ST_330_72_1972_Chevrolet_Truck_Chassis_Service_Manual_10_to_30

EMISSION CONTROL SYSTEMS 6T-7

TRANSM ISSION CONTROLLED SPARK SYSTEM
SMALL BLOCK V 8 ENGINE

GENERAL DESCRIPTION

Control of exhaust em issions em itted by vehicles using The vacuum advance solenoid is controlled by two
the sm all block V8 engine, is accom plished by elim inating switches and a delay relay. The solenoid is energized in
ignition vacuum advance when the vehicle is operating the high forw ard gear (and in re v e rse on H ydra-m atic) by
a transm ission operated switch. A therm ostatic coolant
in low forw ard gears. tem perature switch is used to provide therm al override
Vacuum advance is controlled by a solenoid-operated below 82°F0

switch, which is energized by grounding a norm ally open W ider throttle blade openings at idle are required to
switch at the transm ission. When the solenoid is in the compensate for the retarded spark condition produced by
non-energized position, vacuum to the distributor advance the design of the em ission reduction system . To prevent
unit is denied and the distributor is vented to atm osphere engine dieseling at engine shut down, an idle stop solenoid
through a filte r at the opposite end of the solenoid — is provided. The ignition activated idle stop solenoid
venting the distributor advance unit prevents it from elim inates dieseling tendencies by allowing the throttle
becoming locked at an advanced position. When the sole­ valve to close beyond the norm al idle position when the
noid is energized, the vacuum port is uncovered and the ignition is turned off.
plunger is seated at the opposite end, shutting off the
clean air vent.

SYSTEM THEORY

The system components are shown in their norm al at
r e s t position with the engine off and cold (fig. 6T-11).
The tem perature switch points are closed, making con­
tact with the cold term inal; the delay relay points are
open; transm ission switch points are open; idle stop sole­
noid is de-energized and plunger retracted; vacuum
advance solenoid is de-energized with plunger shutting
off the port to vacuum advance unit.

When the ignition switch is turned on, the idle stop
solenoid is energized, extending the plunger to contact the
throttle lever. A circuit is completed from the ignition
switch through the vacuum advance solenoid and to ground
through the tem perature switch (fig. 6T-12). The solenoid
plunger moves to open the vacuum port and block the
fresh air vent. The vacuum advance solenoid rem ains
energized as long as the tem perature switch cold contact
rem ains closed.

In low gear operation, with engine tem perature above
82 deg rees, the tem p eratu re sw itch cold o verride points

Fig. 6T-11 —Small V8 TCS System (Engine O ff) Fig. 6T-13—Small V8 TCS System (Low G e a r O p eration)

10-30 CHEVROLET TRUCK SERVICE MANUAL

EMISSION CONTROL SYSTEMS 6T-8

BATTERY

E N G IN E IGN. SWITCH
OVERHEAT LIGHT IDLE STOP SO LEN O ID

DIST. V AC UUM
ADVANCE ,
UNIT— V

1 T R A N SM ISSIO N Energized = V.A. 20 SEC.
SWITCH (N.O.) De-energized — Atm . DELAY RELAY (N O.)
TEMPERATURE
SWITCH

Fig. 6T-14—Small V8 TCS System (High G ear Delay Relay) Fig. 6T-15—Small V8 TCS System (High G ear Operation)

open (fig. 6T-13). This breaks the circuit from the vacu­ A fter a delay of 23 seconds (in high gear) the delay
um solenoid, allowing the plunger to block vacuum and relay points close, completing the circuit through the
open the advance unit to atm osphere. vacuum advance solenoid to ground (fig. 6T-15). Vacuum
advance to the distributor is perm itted by energizing the
When the transm ission is shifted into high forward solenoid. Subsequent interruption of the high gear posi­
gear, the transm ission switch points are closed by shift tion will re su lt in a 23 second delay before the vacuum
action or by oil p ressu re as applicable. This completes advance solenoid is energized again.
the circuit from the ignition switch through the delay
relay timing circuit and through the transm ission switch
to ground (fig. 6T-14). Vacuum advance solenoid is not
energized.

COMPONENT DESCRIPTION

IDLE STOP SOLENOID

The idle stop solenoid (fig. 6T-16) is a two position
electrically operated control, used to provide a p re­
determ ined throttle setting. In the energized position
(plunger extended) the plunger contacts the carburetor
throttle lev er and p rev en ts full closing of the c a rb u reto r
throttle plates. This fast idle control when de-energized

.fcr t-

Fig. 6T-16—Small V8 Idle Stop Solenoid Fig. 6T-17—Small V8 Vacuum Advance Solenoid

10-30 CHEVROLET TRUCK SERVICE MANUAL

EMISSION CONTROL SYSTEMS 6T-9

Fig. 6T-20—Turbo Hydra-mafic 350 Transmission Switch

opposite end. In the de-energized position the spring
loaded plunger seats against the vacuum inlet and opens
the distributor advance unit to the clean a ir vent.

