140 EMISSION CONTROLS AND FUEL SYSTEM
This relay closes to complete the TCS circuit
and open the TCS solenoid, allowing vacuum
advance, for 20 seconds after the key is turned
to the "on" position. The operating tempera-
ture of the temperature override switch has
been raised to 93°, and the switch that was
used to engage the A/C compressor when the
key was turned "off' has been eliminated. All
models are equipped with an electric throttle
control solenoid to prevent run-on. The 1974
TCS system is used only on manual transmis-
sion models. System components remain un-
changed from 1973. The vacuum advance so-
lenoid is located on the coil bracket.
All 1973-74 Corvette models are equipped
with a Thermo-Override system instead of the
normal TCS system. This system consists of a
three-position temperature switch, which is
mounted in the right cylinder head and a two-
position vacuum advance solenoid. Three
vacuum lines are connected to the solenoid,
a ported vacuum line from the carburetor, a
vacuum line from the intake manifold, and a
vacuum line that runs to the distributor vac-
uum advance unit. When the engine temper-
ature is between 92°F and 232°F, the tem-
perature switch contacts are open and the
vacuum solenoid is deenergized. This causes
carburetor-ported vacuum to control the op-
eration of the distributor vacuum advance
unit. When the engine temperature is below
93°F or above 232°F, the temperature switch
contacts are closed and the vacuum solenoid
is energized. This moves the plunger in the
solenoid to block the ported vacuum opening
and connect manifold vacuum to the distrib-
utor. When the engine reaches normal tem-
perature, the temperature switch contacts
open and ported vacuum is restored to the
TRANSMISSION
SWITCH (N.O.)
1972 454 TCS system—hot override on
BATTERY
20-SEC. TIME RELAY (N.C.)
ENERGIZED=V
A.
DE-
ENERGIZED=ATM.
BANK
1973 4-speed TCS system
distributor. TCS is not used on 1975 and later
models.
20 SEC
DELAY RELAY (N.O.)
ENERGIZED=V A. DE-
ENERGIZED=ATM.
1972 350 TCS system—hot override on
Exhaust Gas Recirculation
(EGR)
All 1973-84 engines are equipped with ex-
haust gas recirculation (EGR). This system
consists of a metering valve, a vacuum line to
the carburetor, and cast-in exhaust gas pas-
sages in the intake manifold. The EGR valve
is controlled by carburetor vacuum, and ac-
cordingly opens and closes to admit exhaust
gases into the fuel/air mixture. The exhaust
gases lower the combustion temperature, and
reduce the amount of oxides of nitrogen (NOa)
produced. The valve is closed at idle be-
tween the two extreme throttle positions. In
most installations, vacuum to the EGR
BATTERY
SWITCH
HOT
TEMPERATURE
SWITCH
IGNITION SWITCH
HOT
THERMISTOR TEMP.
j \
LEFT BANK SWITCH SWITCH (N.O.)
BATTERY
r
IGN. SWITCH
HOT
TEMPERATURE TRANSMISSION
SWITCH
SWITCH (N.O.)
