gasket too much, the shell may become distorted and
a plug in compressed air at room temperature (which is
damage the insulator.
the condition in the testing unit) and firing a plug in an
operating engine. In the operating engine, voltages are
ELECTRONIC IGNITION SYSTEMS.--As
higher, air-fuel mixtures are present, and the high
more modern and versatile SE is introduced into the
temperatures in the cylinders aid ignition. If these
fleet, electronic ignitions used in units employing
factors are kept in mind, spark plugs can be
gasoline engines have become more common. Most
technicians will agree that the conventional (contact
Do not discard plugs that show a bluish light just
point/condenser) ignition system was the weakest
above the shell while they are being tested. This is a
link in the normally aspirated gasoline engine's
corona discharge caused by the presence of a
dependability chain. And although transistorized
high-tension field. It does not affect plug performance.
ignition has been around for years, it has normally been
Make sure the insulator is clean and dry before testing.
limited to high-performance automobiles and civilian
If a spark comes through the insulator to the plug shell,
equipment. But the dependability of the electronic
it is an indication that the insulator is cracked.
ignition has been proven, and it is becoming more
Sometimes this crack is hidden inside the shell where it
common in support equipment.
cannot be seen. In many cases, movement of the
Because conventional contact points burn easily
insulator can be detected by hand pressure.
and can handle only a limited supply of high voltages,
When you replace plugs, it is important that you
there were disadvantages in the conventional system. If
select replacement plugs with the proper heat range.
a higher primary current could be used without fear of
Check the manufacturer's manual to determine the most
burning the contact surfaces, then a higher secondary
suitable spark plug heat range for the engine. In special
voltage could be induced. Early transistor ignition
cases (or when the proper plug cannot be obtained),
systems used the contact points to trigger the
check with your supervisor to determine which is the
electronics within the module. With that design, the
best plug available for the particular engine.
points were not required to carry the primary's high
current flow, but they were subjected to rubbing block
When you install spark plugs, always use new
wear, which directly effected the saturation of the coil
gaskets, if gaskets are required. A gasket performs two
and the secondary ignition timing.
important functions. It maintains a gastight seal
between the plug and its seat, and it conducts heat from
The transistor ignition system, shown in figures
the electrode tip to the engine block.
6-54 and 6-55, is used in the JG40 shop tractor. The
system is solid-state, which results in longer lasting
When you install spark plugs, use a torque wrench
plugs, less engine misfire under load, longer periods
and tighten them to the torque specified by the engine
between tune-ups, easier starting in the fleet
manufacturer. If you do not seat the gasket tightly, the
environment, and overall lower emissions. The ignition
spark plug becomes overheated. If you flatten the
system is a Chrysler Corporation design, and a
complete tester for the system is available through
Chrysler representatives. However, the theory is
relatively simple, as are the basic troubleshooting
Figure 6-55.--Distributor used with transistorized ignition
Figure 6-54.--Electronic ignition system.