GMC 1976 ZEO 6083 Maintenance Manual page 416

6Y- 36
ENGINE ELECTRICAL
a. If resistance exceeds 2.5 ohms, check wires
and connections for loose or intermittent contact.
Check by-pass resistor and ignition switch for opens .
b. If resistance is 1 .0 to 2.5 ohms, check the
output of the ignition coil .
c. If resistance is less than 1 .0 ohm, replace
shorted by-pass resistor and replace contact points .
C. Engine Runs Rough, Poor Power or Gas Mileage
1 . Check all tune-up specifications (timing, dwell,
carburetion, fouled plugs, etc.) If settings are im-
proper, correct as required .
2. If settings are O.K . check both centrifugal and
vacuum advance of distributor and correct with re-
placement parts, if necessary.
3. If distributor advance mechanisms are within
specifications, check coil available voltage and plug
required voltage.
a. High requirements or low availability of
voltage will require a replacement of parts.
b . If coil and plugs are O.K ., the problem is
not in the ignition system .
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 per-
formance 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 carbu-
retor increase the voltage requirements and lead to
many needless coil changes. Leakage of high tension
current through moisture on an unprotected coil ter-
minal 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 defec-
tive 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, us-
ing 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 termi-
nal, 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 test-
ing ignition coils . One type makes use of an open or
protected spark gap, while the other reports the con-
dition 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 connec-
tion 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 mark-
ings 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 .