Buick LeSabre 1965 Service Manual page 606

1 0 - 3 6 TROUBLE DIAGNOSIS ELECTRICAL SYSTEMS
of the throttle valve to move
above the opening.
The vacuum at the opening acts on
the control diaphragm, causing it
to compress the spring and to
rotate the breaker plate in a
counterclockwise direction. This
moves the contact points so that
the distributor cam lobes open
the points earlier in the com-
pression stroke. The amount of
throttle opening and the engine
load determine the amount of in-
take manifold vacuum and thus
the amount of spark advance ob-
tained. The advance obtained by
the vacuum control is added to the
advance obtained by the centri-
fugal a d v a n c e mechanism as
shown in Figures 10-37 through
10-39.
10-33 IGNITION SYSTEM-
TROUBLE
DIAGNOSIS
If engine trouble has arisen which
seems to be due to improper op-
eration of the ignition system, it
may be desirable to make a quick
preliminary check of the ignition
system before making a complete
analysis, in order to determine
whether the ignition system is
actually at fault. The quick checks
described in this paragraph may
be used but it must be understood
that they are no substitute for the
complete ignition system inspec-
tion. The checks to be made de-
pend on whether the engine will
or will not run.
a. Engine Will Not Run
1. Make quick check of battery
and cables (par. 10-11) if crank-
ing motor does not turn engine at
normal cranking speed.
2. Pull coil high tension cable
from distributor cap and hold the
lead terminal about 3/16" from
a clean ground point on engine.
If a good spark occurs while en-
gine is being cranked, the pri-
mary circuit and the secondary
J ^ L F U L L \
V A C U U M j E 4
PULL ROD W@|
1
«- NO
- ^ ADVANCE
mom
i
Figure 10-36—-Vacuum
Advance Mechanism
circuit to this point may be con-
sidered to be okay. Proceed with
Steps 3 through 6.
3. Remove distributor cap and
check interior for moisture, cor-
roded terminal segments, and
check terminal sockets for cor-
rosion. Check rotor for corro-
sion. Clean off corrosion and wipe
distributor cap dry. Check for a
crack or carbon path in cap or
rotor.
4. Inspect ignition cables for pos-
sible short circuits and corroded
terminals. Remove and inspect
spark plugs (par. 10-36).
5. If cause of trouble has not
been found, check approximate ig-
nition timing (par. 10-35).
6. If engine still fails to run, the
trouble is probably due to causes
other than ignition, such as lack
of fuel, carburetion, or compres-
sion loss.
7. If spark did not occur at coil
high tension lead (Step 2 above)
then connect 12-volt test lamp
between distributor terminal of
coil and ground and crank engine.
If test light flickers on and off as
engine is cranked the primary
circuit is probably okay. Check
ignition coil (par. 10-37) and con-
denser (par. 10-38).
8. If test light remains on as en-
gine is cranked, contact points
are not closing properly; check
point opening and ground con-
nection in distributor (par. 10-39).
9. If test light remains off as
engine is cranked, the primary
circuit is open or the points are
not opening properly. Check for
loose connections, broken leads,
defective switch, contact point
opening, and primary c i r c u i t
winding in coil. Visual inspection
of points and the use of a test
lamp or voltmeter will locate the
source of this trouble.
b. Engine Runs, But Not
Satisfactorily
1. When missing, loss of power,
or hard starting is present a com-
plete checkup of the ignition sys-
tem is in order, since these
conditions may result from any-
thing from a low battery to defec-
tive spark plugs, or from other
engine conditions not related to
ignition. In this case, the com-
plete inspection of ignition system
should be used.
2. Detonation may be caused by
improper timing (par. 10-35),
improper operation of centrifugal
or vacuum advance mechanism
(par. 10-32, d), worn distributor
bearings or a bent shaft, dirty or
wrong heat range spark plugs
(par. 10-36). It may also be
caused by overheating, excessive
carbon in cylinders, or by low
octane fuel.
3. Overheating may be caused by
one or more of the conditions
which contribute to detonation, as
well as by faults in engine cooling
system.
1 0 - 3 4 REPLACEMENT
AND ADJUSTMENT
OF DISTRIBUTOR
CONTACT POINT
SET
When inspection of the contact
points as described in paragraph
10-39 shows replacement to be
advisable, the following procedure
should be used.