GMC 1976 ZEO 6083 Maintenance Manual page 412

6Y- 32
ENGINE ELECTRICAL
ber in the vacuum advance mechanism and this
causes the diaphragm to be moved against the spring .
This motion is transmitted by linkage to the distribu-
tor breaker assembly rotation is governed by the
amount of vacuum in the intake manifold up to the
limit imposed by the design of the vacuum advance
mechanism .
When the distributor breaker plate assembly is
rotated, the contact points are carried around the
breaker cam to an advanced position, so that the
breaker cam contacts the rubbing block and closes
and opens the points earlier in the cycle. This pro-
vides a spark advance based on the amount of
vacuum in the intake manifold . Thus, for varying
compressions in the cylinder the spark advance will
vary, permitting greater economy of engine operat-
ion. It should be recognized that the additional ad-
vance provided by vacuum control is effective in
providing additional economy only on PART-
THROTTLE operation .
At any particular engine speed there will be a
certain definite advance resulting from operation of
the centrifugal advance mechanism, plus a possible
additional advance resulting from operation of the
vacuum advance mechanism . For example, an initial
timing advance of 5 degrees, plus a centrifugal ad-
vance of 10 degrees, makes a total of 15 degrees
advance at 40 miles an hour . If the throttle is only
partly opened, an additional vacuum advance of up
to 15 degrees more may be obtained, making a total
of 30 degrees. When the throttle is wide open there
is no appreciable vacuum in the intake manifold, so
this additional advance will not be obtained . All ad-
vance then is based on engine speed alone and is
supplied by the centrifugal advance mechanism.
The vacuum advance mechanism is an economy
device which will increase fuel economy when prop-
erly used . The driver who drives with wide-open
throttle whether in low or high gear will not obtain
this additional advance with its resulting increased
fuel economy.
Cam Angle
The cam angle, often referred to as contact angle
or dwell angle, is the number of degrees of cam rota-
tion during which the distributor contact points re-
main closed . It is during this period of cam rotation
that the current in the primary winding increases.
Although the cam angle may not change, the length
of time the contacts remain closed becomes less and
less as the engine speed increases. At higher engine
speeds, the ignition coil primary current does not
reach its maximum value in the short length of time
the contacts are closed . In order to store the max-
imum amount of energy obtainable or the coil, and
consequently obtain sufficient energy to fire the plug,
it is necessary to design a breaker lever assembly that
will operate properly at high speeds . The distributor
is equipped with a special-high rate-of-break cam
and a special high speed breaker lever which is capa-
ble of following the cam shape at high speeds without
bouncing . The high rate-of-break cam separates the
contact points faster for each degree of rotation and
permits closing earlier, thus increasing cam angle.
With the special cam and breaker lever combination,
it is possible to obtain the maximum cam angle and
consequently optimum ignition performance at high
speeds .
The point opening is the maximum distance that
occurs between the separated contacts as the cam
rotates . If the cam angle is properly set, the point
opening most likely will also be according to specifi-
cations. In some cases, it may be necessary to meas-
ure point opening in addition to cam angle to insure
that the contacts are properly set. A feeler gauge on
new contacts, or a dial indicator on used contacts
may be used to measure point opening.
Ignition Condenser (Capacitor)
The capacitor consists of a roll of two layers of
thin metal foil separated by a thin sheet or sheets of
insulating material . This assembly is sealed in a
metal can with a flat spring washer providing a tight
seal .
The high voltage induced in the coil primary
causes the capacitor plates to charge when the con-
tacts first separate ; the capacitor acts initially like a
short circuit and current flows into the capacitor to
minimize arching at the contacts .
IGNITION COIL
An ignition coil is a pulse transformer that steps
up the low voltage from the battery or generator to
a voltage high enough to ionize the spark plug gap
and ignite the air-fuel mixture in the cylinder . A
typical coil is made up of a primary winding, consist-
ing of a few hundred turns of relatively large wire,
and a secondary winding, consisting of many thou-
sand turns of a very small wire . These windings are
assembled over a soft iron core and are enclosed by
a soft iron shell. This assembly is inserted into a
one-piece, steel or diecast aluminum coil case, which
is filled with oil and hermetically sealed by a coil cap
made of molded insulating material . The cap con-
tains the primary and secondary high voltage termi-
nals .
The ignition coils are hermetically sealed to pre-
vent the entrance of moisture, which would cause
coil failure. During manufacture, the coil case also is
filled with oil at a high temperature. As the oil tem-
perature decreases to more nearly match the temper-