Dodge RAM TRUCK 1500 1995 Service Manual page 392

mm IGNITION SYSTEMS SD - I I
GROUND
ELECTRODE
CENTER
ELECTRODE
CHIPPED
INSULATOR
timing, detonation and cooling system malfunctions
can also cause spark plug overheating.
BLISTERED
WHITE OR
GRAY
COLOiEP
INSULATOR
J908D-13
Fig. 30 Chipped Electrode insulator
PREIGNITION DAMAGE
Preignition damage is usually caused by excessive
combustion chamber temperature. The center elec­
trode dissolves first and the ground electrode dis­
solves somewhat latter (Fig. 81). Insulators appear
relatively deposit free. Determine if the spark plug
has the correct heat range rating for the engine. De­
termine if ignition timing is over advanced, or if
other operating conditions are causing engine over­
heating. (The heat range rating refers to the operat­
ing temperature of a. particular type spark plug.
Spark plugs are designed to operate within specific
temperature ranges. This depends upon the thick­
ness and length of the center electrodes porcelain in­
sulator.)
GROUND
ELECTRODE
STARTING
TO DISSOLVE
CENTER
ELECTRODE
DISSOLVED
J908D-14
Fig. 31 Preignition Damage
SPARK PLUG OVERHEATING
Overheating is indicated by a white or gray center
electrode insulator that also appears blistered (Fig.
32). The increase in electrode gap will be consider­
ably in excess of 0.001 inch per 1000 miles of opera­
tion. This suggests that a plug with a cooler heat
range rating should be used. Over advanced ignition
J908D-16
Fig. 32 Spark Plug Overheating
SPAM PLUI SECONDARY CABLES
On 3.9L V-6 or 5.2/5.9L V-8 engines, spark plug
heat shields are pressed into the cylinder head to
surround each spark plug cable boot and spark plug
(Fig. 33). These shields protect the spark plug boots
from damage (due to engine heat generated by the
exhaust manifolds) and should not be removed. After
the spark plug cable has been installed, the lip of the
cable boot should have a small air gap to the top of
the heat shield (Fig. 33).
J9314-94
Fig. 33 Heat Shields—3.9L V-6 or 5.2/5.9L V-8
Engines
TESTING
Spark plug cables are sometimes referred to as sec­
ondary ignition cables or secondary wires. The cables
transfer electrical current from the distributor to in­
dividual spark plugs at each cylinder. The spark plug
cables are of nonmetallic construction and have a
built in resistance. The cables provide suppression of
radio frequency emissions from the ignition system.