GMC 4000 Series Service Manual page 412

GENERAL
The ignition system consists of the source of
power (battery or generator) ignition switch, igni­
tion coil, distributor, condenser, spark plugs, and
high and low tension wires.
A special resistance wire is used in the body
wiring harness connecting the ignition terminal of
the ignition switch to the ignition coil on all gaso­
line engine equipped vehicles except "P" models.
The resistance wire is identified on Wiring Dia­
grams in applicable "Wiring Diagrams" booklet as:
20 WHT.-OR.- & PPL. CR. TR. for V-6 gasoline
engine models and 20 WHT.-RED & BLK. Cr. TR.
for In-line gasoline engine models.
This resistance serves to increase ignition
coil efficiency and lengthen distributor point life.
If wire becomes damaged, it should be replaced
with wire of same material and length. The r e s is t­
ance value of the wire can be checked as explained
later.
On "P" models, a resistor is connected into the
ignition circuit between the ignition switch and
starter solenoid. The resistor can be checked as
explained later in this section.
IMPORTANT: Since the ignition coil is fed
through the starter solenoid during cranking, the
engine will have a tendency to start any time it is
cranked, even though the ignition switch is in the
"OFF" position. To prevent this from happening
and possibly causing serious injury, always dis­
connect the ignition primary wire from the coil
before making tests which require cranking the
engine.
NOTE: When using a remote switch to operate
the starter, disconnect primary wire from ignition
coil negative terminal and turn ignition switch "ON."
Failure to do this will result in damaged grounding
circuit in ignition switch.
RESISTANCE WIRE OR UNIT CHECK
NOTE: When replacing a resistive unit, always
use factory recommended parts.
Check resistance value of wire or unit as fol­
lows:
1. Make sure the ignition switch is "OFF."
2. Connect ohmmeter leads to each end of the
resistance wire or to each terminal of the re sist­
ance unit.
3. Resistance value of wire used on V-6 gaso­
line engine models should be 1.52 - .07 ohms. Re­
sistance value of wire or unit used on In-Line gas­
oline engine models should be 1.80 ± .05 ohms.
IMPORTANT: When making the resistance
check, be certain ignition switch is in "OFF"posi­
tion, otherwise damage to ohmmeter will result.
G M C S E R V I C E M A N U A L
ments on the ignition system at regular intervals,
causes of lowered performance will be located and
corrected before they progress to a point where
they will seriously affect engine performance.
directed previously in "BATTERY" section.
mechanism by removing distributor cap and turn­
ing the rotor in clockwise direction. The cam should
rotate freely and when released, it should return
to its original position without sticking or binding.
and clean wire sockets with a small round brush.
Inspect cap and rotor for chips, cracks, and car­
bonized short paths; look for burned metal inserts
in cap and for burned metal segment on rotor.
and roughness is normal. Slight roughness can be
removed with a few strokes of a clean, fine-cut
contact file. If points are badly pitted or burned,
replace points as directed later under "Distributor."
If dwell meter is not available, point opening can
be checked with a dial indicator. Never check open­
ing of used points with a feeler gauge. If dwell
angle (or point opening) requires adjustment, ad­
just as directed later under "Distributor."
directed later under "Ignition Timing."
ed, or oil soaked insulation. Check all connections,
making sure they are clean and tight.
cated. Refer to LUBRICATION (SEC. 0) for inter­
vals and instructions.
adjust point gap as directed later under "Spark
Plugs."
connector features a three tang lock to secure a
firm connection (fig. 1). The switch lock cylinder
and cylinder housing can be removed as follows:
REMOVAL
in "OFF" position and inserting wire in small hole
in cylinder face. Push in on wire to depress plunger
and continue to turn key counterclockwise until
lock cylinder can be pulled from cylinder housing.
IGNITION MAINTENANCE
By making the following inspection and adjust­
1. Check condition of battery and cables as
2. Check operation of centrifugal advance
3. Wipe out distributor cap with a soft cloth
4. Inspect breaker points. Slight discoloration
5. Check cam dwell angle with a dwell meter.
6. Check ignition timing with timing light as
7. Inspect all ignition wiring for brittle, crack­
8. Distributor must be kept properly lubri­
9. Remove spark plugs and clean, inspect, and
IGNITION SWITCH REPLACEMENT
The ignition switch and harness-to-switch
1. Disconnect negative cable from battery post.
2. Remove lock cylinder by positioning switch
Sec. 6Y
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