Fig. 6T-18—Small V8 Time Delay Relay TIME RELAY

(plunger retracted) allows throttle plates to close beyond The tim e relay is a solid state timing device designed
the norm al idle position; thereby shutting off a ir supply to delay circu it continuity for a minimum of 20 seconds,
and in essence starving the engine so that it will shut after the transm ission has been shifted into high forw ard
down without dieseling. The idle stop solenoid is bracket- gear. Delaying circuit completion delays vacuum ad­
attached to the carburetor so that the plunger, when ex­ vance. Any opening of the tran sm issio n switch contacts
tended, contacts the throttle lever. (downshift) of m ore than one or two seconds, causes the
relay to recycle and deny vacuum advance for another
VACUUM ADVANCE SOLENOID 20 seconds. A tra n sm issio n switch interruption of less
than one second will disable the time relay and prevent
The vacuum advance solenoid is located on the right vacuum advance until the next upshift cycle.
re a r portion of the inlet manifold (fig. 6T-17). This
electrically operated two-position plunger controlled The time relay is located in the passenger com part­
valve serves to supply or deny vacuum to the distributor m ent on the dash and toe panel between the fuse block
vacuum advance unit. In the energized position, the and the steering column (fig. 6T-18).
plunger opens the vacuum port from the carburetor to the
vacuum advance unit. In opening the vacuum port the TEMPERATURE SWITCH
plunger sim ultaneously closes the clean a ir port at the
The TCS system tem p eratu re switch is located in the
right cylinder head between the number 6 and number 8
exhaust port (fig. 6T-19).

The switch is a two-position single term inal control
which provides a path to ground, for the cold override
feature, when engine coolant tem p eratu res are below 82
degrees. The "off” or "neutral” position is maintained
at engine coolant te m p eratu res above 82 degrees.

T.C.S. SW ITCH

VALVE BODY

Fig. 6T-19— Small V8 Temperature Sw itch (Right Head) Fig. 6T-21—Turbo Hydra-matic 400 TCS Sw itch Location

10-30 CHEVROLET TRUCK SERVICE MANUAL

EMISSION CONTROL SYSTEMS 6T-10

Fig. 6T-22—Turbo Hydra-matic 400 TCS Connection Fig. 6T-24—Turbo Hydra-matic 400 TCS Switch (Sectional View)

Fig. 6T-23—Turbo Hydra-matic 400 TCS Connection Wnen the transm ission is shifted into high forw ard gear,
Terminal Identification the plunger drops into a re c e ss or flat on the shifter
shaft, causing the plunger to extend and allow the cup
TRANSMISSION SWITCH contact to re s t on the switch housing (fig. 6T-8). A c ir ­
On manual shift synchrom esh tran sm issio n s, both 3- cuit is completed to ground, from the transm ission
through the switch housing to the cup contact and through
and 4 -speed, the sw itch is located on the outside of the the spring to the electrical term inal.
transm ission case in an area adjacent to the 2-3 or 3-4
shifter shaft, as applicable (fig. 6T-7). The mechanically The Turbo H ydra-m atic 350 tran sm issio n s use a p re s ­
operated switch is spring loaded to provide continuity sure sensitive switch, which is activated by transm ission
between the switch term inal and the switch housing. The fluid p ressure. The switch for Turbo Hydra-m atic is
molded top and the plunger are non-conductors and installed externally in the 2-3 direct clutch p ressu re tap
therefore insulate the cup contact and switch term inal (fig. 6T-20). At rest, in or out of the installed position,
when the plunger is depressed. When installed in the the switch is in a norm ally open position. Construction
transm ission, the plunger contacts the shifter shaft, of the switch is such that a spring loaded diaphragm that
which causes the plunger to r e tra c t in low forw ard g ears. contains a m etallic contact, is held away from a cup
contact, which in turn, is in contact with the switch hous­
ing (fig. 6T-10). Transm ission fluid p ressu re, against the
insulated plug, forces the diaphragm upward so that the
diaphragm contact closes the circuit through the spring
to the electrical term inal.

The Turbo H ydra-m atic 400 tra n sm issio n also u ses a
p ressu re sensitive switch which is activated by tra n s­
m ission fluid p re ssu re . However, the TCS switch is
located internally in the transm ission (fig. 6T-21). From
its location in the valve body the switch is connected to
the externally located, combination detent solenoid and
TCS connector (figs. 6T-22 and 6T-23). The norm ally
open switch is constructed so that a spring loaded m etal­
lic diaphragm is held away from the term inal in the
switch. T ransm ission fluid p ressu re forces the dia­
phragm upward so that the spring seat contacts the
electrical term inal to provide continuity through the
grounded housing (fig. 6T-24).

10-30 CHEVROLET TRUCK SERVICE MANUAL