Jeep CJ/Scrambler 1971-1986 Repair Guide

Transmission Controlled Spark System

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OPERATION



See Figures 1, 2, 3 and 4

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.The system incorporates the following components:

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. 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 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.



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Fig. Fig. 1: TCS system electrical schematic



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Fig. Fig. 2: TCS system used on in-line 6 cylinder engines



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Fig. Fig. 3: TCS system used on the 8-304 engine



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Fig. Fig. 4: Emission controlled distributor vacuum application

TESTING



Spark Coolant Temperature Overide (CTO) Switch

Before testing the spark CTO switch, make sure that the engine coolant temperature is below 160°F (71°C).

  1. Remove all the hoses from the CTO switch and plug those which will create a vacuum leak.
  2.  
  3. Connect a vacuum line from a manifold vacuum source to the top port of the CTO switch.
  4.  
  5. Connect a vacuum gauge to the center port.
  6.  
  7. Start the engine. Manifold vacuum should be indicated on the gauge. If not, replace the switch.
  8.  
  9. With the engine still running and the coolant temperature still below 160°F (71°C), disconnect the vacuum line from the top port and connect it to the bottom port.
  10.  
  11. No vacuum should be indicated. Replace the switch if there is vacuum.
  12.  
  13. Allow the engine to run until the coolant temperature exceeds 160°F (71°C). Manifold vacuum should be indicated. If not, replace the CTO switch.
  14.  
  15. Disconnect the hose from the bottom port and connect it to the top port again. With the coolant temperature above 160°F (71°C), no vacuum should be indicated. If there is, replace the CTO switch.
  16.  

 
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