Chevrolet Light Duty Truck 1973, Руководство по обслуживанию

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E N G IN E ELECTRICAL 6 Y -4 7
test show that the resistance can go to .5 ohm before
ignition performance is affected.
B. Insufficient or Excessive Capacity
The condenser specified for use in the ignition system
has a capacity of .18 to .23 microfarads.
If a condenser is used which does not have the specified
capacity of .18 to .23 microfarads, excessive pitting of
one contact point and a corresponding buildup of metal
on the other contact point will result. A condenser having
insufficient capacity will cause build-up of metal on the
breaker arm (positive) point. A condenser having
excessive capacity will cause build-up of metal on the
contact support (negative) point.
In exceptional cases, pitting and metal buildup on
contact points may be experienced even when condenser
capacity is within the specified limits. In such cases the
life of contact points will be improved by installing a
condenser of high-limit capacity if metal build-up is on
breaker arm point, or a condenser of low-limit capacity
if metal build-up is on contact support point. There is
usually sufficient variation in the capacities of stock
condensers to permit selection of a high or low limit
condenser by testing the available stock.
C. Low Insulation Resistance
A weak or leaking condenser is usually one that has
absorbed water so that the insulation resistance of the
winding is lowered to the extent that the condenser will
not hold a charge satisfactorily. A condenser with low
insulation resistance will drain sufficient energy from the
ignition system to lower the secondary voltage seriously.
The condenser specified for use in the ignition system is
sealed to prevent absorption of water, and no other type
should be used.
A leaky condenser usually does not affect engine
performance except when hot. It is unlikely that a
condenser with low insulation resistance would cause
missing at low or medium speeds under conditions where
the condenser does not get hot. A condenser that has low
enough resistance to affect engine performance when
cold would probably be indicated as broken down on
most condenser testers.
Condenser testers equipped to check condensers for low
insulation resistance usually give a reading megohms, a
megohm being one million ohms. The scale is marked to
indicate whether the condenser is good or bad.
When testing a condenser for low insulation resistance
the lead should always be disconnected from the
distributor. Since the distributor terminals and the
connected circuit have much lower insulation resistance
than the condenser, failure to disconnect the condenser
lead will give a reading much too low.
IGNITION SYSTEM RESISTANCE TEST
Check for proper functioning of the resistance in the
primary ignition circuit by turning on the ignition. With
the engine not running, a voltmeter connected from the
battery side of the coil to ground should read
approximately 5 to 5.5 volts. If the reading is a full 12
volts, the ignition points may be open; "bump" the
starter a few times until the engine comes to rest with the
ignition points closed and again check for a 5.5 volt
reading. A reading of 12 volts or over for all engine
positions would indicate that the shorting switch is
making contact all the time; this condition must be
corrected immediately or ignition point life will be very
short.
Check for proper closing of the shorting switch and also
for proper functioning of the complete starting circuit by
grounding the secondary coil wire so the engine won’t
start. With the engine cranking, a voltmeter connected
from the battery side of the coil to ground should read at
least 9 volts. A reading of under 5 bolts would indicate
that the shorting switch is not closing; this condition
would result in hard cold starting.
Briefly, the advantages of our resistance with shorting
switch system are: it sends full battery voltage to the coil
for good cold weather starting, and it cuts down the
voltage to the coil with the engine running for long
ignition point life.
NOTE: Discourage any attempts to measure
voltage at the coil with the engine running;
because of variations in current flow at high
speeds and in regulated voltage, this check
would be meaningless. Voltage readings on a
perfectly-functioning ignition system may go
over 11 volts.
SPARK PLUGS
Under normal operating conditions, spark plugs wear out
due to the destructive action, under intense heat, of
sulphur and lead compounds in the fuel and the
bombardment of the electric spark on the electrodes.
It is reasonable to expect 12,000 miles of useful life from
a spark plug which has been cleaned and regapped at
regular intervals. However, operating conditions are an
important factor and life expectany of the spark plug
will vary with the type of service in which the engine is
used.
The same type of spark plug used in two different
engines of the same make and model may frequently
show wide variation in appearance. The cause of such
differences lies in the condition of the engine, its piston
rings, carburetor setting, kind of fuel used, and under
what conditions the engine is operated, namely,
sustained high speeds or heavy loads; or continual low
speed, stop-and-go driving or light loads.
Spark Plugs are frequently blamed for faulty engine
operation which they do not cause. Replacement of old
spark plugs by new may temporarily improve poor
engine performance because of the lessened demand new
plugs make on the ignition system. This cannot
permanently cure poor engine performance caused by
worn rings or cylinders, weak coil, worn contact points,
faulty carburetion or other engine ills.
On the following pages are pictures of some commonly
encountered appearances and causes • of spark plug
problems.
LIGHT DUTY TRUCK SERVICE MANUAL