DeTomaso Mailing List: April 2001, Message #638

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When the starter is cranking the engine, the battery voltage will drop and 
the voltage to the coil of a standard breaker point/coil ignition system will 
be reduced.  This will reduce the ignition energy at the spark plugs and make 
it difficult to start the engine.

The primary resistance of the standard ignition coil is designed to produce 
an adequate spark with this reduced battery voltage during engine cranking.

When the engine starts and the battery voltage rises, the same coil design 
which is adequate for starting the engine will draw too much current and 
overheat as well as tend to quickly degrade the ignition breaker points.

So two circuits are used.  The standard ignition coil gets direct battery 
voltage when the key is in the "Start" position and a resistance in the form 
of an actual ballast resistor (that wire wound thing on a white ceramic base) 
or a resistance wire (later Panteras) is inserted between the battery and the 
ignition coil when the key is in the "Run" mode.

Most electronic ignition systems operate without ballast resistors.  The Ford 
Duraspark II has been used by Ford with and without ballast resistors, 
although the later schematics show a resistor.  I am personally aware of a 
Duraspark II operating just fine for over a year without a ballast.

Be aware that later model Panteras have a resistance wire between the 
ignition switch and the coil in the "Run" mode. If this same resistance wire 
is used to power an electronic ignition, the ignition will be operating on a 
reduced input voltage.  In addition, OLD resistance wires (and ballast 
resistors) can exhibit truly exasperating intermittent problems (just like 
OLD ignition wires).  You can add a new wire to the system (Fuse 13 is a good 
source) to power an electronic ignition so the system will be operating on 
full input voltage in "Run" mode.

SOBill Taylor




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