1977 F100, I-6 4.9L, working my way through piece by piece. Was tracing down wiring with the engine off in the "run" position (so I had voltage). Noticed that the ballast wire (the one from the key cylinder to the coil, the pink guy) was warm. Not hot, not burning up, but it was 95 degrees outside and I noticed it was warm to the touch.

OK, in the "run" position it is supplying voltage to the coil and dropping about 6 volts across it (12v battery to 6 volt coil), so I can see it getting warm. But after I start the vehicle, isn't the voltage to the coil fed from somewhere else? Shouldn't the wire eventually cool down? When I started the truck and ran it for a while, the wire stayed warm.

Is this normal or do I have a problem??

 

Coil is fed full 12v in start position with resistor wire bypassed. When in run position, coil is fed thru resistor wire. Your indications are normal.

 

Just as a sanity check, check the resistance between the two small terminals on the coil (BATT and TACH TEST). It shouldn't be less than an ohm. My interpretation of "warm" may be different than yours.

 

That is basically the nature of a resistor to give up the voltage as heat dropping down to 6volts. You need to test it as stated above to make sure it doesn't need to be replaced. Old resistors used to be a ceramic block on the firewall, I remember those getting warm in the vehicles that I had that usd them.

 

There are three possible resistor wires, the only difference is the length and the color code.

COLF-12250-A .. Resistor Wire-Ignition Coil (Motorcraft DY-37).

61.49" long / Color coded pink / 1.30-1.40 ohms resistance / #16 guage wire.
---------------------------------------------------------------------------
D1AZ-12250-A Resistor Wire-Ignition Coil (Motorcraft DY-84A).

60.00" long / Color coded red w/green stripes / 1.30-1.40 ohms resistance / #16 guage wire.
-------------------------------------------------------------------------------------------
D7AZ-12250-A .. Resistor Wire-Ignition Coil (Motorcraft DY-213).

59" long / color coded red / 1.30-1.40 ohms resistance / #16 gauge wire.

 

The test I described is not to test the resistor wire; it's to test the coil primary winding resistance. If the coil is going failed-short, it will draw excess current, which will dissipate more heat than normal across the length of the resistor wire. That is to say, a bad coil will make the resistor wire get too hot because it's downstream in the circuit.

 

Fmc400: understood, I'll check it this morning. Thanks for the tip.

 

Primary coil ohms out to 1.5. I'm guessing the heating I felt in the ballast cable is normal. FWIW, I'm not sure I understand putting a 6V coil in a 12v system, but Ford didn't ask me.

 

The reason for having a sub-12 volt ignition system is because when you turn the key to START to crank the engine, the battery voltage drops below 12 volts as the starter pulls massive current. So the ignition system has to be designed to run on this lower voltage, in order to fire up.

The problem is that once the key goes back to RUN and the engine is running normally, the voltage can go past 14 volts at idle. Since the ignition system is designed to run at the lower voltage, the ballast resistor drops the excess voltage, so that the voltage at the coil in RUN remains at what it would be in START.

The ballast resistor is bypassed when the key is turned to START. So the voltage at the coil remains consistent - in START it's below 12 volts because the battery voltage is low while the engine is cranking; in RUN it's below 12 volts because the ballast is in the circuit.