GM Full Size Vans 1987-1997 Repair Guide

Throttle Return Control System

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See Figure 1



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Fig. Fig. 1: Typical throttle return control system schematic

This system consists of a throttle lever actuator, a solenoid vacuum control valve, and an electronic speed sensor. The throttle lever actuator, mounted on the carburetor, opens the primary throttle plates a preset amount above normal engine idle speed, in response to a signal from the solenoid vacuum control valve. The valve is mounted on the thermostat housing mounting stud. It is held open in response to a signal from the electronic speed sensor. When open, the valve allows a vacuum signal to be sent to the throttle lever actuator. The speed sensor monitors engine speed at the distributor. It supplies an electrical signal to the solenoid valve, as long as a preset engine speed is exceeded. The object of this system is the same as that of the earlier system.

THROTTLE LEVER ACTUATOR



The checking procedure is the same as for earlier years. Follow Steps 1-9 of the Throttle Valve procedure. Adjustment procedures are covered in the carburetor portion of Fuel System .

TRC SYSTEM CHECK



  1. Connect a tachometer to the distributor TACH terminal. Start the engine and raise the engine speed to 1890 rpm. The throttle lever actuator on the carburetor should extend.
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  3. Reduce the engine speed to 1700 rpm. The lever actuator should retract.
  4.  
  5. If the actuator operates outside of the speed limits, the speed switch is faulty and must be replaced. It cannot be adjusted.
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  7. If the actuator does not operate at all:
    1. Check the voltage at the vacuum solenoid and the speed switch with a voltmeter. Connect the negative probe of the voltmeter to the engine ground and the positive probe to the voltage source wire on the component. The positive probe can be inserted on the connector body at the wire side; it is not necessary to unplug the connector. Voltage should be 12 to 14 volts in both cases.
    2.  
    3. If the correct voltage is present at one component but not the other, the engine wiring harness is faulty.
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    5. If the voltage is not present at all, check the engine harness connections at the distributor and the bulkhead connector, and repair as necessary.
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    7. If the correct voltage is present at both components, check the solenoid operation: ground the solenoid-to-speed switch connecting wire terminal at the solenoid connector with a jumper wire. This should cause the throttle lever actuator to extend, with the engine running.
    8.  
    9. If the lever actuator does not extend, remove the hose from the solenoid side port which connects the actuator hose. Check the port for obstructions or blockage. If the port is not plugged, replace the solenoid.
    10.  
    11. If the actuator extends in Step 4d, ground the solenoid-to-speed switch wire terminal at the switch. If the actuator does not extend, the wire between the speed switch and the solenoid is open and must be repaired. If the actuator does extend, check the speed switch ground wire for a ground; it should read zero volts with the engine running. Check the speed switch-to-distributor wire for a proper connection. If the ground and distributor wires are properly connected and the actuator still does not extend when the engine speed is above 1,890 rpm, replace the speed switch.
    12.  

  8.  
  9. If the actuator is extended at all speeds:
    1. Remove the connector from the vacuum solenoid.
    2.  
    3. If the actuator remains extended, check the solenoid side port orifice for blockage. If plugged, clear and reconnect the system and recheck. If the actuator is still extended, remove the solenoid connector; if the actuator does not retreat, replace the vacuum solenoid.
    4.  
    5. If the actuator retracts with the solenoid connector off, reconnect it and remove the speed switch connector. If the actuator retracts, the problem is in the speed switch, which should be replaced. If the actuator does not retract, the solenoid-to-speed switch wire is shorted to ground in the wiring harness. Repair the short.
    6.  

  10.  

 
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