Intro. to Ignition Feed Circuit
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
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
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 also 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.
Remove instrument cluster, locate pink resistor wire and test for voltage where
it plugs into the black pigtail lead from the ign switch. 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. Check with
a test light with the key in the on position. If power is at the switch, next step
is to check at the firewall plug connector then either the coil positive terminal
or the starter solenoid small terminal ( brown wire) in the same manor.
....or go see Dave at Shasta Mustang Supply for advice or perhaps a $5.00 wire