GMC 1976 ZEO 6083 Maintenance Manual page 410

6Y- 3 0 ENGINE ELECTRICAL
higher secondary voltage output throughout the
speed range. The primary current from the ignition
switch passes through a resistance wire which lowers
the voltage to approximately 8 volts. This lower volt-
age provides for longer contact life .
For optimum starting performance, the resist-
ance is bypassed during cranking, thereby connect-
ing the ignition coil directly to the battery. This
provides full battery voltage at the coil and keeps
ignition voltage as high as possible during cranking .
The resistance is bypassed automatically through the
ignition and starting switch when the switch is in the
"Start" position .
The secondary ignition cables in the secondary or
high tension system (coil to distributor and distribu-
tor to plugs) are resistant to grease, battery acid and
road salt, and offers resistance to corona breakdown.
Ignition cables have a multiple cloth thread core
impregnated with a graphite solution to give the cor-
rect conductivity .
The spark plugs used are a resistor type plug . The
plugs have a type number on the insulator which
designates thread size as well as relative position of
the plug in the heat range. The last digit of the type
number indicates the heat range position of the plug.
The higher the number the hotter the plug . Spark
plugs should be replaced at least every 12 months or
12,000 miles depending on driving conditions with
unleaded fuels or at 6 months or 6,000 miles with
leaded fuels.
THEORY OF OPERATION
The basic ignition system consists of the ignition
coil, condenser, ignition distributor, ignition switch,
low and high tension wiring, spark plugs, and a
source of electrical energy (battery or generator) .
The ignition system has the function of producing
high voltage surges and directing them to the spark
plugs in the engine cylinders. The sparks must be
timed to appear at the plugs at the correct instant
near the end of the compression stroke with relation
to piston position . The spark ignites the fuel-air mix-
ture under compression so that the power stroke
follows in the engine .
There are two separate circuits through the igni-
tion system . One of these is the primary circuit which
includes the ignition switch, primary winding of the
ignition coil, distributor contact points and con-
denser . The other is the secondary or high tension
circuit which includes the secondary winding of the
ignition coil, the high tension lead, distributor cap,
rotor and spark plugs.
The basic operation is described as follows: With
the switch closed, current flows through the primary
circuit, that is from the battery through the primary
winding of the ignition coil and closed distributor
contacts to ground, and then back to the battery. A
cam mounted on the rotating distributor shaft causes
the distributor contacts to open and close. When the
contacts open, the current decreases very rapidly in
the ignition coil primary winding, and a high voltage
is induced in the coil secondary winding.
This high voltage is impressed through the dis-
tributor cap and rotor across one of the spark plugs.
As the voltage establishes an arc across the spark
plug electrodes, the air-fuel mixture in the cylinder
is ignited to provide the power stroke .
The secondary electrons flow from the coil sec-
ondary winding, across the distributor rotor gap and
spark plug gap, and then back to the secondary wind
ing through ground, the battery and switch . The
distributor contacts then reclose, and the cycle re-
peats . The next-firing spark plug then will be the one
connected to the distributor cap insert that is aligned
with the rotor when the contacts separate . With the
engine running, current flows through the coil pri-
mary calibrated resistance wire ; the other lead con-
nected between the coil and solenoid terminal is a
by-pass feature that will be covered in the section
entitled "Ignition Coils" .
When the contacts separate, a high voltage is
induced in the coil primary winding. This voltage
may be as high as 250 volts, which causes an arc to
form across the distributor contacts . To bring the
primary current to a quick controlled stop, and in
order to greatly reduce the size of the arc and thereby
insure prolonged contact point life, a capacitor (con-
denser) is connected across the distributor contacts .
DISTRIBUTOR
The distributor has three jobs . First, it opens and
closes the low tension circuit between the source of
electrical energy and the ignition coil so that the
primary winding is supplied with intermittent surges
of current. Each surge of current builds up a mag-
netic field in the coil . The distributor then opens its
circuit so that the magnetic field will collapse and
cause the coil to produce a high voltage surge. The
second job that the distributor has is to time these
surges with regard to the engine requirements . This
is accomplished by the centrifugal and vacuum ad-
vance mechanisms . Third, the distributor directs the
high voltage surge through the distributor rotor, cap
and high tension wiring to spark plug which is ready
to fire .
The typical contact point type ignition distribu-
tor consists of a housing, shaft, centrifugal advance
assembly, vacuum advance assembly, breaker plate
assembly, capacitor or condenser, and rotor.