Current Ramping – Ignition Coil
Regardless of design configuration, the role of the ignition
coil is to multiply battery voltage into high voltage, which will provide the
necessary spark. Following Ohm’s law for the conversion of volts to amperes,
oil-filled coils generally require from 3 to 5 amperes of primary current to
produce 20,000-30,000 volts of secondary current. Modern E-core and COP
(coil-over-plug) ignitions require as much as 7 amperes of primary current in
order to produce 30,000-60,000 volts of spark output. The end result is the
coil producing enough voltage to arc across a spark plug gap of 0.035 in. to
0.060 in. u
nder cylinder pressures as high as several hundred pounds per square
inch.
Although the resistance test is not a definitive measure of
a coil’s electrical integrity, a coil should be replaced if the resistance
values don’t fall within specifications.
The diversity of modern ignition coil designs has made
ignition oscilloscope analysis more difficult because many ignition coil
waveforms deviate from the conventional norm. Consequently, oscilloscope
diagnosis shouldn’t be considered a definitive test of ignition coil condition
unless it can be compared with a known-good waveform. See Fig. 1 and Fig. 2
On the other hand, using a low-amperage current probe to
measure the current “ramp” through the primary ignition circuit is perhaps the
most definitive method of determining the electrical integrity of the coil and
the quality of the triggering action. Many defective ignition coils, for
example, will pass a resistance test, but fail a current ramp test. When
testing multiple coil systems, the current ramp gives an excellent comparison
of current flow through each coil in the ignition system and usually helps the
technician arrive at a more accurate diagnostic conclusion.
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about this training course or other Delphi training.
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