DESCRIPTION & OPERATION
General Motors vehicles use a variety of vacuum and thermal operated switches, solenoids and valves to control the various emission systems. Some of the most commonly used of these are listed below. Some actual valve designs may vary from those shown. The chart can be used to identify and check operation of the various valves and switches. Emission Control Application Tables, located at the beginning of this manual, will help to determine component usage on specific engines. When calibration temperature has not been indicated, the temperature information may be found stamped on the base of the component.
CANISTER PURGE THERMAL VACUUM SWITCH
Port 3 is connected to ported vacuum source. Port 2 is connected to vacuum controlled component. Port 1 is vented to atmosphere. Vacuum is metered though valve when temperature is below 170°F (77°C). When temperature is above 170°F (77°C), both ports are fully open. (Scheme 1)
Scheme 1
CHOKE VACUUM BREAK THERMAL VACUUM SWITCH
Port 1 is connected to thermostatic air cleaner temperature sensor. Port 2 is connected to manifold vacuum. Port 3 is connected to choke vacuum break. When temperature of switch is less than 70°F (21°C), ports 1 and 2 are connected and port 3 is blocked off. This results in richer choke operation. When temperature is above 70°F (21°C), ports 1 and 3 connect with port 2, pulling choke to leanest position. Port 1 is always open to port 2. (Scheme 2)
Scheme 2
DISTRIBUTOR & EARLY FUEL EVAPORATION THERMAL VACUUM SWITCH
Port 1 is connected to manifold vacuum. Port 2 is output port to EFE valve. Port 3 is ported vacuum source. Port 4 is output to distributor spark vacuum regulator valve. (Scheme 3)
Scheme 3
EARLY FUEL EVAPORATION CHECK VALVE
Port 1 is connected to ported vacuum. Port 2 is connected to EFE/EGR solenoid. When vacuum is applied to port 1 (tapered end), vacuum should flow freely though valve. When vacuum is applied to port 2 (squared off end), vacuum should not pass through or leak down in less than 60 seconds. (Scheme 4)
Scheme 4
EARLY FUEL EVAPORATION THERMAL VACUUM SWITCH
Port 1 is a filtered vent. Port 2 is connected to EFE valve. Port 3 is connected to manifold vacuum. When temperature is below 105° F (40° C), manifold vacuum is directed through ports 2 and 3 to EFE actuator, which closes EFE valve. When temperature is above 105° F (40° C), port 3 is blocked and EFE vacuum is vented to atmosphere through port 1. (Scheme 5)
Scheme 5
RESPONSE VACUUM REDUCER
Port 1 is a vent. Port 2 is connected to EGR valve. Port 3 is connected to vacuum modulator valve (VMV). Connect vacuum gauge to port 3. Connect vacuum pump to port 2 and apply 15 in. Hg. Vacuum gauge should read not less than 14.25 in. Hg. (Scheme 6)
Scheme 6
SECONDARY VACUUM BREAK THERMAL VACUUM SWITCH
Port 1 is connected to manifold vacuum. Port 2 is connected to vacuum controlled component. Port 3 is a filtered vent. When temperature is below 70°F (21°C), manifold vacuum is directed through ports 2 and 3 to EFE actuator which closes EFE valve. When temperature is above 70°F (21°C), port 3 is blocked and EFE actuator vents to atmosphere through port 1. (Scheme 7)
Scheme 7
DISTRIBUTOR THERMAL VACUUM SWITCH
Port 1 switches to port 2 at 120°F (49°C), port 3 is opened to the EGR valve and port 4 is opened to the canister purge. Port 5 is connected to ported vacuum at the carburetor. (Scheme 8)
Scheme 8
VACUUM MODULATOR VALVE
Vacuum modulator valve (VMV) helps regulate EGR valve opening. The VMV allows increase in vacuum to EGR valve as throttle closes. To test VMV, connect vacuum gauge to port 2. With engine idling, vacuum reading should be about 14 in. Hg. Connect vacuum gauge to port 1. Vacuum reading should drop to about 13 in. Hg. (Scheme 9)
Scheme 9
VACUUM REGULATOR VALVE
Vacuum is supplied at port 1 and reduced at port 2 as throttle is opened. At closed throttle, vacuum at port 2 should be 15 in. Hg. At wide open throttle, vacuum at port 2 should be zero. Vacuum from port 2 is directed to EGR vacuum switch. (Scheme 10)
Scheme 10
FUEL VAPOR CONNECTOR
Port 1 is connected in series between ported vacuum and fuel tank pressure valve. Port 2 and 4 are connected to canister purge solenoid. Port 3 is connected in series between float bowl vent solenoid and canister. When canister purge solenoid is closed, purge vapors pass through.030" (.8 mm) restriction. When canister purge solenoid opens, purge vapors pass through.080" (2 mm). (Scheme 11)
Scheme 11
CANISTER CONTROL VALVE (TYPE 1)
Port 1 is connected to manifold vacuum. Port 2 is connected to float bowl. Port 3 is connected to purge vacuum. Port 4 is connected to canister. When engine is running, vacuum at port 1 closes float bowl vent port 2. Port 3 then pulls fuel vapors through port 4. (Scheme 12)
Scheme 12
CANISTER CONTROL VALVE (TYPE 2)
Port 1 is connected to ported vacuum. Port 2 is connected to manifold vacuum. Port 3 is connected to float bowl. Port 4 is connected to canister. When engine is running, manifold vacuum at port 2 closes float bowl vent port 3. When engine speed rises above idle, port 1 opens passage between port 2 and port 4. Result is canister purging when engine is warmed above 170° F (76° C). (Scheme 13)
Scheme 13
ANTI-DIESEL VACUUM SOLENOID VALVE
Port 1 is connected to vacuum delay valve. Port 2 is connected to idle load compensator. Port 3 is connected to vacuum tank. When solenoid is energized, port 3 will hold vacuum. When solenoid is de-energized, vacuum will hold at port 1 and 2. (Scheme 14)
Scheme 14
FUEL TANK PRESSURE CONTROL VALVE
Port 1 is connected to manifold vacuum. Port 2 is connected to fuel tank vent. Port 3 is connected to canister. When engine is running, manifold vacuum at port 1 opens passage between port 2 and 3. When engine is not running, diaphragm closes port 2 and 3. Result is fuel tank venting through restriction. (Scheme 15)
