Engine Spark Knock (Ping) Diagnosis; Distributor Advance Tests; Coil Tests - Jeep 1979 CJ-5 Technical & Service Manual

10-14
IGNITION SYSTEM
TESTING .
Electrical Tests
Refer to Troubleshooting for test procedures.
Distributor livance Tests
Centrifugal Advance
(1) Disconnect vacuum line from vacuum advance
unit and plug.
(2) Connect timing light and tachometer.
(3) Start engine and observe timing mark while en-
gine is idling.
(4) Slowly increase engine speed to 2000 rpm. Tim-
ing should advance smoothly as engine speed increases.
Refer to Chapter 1A—General Service and Diagnosis for
advance curve information.
Vacuum Advance
NOTE: Engine must be warmed up to operating
temperature.
(1) Connect vacuum line to vacuum advance unit.
(2) Observe timing mark while engine is idling.
(3) Slowly increase engine speed to 2000 rpm. With
vacuum applied, timing should advance more than with
centrifugal advance alone. At 2000 rpm, vacuum ad-
vance should cause total advance to be higher than cen-
trifugal advance alone. Refer to Chapter 1A—General
Service and Diagnosis for advance curve information.
Coll Tests
The coil can be tested on any conventional coil tester
or with an ohmmeter. A coil tester is preferable as it will
detect faults that an ohmmeter will not.
Primary Resistance Test
(1) Remove connector from negative and positive
terminals of coil.
(2) Set ohmmeter to low scale and calibrate to zero.
(3) Connect ohmmeter to negative and positive ter-
minals. Resistance should read 1.13 to 1.23 ohms at
75°F. At temperatures above 200°F, 1.50 ohms is
acceptable.
Secondary Resistance Test
(1) Remove ignition wire from center terminal of
coil.
NOTE: Ignition must be off.
(2) Set ohmmeter to 1000 scale and calibrate to zero.
(3) Connect ohmmeter to brass contact in center
terminal and to either primary terminal. Resistance
should read 7700 to 9300 ohms at 75°F. A maximum of
12,000 ohms is acceptable if coil temperature is 200°F or
more.
Current Flow Test
(1) Remove connector from coil.
(2) Depress plastic barb and withdraw positive wire
from connector. Barb is visible from coil side of
connector.
Eigne Spark Knock (Ping) Piaposis
Spark knock can be attributed to a number of factors.
The most common are climatic factors such as temper-
ature, air density and humidity.
• High Underhood Temperature
Underhood temperature is increased by the use of
air conditioning (especially during long periods of
idling), overloading (trailer pulling, 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 temperature drops. A denser than normal
mixture of air and fuel drawn into the cylinder has
the same effect as raising the the compression
ratio. This increases the possibility of spark knock.
• Humidity
Low humidity increases the tendency to spark
knock. High humidity decreases spark knock.
• Fuel Octane Rating
Fuels of equivalent research octane rating may
vary in their knocking characteristics in a given
engine. It may be necessary to reduce initial tim-
ing (not more than 2 degrees from specifications)
or select an alternate source of fuel.
• Ignition Timing
Ignition timing should be checked to be sure it is
set within specifications.
NOTE: The white paint mark on the timing degree
scale represents the specified spark setting at idle speed,
not TDC (Top Dead Center).
• Combustion Chamber Deposits
An excessive build-up of deposits in the com-
bustion chamber may be caused by not using rec-
ommended fuels and lubricants, prolonged engine
idling or continuous low speed operation. These
deposits may be reduced by the occasional use of
Carburetor and Combustion Area Cleaner or its
equivalent, or by operating the vehicle at turnpike
speeds.
• Distributor Advance Mechanism
The centrifugal and vacuum advance units should
be checked to be sure they are operating freely.
• Exhaust Manifold Heat Valve
If the heat valve sticks in the heat ON position, the
intake manifold is heated excessively.