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Most 12v auto ignition systems use a ignition coil that has no internal resistor, the resistance must be provided for it externally.

If an ignition coil has no resistance in it's ''feed'' wire, overheating of the coil will occur. I've witnessed a coil explosion that had been ''hotwired'' from batt to coil directly . The oil used inside the coil who's purpose is to cool the coil, became overheated as the engine was not running.

With some auto manufacturers, the ballast resistor resembles a spring in a porcelain block normally mounted on the outer firewall area . Ford used a ballast resistor wire between the ignition switch and the firewall. This special pink wire is of a different texture than any wire in the loom and can be easily recognized by it's non-glossy finish. It is not bundled and taped with the other wires behind the instrument cluster. Look for the black woven sheath that covers the majority of its length.

Because resistance causes heat in an electrical circuit, this wire can get pretty hot. Be certain to inspect the routing of this special wire! Always notice windshield wiper linkage ''swath'' area, to avoid damage to wires by incorrect routing of loom.

Rarely this wire can develop an intermittent open as the wire heats up. With an open circuit, electricity can no longer reach the coil, and your engine stops running!

In the ''start mode'', when the solenoid is engaged, the resistor is by-passed and the coil receives the full complement of power.

Ignition switches are known to develop malfunctions, as the bakelite portion of the switch which is staked onto the switches aluminum housing, loosens up. Re-staking the switch will often cure your problems.

Ascertain the presence of ''pigtail'' lead from ign switch that feeds the resister wire to coil. This is the point to plug-in the factory tachometer ( rally pac and other Ford ). Factory tachometers are wired in series, most after market tachs are wired in parallel at the coil.

Remove instrument cluster, locate pink resistor wire. Check with a test light at either the coil positive terminal or the starter solenoid small terminal ( brown wire).

Go see Dave at Shasta Mustang Supply for advice or perhaps a $5.00 wire schematic.