I apologize to someone.

I got an instant message asking for help on a electrical problem. It involved a resistor wire. I got side tracked and my log in timed out.

Long story short, I lost it, don't know how to get it back, and don't remember who it was. Started with an F.

Let me know.

A resistor wire is used in place of a resistor. Chrysler used the ballast resistor. GM and Ford used a resistor wire. This went from the ignition switch to the positive side of the coil. The connection was actually under the dash or in the harness under the hood. When the points were closed, there was about 9 volts at the coil positive. This was bypassed when the car was turned to start postion. Starter draw drops system voltage, so the spark gets weaker. To stop this fom happening, the coil is operated at 9 volts, and at battery voltage when cranking. You now have strong spark when cranking.

If you measure the voltage at the coil+ with the key on, engine off, and the points are open, it will read battery voltage. Don't let this fool you. Start the car and coil positive won't be 9 volts. This is due to part of the time the points are open, and the voltage will be system voltage, the go to 9 when the points close. A volt meter will read and average of the two, usually around 11 volts.

Resistor wires can be used in other places for specfic reasons. When a resistor wire burns it has too much current flow. HEI did away with this, it became a function of the module.

Hey ken,
I think if you click send private message next to anyone's name, it also displays your previous messages, maybe it will be there,

Fortdoggy.

Fortdoggy ask the question!

The resistance wire to the coil is fine. My problem is a resistance wire from the dash harness starting point to the ignition switch. It is a 24 Gage brown that connects with a 12 gage orange wire at the ignition switch connector. (The orange wire terminates at the heater fuse and the other side of the fuse is a brown wire that connects at the blower / heater switch).

The resistance wire gets so hot it has melted its insulation and is trying to melt others...

What is this?

Okay, I got it. Thanks everyone.

I don't have any diagams in front of me at the moment, but try this.

Disconnect the alternator plug. The two wire plug on the side of the alternator, and see if the high amperage draw goes away. This bit of information will help. While you try that and let me know, I will do a bit of research. We will find it.

|UPDATED|5/5/2004 6:11:14 PM (AZT)|/UPDATED|

OK. I put the ignition switch in the run position, without starting the car and the wire started heating up. I started the car and the wire steadily became hotter. I pulled the alternator plug and the heat went away. The heater blower also would not run with the alternator plug removed. I found in the 1972 service manual that the resistance wire rated 10 ohm/6 watt. My wire has been cut and spliced by a previous owner. My next plan is to remove all the resistance wire (which is ruined) and replace it with a regular wire and a wire-wound resistor. If you have any other ideas let me know. I will keep you informed.

As long as you have the correct resistance, that will work fine. I just checked my wiring diagrams. Sorry it took me a bit to dig them up.

The resistor wire you have is the one going to the alternator. The fact that it quit heating with the alternator was unplugged means there is a short circuit inside the alternator. This is likely a bad diode, but could be something else internal.

The reason for the resistor is odd, but simple. If you wire the car without the resistor, everything will work fine, but the car won't shut off. This resistor wire supplies an ignition feed to the alternator to signal the ignition is on, and for the alt to start charging. Without the resistor, when the key is shut off, the current for the alternator feeds backwards up the wire, supplies the ignition system, and the car keeps running when the key is shut off. It feeds back with the resistor as well, but the current is too small for the ignition to operate, so the car shuts down. Once the engine stops, the alt is no longer producing power, so no feedback and everything shuts down.

Many other cars besides our C3s just use the light for the alterntor. The bulb is the resistor. If you wire a light bulb in place of the wire it will operate, and act as a charging system light. A dash light bulb or side marker bulb will work fine. Don't use a heaver bulb such as a backup or tail light. When the key is turned on and the alt is not turning, there is power on from the ign switch, and the alt is the ground, so the light comes on. Once charging there is equal power on both sides of the light, so no current flow, and the light goes out.

Before you replace the wire with a resistor, fix the alternator or you will have the same problem again. If you use too small of a wattage resistor, it won't handle the normal current flow, and will blow out. This won't blow out a light bulb, but the alternator won't charge properly, if at all, unless the internal short is repaired.

You may want to double check the fuse for the heater. There is a tie in, and it could have blown while checking the original problem. Where you brown/white wire attaches to the ignition switch, there is also a 12 gauge orange wire which goes to the heater fuse, then travels as a 14 gauge brown wire to the heater switch.

|UPDATED|5/11/2004 1:40:07 PM (AZT)|/UPDATED|

Very good info Ken. Just to add one electrical note, the resistor you use should not have a smaller wattage rating than the resistor wire. Higher is ok. You will not find a 10 ohm 6 watt resistor. 10 ohms at 10 watts is a common size and will work fine.

ok. I'm on it.
Thanks again.

Ken - you are the man!

I took the alternator to AutoZone to have the output checked (14 Volts)- it was good.

Replaced the trio diode - Part number is a Wells DO306, the cost is 5.99. (In stock at AutoZone)
Cleaned everything up, reinstalled and everything in the system is working again.

I have decided to keep my 10 Oh 10 watt resistor in the circuit at this time. When the key is in Run, the resistor will heat up, but once the car is running the resistor cools off - when the diode was bad, the resistor would keep heating up once the key was moved from run to start and back to run. I like having a resistor on the engine side of the firewall, instead of the resistance wire in the dash harness.

I will now be moving on to my next issue.....
Thanks again.