Good Bill, but it’s actually not that “the EI compensates and reduces the
voltage to the primary side of the coil”. It controls the current during
dwell time to guarantee the proper energy storage in the coil’s magnetic
field. Remember the energy in the coil is 1/2 times its inductance times the
current squared. Directly analogous to 1/2 mass times velocity squared, etc.
Storing “constant energy” regardless of rpm, is what EI modules do. I really
suggest getting the design data that folks at GM, etc. use to implement an EI
system. As I’ve long said, I happen to have one of Motorola’s, that someone
else passed to me, and anyone else is welcome to. It may be eye opening just
to see how much circuitry is inside the modules and see the functions
performed by the different circuits within them.
Also, remember that the ballast was added to protect points & coil from
overheating when the ignition was left on, but the engine had stopped with
points closed. And we know that some Hall-Effect & other EI pickups can do
similar, just having only the coil to overheat. For our case, there’s no
possibility of this because our EIs depend on the rotor turning.–
79xj6L SII (BRG + wires)
86xj6 SIII (Black)
61 Sprite MkII (Red)
Menlo Park, Calif.
Wild Bill wrote:
In reply to a message from Frank Andersen sent Tue 19 Feb 2008:
If I could throw in my $.02 on the ballast resistor discussion:
A short history of the balast resistor.
In the old days of point type ignitions and carburetors with
chokes to start cold engines, manufacturers found that they
needed a really hot spark to fire a cold engine with a
really rich mixture turning slowly with the starter. On top
of this, the starter was drawing a lot of current and
dropping the voltage to the coil (Note how your lights dim
when you crank your engine over) making the spark even
weaker. So, they used a coil with a very low primary
winding so they got a really strong spark. Problem was,
this really strong spark wreaked havok on the rotor and
distributor cap and the low primary winding on the coil made
a really big spark when the points contacted and seperated,
burning out points rather quickly.
So, they used these low primary winding coils to make
engines start better, but put a ballast resistor in series
with the primary winding to make them play nice with the
points, cap, and rotor. Then in order to get the hot spark
they needed to start the car, they ran a second wire to the
primary winding on thhe coil, usually from the starter relay.
So, when you turn the key to start and crank the engine, the
starter solenoid sends the full 12V to the coil and you get
a hot spark to start the engine. Once the engine starts and
the starter is disengaged, the 12V to the coil is attenuated
through the ballast resistor and the current flow is reduced
to save the points, cap & rotor.
In a car with electronic ignition, you may or may not have a
ballast resistor depending in what the manufacturer designs.
As a general rule of thumb, bypassing the ballast resistor
will give you a stronger spark at the expense of more rapid
cap & rotor wear unless the EI compensates and reduces the
voltage to the primary side of the coil. Whether the Jag’s
module actually does this, I don’t know.
The archives and FAQ will answer many queries on the XJ series…
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