupled to addressing the vacuum advance curve of a mild build. You need to slow down the vacuum advance and reduce its total. This can be done with springs and shims in a factory style vacuum advance or the use of a newer adjustable advance. To tune out low load pinging under partial throttle operation.

I submit this telling article from the early 2000’s. It was the results of multiple dyno pulls on todays gas and todays gas with octane booster. Also it addresses similar blends prevalent in the late 60’s. it supports that the earlier blends were so much more “explosive” that they had to actually reduce total timing in this Mopar 360 engine build. It display facts that I have experienced in many of my engine builds.

I am currently building a 53 CC Twisted Wedge combustion chamber, flat top 302, with a roller cam. It will run just fine on 91 octane gas with 12 to 16 degrees of total advance and 38 total. Fine tuning the vacuum advance total and start point. Also cam selection can greatly affect pinging as it will blead off static low RPM compression.

Please digest this article to alter your thoughts on todays gas and high compression engines. Yes, old school engines. It is hugely revealing! And don’t get me started on hardened valve seat! What a bunch of baloney!


The voltage with the vehicle running is 6.5-8V. Not a question of if. It is. The Pertronix II, III will NOT run on the voltage through the resistor wire. The Pertronix I will. There is a trade-off. The erratic voltage destroys the ignition module. I always see dunderheads commenting about how the Pertronix units fail. Then find out they never pulled a 12V constant and run it directly. This is absolutely crucial with voltage sensing factory tachometers.
I had to do this on my 67 Shelby and it has to be before it passes through the tach.

Thx Keith.
Pertronix does sell a “power relay” for this situation, supplying a constant 12 volts to the coil.
Have you used this part?

I cannot speak to a factory tach, but the Pertronix will run on the resister wire as stated. But it does need the Pertronix matching coil. I will post that part # tomorrow. Keith, please give some detail on the way you wired your car. I have a 68 Shelby GT-350 customer car in my garage as we speak. The owner wants me to upgrade to a Pertonix system for his car. Please provide the Pertronix parts you purchased and the hot wire source you used and where to hook it up. We all learn when you learn!! This is wonderful that you chimed in!!