AMC Coupes/Sedans/Wagons 1975-1988 Repair Information

Transmission Controlled Spark (TCS) System

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OPERATION



See Figures 1, 2 and 3

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Fig. Fig. 1: Electrical schematic of the Transmission Controlled Spark (TCS) system operation



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Fig. Fig. 2: TCS system component locations6 cylinder engines



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Fig. Fig. 3: TCS system schematic and component locations used on 8 cylinder engines

The purpose of this system is to reduce the emission of oxides of nitrogen by lowering the peak combustion pressure and temperature during the power stroke.

Ambient Temperature Override Switch

This switch, located at the firewall, senses ambient temperatures and completes the electrical circuit from the battery to the solenoid vacuum valve when the ambient temperatures are above 63°F (17°C).

Solenoid Vacuum Valve

This valve is attached to the ignition coil bracket at the right side of the engine (V8 engines) or to a bracket at the rear of the intake manifold (Sixes). When the valve is energized, carburetor vacuum is blocked off and the distributor vacuum line is vented to the atmosphere through a port in the valve, resulting in no vacuum advance. When the valve is de-energized, vacuum is applied to the distributor resulting in normal vacuum advance.

Solenoid Control Switch

This switch is located in the transmission valve body. It opens or closes in relation to car speed and gear range. When the transmission is in high gear, the switch opens and breaks the ground circuit to the solenoid vacuum valve. In lower gear ranges the switch closes and completes the ground circuit to the solenoid vacuum valve. With a manual transmission, the switch is operated by the transmission shifter shaft. With automatic transmissions, the switch is controlled by the speedometer gear speed. Under speeds of 25 mph, the switch is activated.

Coolant Temperature Override Switch

This switch is used only on the 8-304 engine. It is threaded into the thermostat housing. The switch reacts to coolant temperatures to route either intake manifold or carburetor vacuum to the distributor vacuum advance diaphragm.

When the coolant temperature is below 160°F (71°C), intake manifold vacuum is applied through a hose connection to the distributor advance diaphragm, resulting in the full vacuum advance.

When the coolant temperature is above 160°F (71°C), intake manifold vacuum is blocked off and carburetor vacuum is then applied through the solenoid vacuum valve to the distributor advance diaphragm, resulting in decreased vacuum advance.

The relationship between distributor vacuum advance and the operation of the TCS system and coolant temperature override switch can be determined by referring to the Emission Control Distributor Vacuum Application Charts.

TESTING - System Test



MANUAL TRANSMISSION
  1. Connect a vacuum gauge between the distributor and the solenoid vacuum valve, using a T-connector.
  2.  

This won't work unless the temperature switch on the front crossmember is above 63°F.

  1. Start the engine. With the transmission in Neutral, the vacuum gauge should ready zero.
  2.  
  3. Increase engine speed to between 1000-1500 rpm with the clutch pedal depressed. The vacuum reading should remain at zero.
  4.  
  5. With the clutch still depressed, place the transmission in High gear. Increase engine speed, as before. The vacuum gauge should now read at least 6 in. Hg. If it does not, proceed further with testing.
  6.  
  7. Unfasten the transmission switch lead from the solenoid vacuum valve terminal. Connect the lead in series with a low amperage test lamp and the positive side of the battery.
  8.  
  9. Move the shift lever through all of the gears. The test lamp should remain on until high gear is entered.
  10.  
  11. If the lamp stays on when the transmission is in High, either the switch is defective or the circuit is grounded. If it fails to come on at all, the switch is defective or the circuit has a loose wire.
  12.  
  13. If the transmission switch is functioning properly but the system check indicates that something is still wrong, check the temperature switch.
  14.  

AUTOMATIC TRANSMISSION

The spark control system is controlled by a switch which operates on transmission governor oil pressure.

  1. Disconnect the electrical lead from the terminal of the governor pressure switch.
  2.  

The switch may easily be reached with the hood opened. On sixes, the switch is located on the right rear of the cylinder block; on V8's, it is attached to a bracket at the rear of the right-hand rocker cover.

  1. Connect a 12V test light in series, between the lead and the terminal on the switch.
  2.  


WARNING
Use a low amperage test light, so that the switch contacts will not be damaged.

  1. Raise the car, block the front wheels (if they are not off the ground), and securely support it so that the rear wheels are free to turn.
  2.  
  3. Apply the service brakes. Start the engine. The test light should glow.
  4.  
  5. Place the gear selector in Drive, release the brake pedal and slowly depress the gas pedal.
  6.  
  7. Watch the speedometer and the test light; between 33-37 mph the switch should open and the test light should go out.
  8.  
  9. If the light does not go out within this speed range, adjust the switch by turning the 1 / 16 in. allen screw on the switch terminal. Turn the screw clockwise to increase or counter-clockwise to decrease the switch cut-out sped. The switch should be adjusted to open at 35 mph.
  10.  
  11. If the switch cannot be adjusted, replace it.
  12.  
  13. If the switch is working properly, but the TCS system is not working, the solenoid vacuum valve is probably defective.
  14.  

Transmission Switch

  1. Leave the rear wheels of the car off the ground as in the system test above.
  2.  
  3. Disconnect the transmission switch leads. Connect a low amperage test lamp in series with the switch and the positive side of the battery.
  4.  
  5. Accelerate to the speed specified in Step 3 of the system test and watch the test lamp. It should remain on until the specified speed is reached. If the lamp fails to go out or if it does not light at all, the switch is defective and must be replaced.
  6.  
  7. If the switch is working properly, reconnect it and go on with the next test.
  8.  

Vacuum Advance Solenoid

  1. Disconnect the vacuum advance solenoid leads. Connect a vacuum gauge to the solenoid hose as in the system test.
  2.  
  3. Place the transmission in Neutral and start the engine. Increase engine speed. The gauge should indicate the presence of a vacuum.
  4.  
  5. Connect the hot lead to a 12 volt power source. Ground the other lead. Increase the engine speed again. The solenoid should energize, resulting in a vacuum reading of zero.
  6.  
  7. Replace the vacuum advance solenoid if it is faulty. If it is not, reconnect the wiring and go on with the next appropriate test.
  8.  

Ambient Temperature Override Switch

  1. Disconnect the ambient temperature switch leads.
  2.  
  3. Replace the switch in the circuit with a jumper wire.
  4.  
  5. Repeat the system test. If the vacuum gauge now reads zero or below the specified speed, i.e., the solenoid energized, the temperature switch is defective.
  6.  
  7. If the switch proves not to be defective when tested in Step 3, reconnect it after removing the jumper lead.
  8.  
  9. Cool the switch, using either ice, cold water, or an aerosol spray, to below 63°F. Repeat the system test. If there is no vacuum below the specified speed, the switch is stuck closed and must be replaced.
  10.  

 
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