GMC 4000 Series Service Manual page 415

Sec. 6Y
Page 404
IGNITION SYSTEM
Distributor used on In-line engines is mounted
on right side of engine and is driven by the engine
camshaft by spiral cut gears. A gasket is used be­
tween flange on distributor housing and cylinder
block. The distributor is held in place by a mount­
ing clamp and cap screw (fig. 4). Lower end of dis­
tributor shaft is tongued and engages a slot in up­
per end of oil pump drive shaft to drive the oil
pump.
Distributor used on V-6 engines is mounted on
top center of cylinder block at rear end and is
driven from the engine camshaft by spiral cut
gears. A gasket is used between flange on distrib­
utor housing and cylinder block. The distributor is
held in place by a mounting clamp and cap screw
(fig. 5). The drive gear, secured on lower end of
distributor shaft, by a rivet or roll pin, has a hex­
agonal opening in lower end which engages end of
oil pump shaft to drive the oil pump. Model number
is stamped on the distributor housing.
DISTRIBUTOR CONTACT POINTS
CLEANING
Dirty contact points should be dressed with a
few strokes of a clean, fine-cut contact file. The
file should not be used on other metals and should
be kept free of grease and dirt. Never use emery
cloth to clean contact points. Do not attempt to re­
move all roughness or dress point surfaces down
smooth; merely remove scale or dirt.
Replace contact points which are badly pitted
or burned. High resistance or loose connection in
the condenser circuit, oil or foreign materials on
contact surfaces, improper point adjustment or
C A M LUBRICATOR;
\C '" ATTACHING SCREW
BREAKER PLATE
ATTACHING SC REW S
Figure 6 — Breaker Plate a nd Attaching Parts
G M C S E R V I C E M A N U A L
DISTRIBUTOR
C O N TAC T SET
QU ICK D ISC O N N EC T
TERMINAL
excessively high voltage may cause oxidized con­
tact points. If excessive point pitting is encountered,
check for an out-of-balance condition in the ignition
system , often caused by improper condenser ca­
pacity.
CONTACT POINT REPLACEMENT
(IN-LINE AND V-6 ENGINES)
Removal
1. Release distributor cap hold-down screw s,
remove cap and place it out of work area.
2. Remove rotor.
3. Pull primary and condenser lead wires
from contact point quick-disconnect terminal (fig.
6).
4. Remove contact set attaching screw, lift
contact point set from breaker plate.
5. Clean breaker plate of oil, smudge and dirt.
Installation
1. Carefully wipe protective film from contact
set, then place new contact set assembly on breaker
plate and install attaching screw.
NOTE: Pilot on contact set must engage match­
ing hole in breaker plate.
2. Connect primary lead and condenser lead
to terminals (fig. 6). Lead clips must be assem ­
bled "back-to-back." Push clip nearest the contact
lever down between the spring and locator; then
push the remaining clip down between the first clip
and locator. Do not push on the spring.
3. Apply a slight trace of petroleum jelly to
the breaker cam and a few drops of S.A.E. #20 oil
on top of the shaft.
4. Check and adjust points for proper align­
ment and breaker arm spring for proper tension
(fig. 7). Use an alignment tool to bend stationary
contact support if points need alignment.
NOTE: The contact point pressure must fall
within specified lim its. Weak tension will cause
chatter resulting in arcing and burning of the points
and an ignition m iss at high speed, while excess­
ive tension will cause undue wear of the contact
points, cam and rubbing block. Breaker arm spring
tension should be 19-23 ounces. The contact point
pressure should be checked with a spring gauge.
The scale should be hooked to the breaker lever
and the pull exerted at 90 degrees to the breaker
lever as shown in figure 7. The reading should be
taken just as the points separate. The pressure
can be adjusted by bending the breaker lever spring.
If pressure is excessive, it can be decreased by
pinching the spring carefully. To increase p res­
sure, the lever must be removed from the distrib­
utor so the spring can be bent away from the lever.
Avoid excessive spring distortion.
5. Set point opening to dimension listed in