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1G-24 IGNITION SYSTEMS
Engine Spark Knock (Pre-lgnltlon) Diagnosis
Spark knock (pre-ignition) can be attributed to several
factors. The most common are ambient air conditions,
such as air temperature, density and humidity.
• High Underhood Air Temperature
Underhood air temperature is increased by the use
of air conditioning (especially during long periods of
idling), overloading (trailer pulling or operating in
too high a gear), and the installation of accessories
that restrict airflow.
• Air Density
Air density increases as barometric pressure rises or
as the air temperature decreases. A denser than
normal mixture of air and fuel drawn into the cylin
der has the same effect as increasing the engine
compression ratio and this increases the possibility
of spark knock.
•Humidity
Low humidity increases the tendency for engine
spark knock. High humidity decreases the tendency
for spark knock.
• Fuel Octane Rating
Fuels of an equivalent research octane rating may
vary in their antiknock characteristics for a given
engine. It may be necessary to retard the initial
ignition timing (not more than 2 degrees from the
specification) or select an alternate source of fuel.
•Ignition Timing
Ignition timing should be checked to ensure it is
adjusted to the specification.
NOTE: The white paint mark on the timing degree
scale identifies the specified ignition timing degrees at
idle speed, it does not indicate TDC(Top Dead Center).
• Combustion Chamber Deposits
An excessive build-up of deposits in the combustion
chamber may be caused by not using the recom
mended fuels and lubricants, prolonged engine id
ling or continuous low speed operation. These
deposits can be reduced by the occasional use of
Carburetor and Combustion Area Cleaner 8992352,
or equivalent, or by operating the engine at high
speeds.
• Distributor Ignition Advance Mechanisms
The centrifugal (mechanical) and vacuum ignition
advance mechanisms should be inspected to ensure
they are operating correctly.
• Exhaust Manifold Heat Valve
This is applicable to eight-cylinder engines only. If
the heat valve sticks in the heat ON position, the
intake manifold will be heated excessively.
TEST PROCEDURES
Primary and Secondary Circuit Electrical Tests
Refer to Diagnosis for electrical test procedures.
Distributor Ignition Advance Tests
Ceotritogil (Mechanical) Ignition Advance Test
(1) Disconnect vacuum hose from vacuum advance
mechanism and plug hose opening.
(2) Connect timing light to No. 1 spark plug and
tachometer to ignition coil "tach" terminal (fig. 1G-14).
WARNING: Use extreme caution when engine is oper
ating. Do not stand in direct line with Jan. Do not put
hands near pulleys, belts or fan. Do not vear loose
clothing.
(3) Start engine and observe timing degree scale
and vibration damper index with timing light while en
gine is idling.
(4) Slowly increase engine speed to 2000 rpm. Tim
ing should advance (increase in degrees BTDC)smoothly
as engine speed increases. Refer to Chapter
1A—General Service and Diagnosis for centrifugal (me
chanical) advance curve information.
Vacuum Ignition Advanco Test
NOTE: Engine must be warmed up to normal oper
ating temperature.
(1) Connect vacuum hose to vacuum advance
mechanism.
(2) Observe timing degree scale and vibration
damper index with timing light while engine is idling.
(3) Slowly increase engine speed to 2000 rpm. With
vacuum applied, ignition timing should advance sooner
than with centrifugal advance alone. At 2000 rpm, vac
uum advance should cause total advance to be more than
with centrifugal advance alone. Refer to Chapter
1A—General Service and Diagnosis for vacuum advance
curve information.
Electronic Ignition Retard Test
If the vehicle (six-cylinder engine only) is equipped
with electronic ignition retard and a feedback system,
refer to Chapter U-Fuel Systems for test procedure.
Ignition Coil Tests
The ignition coil can be tested on any conventional coil
tester or with an ohmmeter. A coil tester is preferable
because it can be used to detect faults that are impos
sible to detect with an ohmmeter.
Primary Winding Resistance Test
(1) Remove connector from negative (-) and positive
(+) terminals of coil.
(2) Set ohmmeter for low scale and adjust pointer to
zero.
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Troubleshooting
