GMC 1976 ZEO 6083 Maintenance Manual page 415

NOTE : The instrument panel warning lights are
fed from the ignition terminal of the ignition
switch and have battery voltage applied to them
when the ignition switch is in the START and
RUN position . These circuits are explained in
Chassis Electrical, Section 12 .
When the ignition switch is released from the
START to the RUN position, the IGN. terminal is
still connected to the BAT. terminal, but the solenoid
is no longer energized and so the feed for the coil
from the IGN. terminal on the ignition switch,
through the ignition resistor wire and to the coil,
dropping the battery voltage at the coil to approxi-
mately nine volts. With the ignition switch in the
RUN position, the BAT . terminal is connected to the
IGN. terminal and the ACC. terminal . This permits
operation of all accessories and the ignition system .
TROUBLE DIAGNOSIS
IGNITION SYSTEM
A. Engine Will Not Start But Cranks O.K.
1 . Disconnect a spark plug wire and hold 1/4"
away from the engine block, then crank engine .
a. If strong spark is seen, check timing . Ad-
just as necessary. If timing is correct, trouble is not
in ignition system .
b. If no spark or an intermittent spark is seen,
reconnect plug wire and proceed to step 2.
2 . Disconnect distributor cap-to-coil lead from
coil and place screwdriver blade across coil tower to
engine block and crank engine .
a. If strong spark is seen between coil tower
and metal bar, check distributor cap and rotor for
cracks or carbon tracking . Check lead between dis
tributor and coil for broken or burned terminals or
cracks in insulation . Replace defective parts.
b. If no spark or intermittent spark is seen,
proceed to step 3.
3. Connect jumper wire from battery plus (+)
terminal to coil plus (+) terminal . Place a screw-
driver blade across coil tower to engine block and
crank engine .
a . If strong spark is seen, remove jumper wire
and check wiring connections and switches between
battery plus (+) terminal and coil (+) terminal .
Opens, high resistance or intermittent contact will
require repair or replacement.
ENGINE ELECTRICAL
6Y- 3 5
b. If no spark or intermittent spark is seen,
remove jumper wire and proceed to step 4.
4. Disconnect distributor lead from coil minus
-) terminal and connect test light from coil minus
(-) terminal to engine block. Turn ignition switch
to crank position .
a. If lamp does not light, replace coil .
b. If lamp lights proceed to step 5.
5 . Connect test light from battery plus (+) termi-
nal to distributor lead which is still detached from
the coil . If necessary, rotate distributor until points
close.
a. If lamp lights, check condenser and points .
Replace defective parts.
b. If lamp does not light, proceed to step 6.
6. Connect test lamp from battery plus (+) ter-
minal to connection of distributor lead and contact
points . Make sure points are closed .
coil .
a. If lamp lights, replace distributor lead to
b. If lamp does not light, proceed to step 7 .
7. Connect test lamp from battery plus (+) ter-
minal to screw holding points in place.
a. If lamp lights, replace points and check
capacitor.
b. If lamp does not light, breaker plate or
distributor is not grounded . Check plate-to-distribu-
tor ground wire or distributor-to-engine block con-
nector .
B. Engine Starts But Will Not Continue to Run
1 . Connect jumper wire from battery plus (+)
terminal to ignition coil plus (+) terminal and start
engine .
a. If engine does not continue to run, problem
is not ignition .
b. If engine runs, proceed to step 2.
2 . Remove jumper and disconnect leads from bat-
tery plus (+) terminal and coil (+) terminal. Con-
nect ohmmeter and measure resistance between the
ends of the leads just detached . Ignition switch
should be in the run position .