Wagoneer L6-258 4.2L VIN C 2-bbl (1982)
Spark Control System: Description and Operation
Spark Coolant Temperature Override (CTO) Valve
Heavy Duty Cooling System
PURPOSE
This is a single function valve used in conjunction with a heavy duty cooling system to prevent engine overheating. This valve is installed in the
same location as that used with a standard cooling system.
OPERATION
When coolant temperature is below 220° F, ported vacuum is exposed at port 1 and applies to port D to control the distributor vacuum advance.
When coolant temperature exceeds 220° F, port 1 closes and port 2 is connected to port D to allow manifold vacuum to control the distributor
vacuum advance.
With manifold vacuum applied to the vacuum advance mechanism, engine idle speed increases and engine cooling efficiency is improved.
NOTE: Some engines use both standard and heavy duty cooling system type CTO valves.
Standard Cooling System
Fig 23 CTO Valve
On engines equipped with a spark (vacuum advance) CTO valve, the distributor vacuum spark advance is controlled by the carburetor after the
engine coolant heats to a specified temperature. Driveability during warm-up is improved by controlling the vacuum spark advance by manifold
vacuum when the engine is cold. This is accomplished by the spark control system.
The CTO valve, Fig. 23, is screwed into the intake manifold coolant passage on six cylinder engines. On all V8 engines, the CTO valve is screwed
into the thermostat housing or intake manifold coolant passage. A thermal sensor on the valve is in contact with the engine coolant. The CTO
valve allows either manifold vacuum or carburetor ported vacuum to control the vacuum spark advance, according to coolant temperature.
When coolant reaches the specified temperature, the valve moves upward and blocks manifold vacuum at port 1. Carburetor ported vacuum is now
exposed at port 1 and applied to port D. Ported vacuum now controls the vacuum advance mechanism diaphragm. This is the normal operating
condition.
