GMC 4000 Series Service Manual page 419

Sec. 6Y
Page 408
IGNITION SYSTEM
Low Insulation Resistance. This condition per­
mits leakage which prevents condenser from hold­
ing its charge. A condenser with low insulation
resistance is said to be weak.
High Series Resistance. This is excessive re­
sistance in condenser circuit due to broken strands
in condenser lead or to defective connections. This
will cause burned contact points and ignition fail­
ure upon initial start and at high speeds.
Capacity. Capacity is built into a condenser
and is determined by the area of the metallic ele­
ments, and the insulating and impregnating mater­
ials. A condenser of incorrect capacity will result
in point pitting.
IGNITION COIL
On In-line engine models, the ignition coil is
mounted on side of cylinder block and on V-6 en­
gine models, the ignition coil is mounted on top of
block near front of distributor. Ignition coil prim ­
ary terminals are marked positive (+) and negative
(-).
Wire from distributor primary wire (black)
must be connected to negative (-) terminal of coil.
The light green feed wire from starter solenoid
and the special resistance wire must be connected
to positive (+) terminal of coil.
IGNITION COIL TEST
If there is any doubt as to the condition of the
coil, it should be tested with a conventional coil
tester, following instructions furnished by the
manufacturer of the testing equipment. Defects
indicated by the test are:
1. Weak coil.
2. Open primary circuit.
GENERAL INFORM ATION
Spark plug life is governed to a large extent
by operating conditions, and plug life varies ac­
cordingly. To insure peak performance, spark
plugs should be checked, cleaned, and regapped
every 5,000 m iles.
Worn and dirty plugs may give satisfactory
operation at idling speed, but under operating con­
ditions they frequently fail. Faulty plugs are evi­
dent in a number of ways such as wasting gas,
power loss, lo ss of speed, hard starting, and gen­
eral poor engine performance.
Spark plug failure, in addition to normal wear,
may be due to dirty or leaded plugs, excessive
gap, or broken insulator.
Dirty or leaded plugs may be evident by black
carbon deposits, or red, brown, yellow or blistered
G M C S E R V I C E M A N U A L
If any of the above conditions are evident, coil
must be replaced.
test points of a 110-volt test lamp to both primary
term inals of coil. If test lamp does not light, the
primary circuit is 'open.
high tension terminal and the other test point to
one of the primary term inals. If secondary circuit
is not open, the lamp will not light but tiny sparks
will appear at test points when they are rubbed
over term inals. If secondary circuit is open, no
sparks will occur.
and touch the other point to the primary and high
tension terminals. If the lamp lights, or if tiny
sparks appear at point of contact, the coil windings
are grounded.
be replaced. It is unnecessary to test such a coil
with a coil test instrument.
test for short circuits and other internal defects
should be made using a reliable coil test instru­
ment, following instructions furnished with the in­
strument. The instrument must be frequently
checked to make certain that it is accurately cal­
ibrated.
must be at normal operating temperature since
internal defects often fail to show up on a cold coil
test.
SPARK
PLUGS
oxide deposits on the plugs. The black deposits are
usually the result of slow speed driving and short
runs where sufficient engine operating temperature
'is seldom reached. Worn piston rings, faulty igni­
tion, over-rich fuel mixture and spark plugs which
are too "cold" will also result in carbon deposits,
red, brown, or yellow oxide deposits, a consequence
of the combustion of leaded fuel, usually result in
spark plug failure under severe operating condi­
tions.
operation as long as they remain in a powdery
state. But, under high speed or hard pull, the pow­
der oxide deposits melt and form a heavy glaze
coating on the insulator which, when hot, acts as
a good electrical conductor, allowing current to
follow the deposits and short out the plug.
3. Open secondary circuit.
4. High voltage breakdown in secondary circuit.
5. Shorted turns in primary or secondary.
6. High resistance in primary connections.
Before using a coil test instrument, connect
Apply one te'st point of the test lamp to the
Apply one test point of test lamp to coil case
A coil with open or grounded windings must
If coil windings are not open or grounded, a
NOTE: When using a coil test instrument, coil
The oxides have no adverse affect on plug
E xcessive gap wear on plugs of low mileage