GM Cutlass RWD 1970-1987 Repair Guide

Thermal Vacuum Switch

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This switch was introduced in 1973 V8 engines. Vacuum hoses from the carburetor, intake manifold and distributor connect to this switch which is controlled by engine coolant temperature. During normal engine operation, vacuum from the carburetor passes through the TVS to the distributor. If the engine should overheat while idling, the TVS connects intake manifold vacuum to the distributor which lowers coolant temperature by advancing the ignition timing.

COMPONENT TESTING



Thermal Vacuum Switch
  1. Start the engine and allow it to reach normal operating temperature.
  2.  
  3. With the engine idling, disconnect the distributor hose at the D (top) port on the TVS.
  4.  
  5. Connect a vacuum gauge to the switch. There should be 2-5 in. Hg (13-34 kPa) of vacuum at idle. If this is not the case, and the hoses are unobstructed and connected to the proper ports, the switch is defective and must be replaced.
  6.  

REMOVAL & INSTALLATION



  1. Drain the radiator until the level is below the coolant outlet housing..
  2.  
  3. Disconnect the vacuum lines from the switch noting their locations.
  4.  
  5. Remove the switch using the proper size wrench.
  6.  

To install:
  1. Apply sealer to the threaded portion of the new switch, and install it, torquing to 15 ft. lbs. (20 Nm)
  2.  
  3. Rotate the head of the switch to a position that will permit easy hookup of vacuum hoses. Then install the vacuum hoses to the proper connectors.
  4.  
  5. Refill the cooling system to the proper level
  6.  

Thermal Check and Delay Valve

On 1973 and later 350 and 455 4-bbl carbureted engines (except 350 cu. in manual transmission models) have a thermal check and delay valve. This valve is in the vacuum line which runs between the carburetor spark port and the TVS.

When the underhood (or engine block) temperature is below 50°F (10°C), full vacuum is supplied to the distributor. Above 50°F (10°C), the valve blocks full vacuum for up to 40 seconds. If ported vacuum drops, the valve opens, causing the distributor vacuum advance to be retarded. As vacuum increases, the valve closes, blocking full vacuum again.

Cars manufactured after March 15 1973 have a cover over the valve so that it is more dependent upon engine block temperature.

COMPONENT TESTING



  1. Start the engine and allow it to reach operating temperature. Underhood temperature must be at least 50 °F. (10°C)
  2.  
  3. Connect the vacuum gauge to the TVS port of the valve, and a hand-operated vacuum pump to the "CARB" port. This should be marked on the valve.
  4.  
  5. Operate the pump to create a vacuum. The vacuum gauge on the TVS side should show a slight hesitation before registering.
  6.  
  7. The gauge reading on the pump should drop slightly, taking 3-4 seconds for it to balance with the reading on the other gauge.
  8.  
  9. If steps three and four are negative, replace the valve.
  10.  
  11. Cover the TVS port on the valve with your fingers and operate the pump to create a vacuum of 15 in. Hg (103 kPa).
  12.  
  13. The reading on the gauge should remain steady. If the gauge reading drops, replace the valve.
  14.  
  15. Remove your finger. The reading on the gauge should drop slowly. If the reading goes to zero rapidly, replace the valve.
  16.  

 
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