Chevrolet Light Duty Truck 1973, Service Manual

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6 Y - 4 6 EN GINE ELECTRICAL
IGNITION COIL TEST
A. Weak Coils
Most ignition coils that are replaced are classified as
weak. Many coils rejected as weak actually test up to
specifications and give normal performance. A coil that
actually is weak will first effect engine performance when
the ignition reserve is at a minimum. This may be in
starting, low speed acceleration or top speed. Eventually
the engine will fail to start.
High resistance connections in either the primary or
secondary circuit wiring will react the same as a weak
coil. Wide spark plug gaps, which require higher voltage
than the coil can produce, put the coil under suspicion.
High compression and lean carburetion increase the
voltage requirements and lead to many needless coil
changes. Leakage of high tension current through
moisture on an unprotected coil terminal may produce
carbon tracks which weaken the coil output voltage. For
this reason the nipple on coil high tension terminal must
be properly installed and in good condition.
When an ignition coil is suspected of being defective it
should be tested as described below before being
replaced.
B. Testing Coil for Open and Grounded
Circuits
Before using a coil test instrument, the coil should be
tested for open and grounded circuits, using a 110-volt
test lamp and test points.
1. Apply test points to both primary terminals of coil.
If test lamp does not light, the primary circuit is
open.
2. Apply one test point to the high tension terminal,
and the other test point to one of the primary
terminals. If secondary circuit is not open, the lamp
will not light but tiny sparks will appear at test
points when they are rubbed over terminals. If
secondary circuit is open, no sparks will occur.
3. Apply one test point to a clean spot on the metal
coil case and touch the other point to the primary
and high tension terminals. If the lamp lights, or
tiny sparks appear at the points of contact, the coil
windings are grounded.
4. A coil with open or grounded windings must be
replaced since internal repairs cannot be made. It is
unnecessary to test such a coil with instruments. If
windings are not open or grounded, a test for short
circuits and other internal defects should be made
with a reliable coil test instrument.
C. Coil Test Instruments
Two general type of instruments are used in testing
ignition coils. One type makes use of an open or
protected spark gap, while the other reports the
condition of the coil on a meter.
The spark gap type of tester should always be used
comparatively, that is, the questionable coil should be
compared with a coil of same model that is known to be
good. Both coils must be at the same temperature and
identical test leads must be used.
Certain variables caused by altitude, atmosphere or
spark gap electrode conditions are usually present in the
spark gap type of test.
The meter type testers are usually designed to permit
testing the coil without making any connection to the
secondary terminal. This eliminates the variables usually
present in the spark type of test and avoids the necessity
for comparison with a good coil.
Some different makes and models of coil testers differ in
their methods of use, as well as in the markings on
meters, the instructions of the manufacturer must be
carefully followed when using any coil tester. The
instrument must be frequently checked to make certain
that it is accurately calibrated.
Regardless of instrument or method used, the coil must
be tested at normal operating temperature because
internal defects often fail to show up on a cold test.
DISTRIBUTOR CONDENSER TEST
When a condenser is suspected of being faulty it should
be tested with a reliable condenser tester to determine
whether it is actually the cause of ignition trouble. The
condenser should be tested for (a) high series resistance
(b) insufficient or excessive capacity (c) low insulation
resistance.
A special condenser tester is required to make these tests.
When using a condenser tester the instructions of the
manufacturer must be carefully followed.
NOTE: The condenser must be at normal
operating temperature when it is being
tested.
A. High Series Resistance
High series resistance in the condenser causes the
condenser to be slow in taking the charge and,
consequently, a higher than normal voltage is developed
across the contact points when they first start to open.
The higher voltage causes more distrubance at the
contact points, which in turn causes more rapid wear and
more tendency toward oxidized surfaces. The condition
can become severe enough to cause complete failure of
the ignition system. It would first show up during starting
and low speed operation.
High series resistance may be caused by internal
resistance in condenser or by resistance in the
connections. Any defect caused by internal resistance
should show up at low mileage since this does not change
very much with time or use. The damaging changes are
in the connections, in which looseness, corrosion, or
broken strands may develop.
New condensers may have a series resistance as low as
.05 ohm. Some condenser testers are set to reject
condensers which have a resistance of .3 ohm; however,
LIGHT DUTY TRUCK SERVICE MANUAL