GM Full-Size Trucks 1980-1987 Repair Guide

Throttle Return Control

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See Figures 1, 2, 3 and 4

The purpose of the throttle return control system is to prevent an over rich condition during periods of deceleration. This is accomplished by keeping the throttle plate slightly open during deceleration, which helps maintain the air/fuel ratio at optimum efficiency. The Throttle Return Control (TRC) system consists of a throttle lever actuator, a solenoid vacuum control valve, and an engine speed switch.

The engine speed switch monitors the engine speed (rpm) at terminal "B" from the tach signal generated by the distributor. When a drop in engine speed (rpm) is detected the engine speed switch will apply a ground to the solenoid vacuum control valve. The solenoid vacuum control valve will now open and apply vacuum to the throttle lever actuator. When vacuum is applied to the throttle lever actuator, the plunger extends to hold the throttle plate slightly open. The system is designed to operate only when engine speed falls between 1700 and 1890 rpm.

The throttle lever actuator is mounted on the carburetor. The solenoid vacuum control valve is mounted at the left rear of the engine above the intake manifold on the 6-cylinder, or on the thermostat housing mounting stud on the V8 engine. The engine speed switch can be found on the valve cover on V8 engines and on the right side of the engine block on the in-line six cylinder engine.



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Fig. Fig. 1: TRC system schematic



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Fig. Fig. 2: Engine speed switch locations



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Fig. Fig. 3: TRC valve assembly-inline six cylinder engine



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Fig. Fig. 4: TRC valve assembly-V8's

TESTING



TRC System Test
  1. Connect a tachometer to the distributor TACH terminal.
  2.  
  3. Start the engine and raise the engine speed to 1890 rpm. Observe the throttle lever actuator position. The actuator should be extended.
  4.  
  5. Reduce the engine speed to 1700 rpm. Observe the throttle lever actuator position. The actuator should be retracted.
    1. If the throttle lever actuator operated as described, the TRC system is operating correctly. No further testing is necessary.
    2.  
    3. If the throttle lever actuator operated as described but moved outside of the engine speed limits given, the speed switch is faulty and must be replaced. It cannot be adjusted.
    4.  
    5. If the throttle lever actuator did not operate at all, further testing is required. Proceed to the next Step.
    6.  
    7. If the throttle lever actuator is always extended, proceed to Step 7.
    8.  

  6.  
  7. 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 (see schematic). 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.
    1. If the correct voltage is present at one component but not the other, the engine wiring harness is faulty.
    2.  
    3. If voltage is not present at all, check the engine harness connections and related wires for an open wire or connection. Repair as necessary.
    4.  
    5. If the correct voltage is present at both components, go to the next step.
    6.  

  8.  
  9. Check for solenoid vacuum control valve operation by grounding the solenoid-to-speed switch wire terminal at the solenoid connector with a jumper wire. This should cause the throttle lever actuator to extend, with the engine running.
    1. If the lever actuator does not extend, remove the hose from the solenoid side port which connects to the actuator hose. Check the port for obstructions or blockage. If the port is not plugged, replace the solenoid.
    2.  
    3. If the lever actuator extends, go to the next step.
    4.  

  10.  
  11. Ground the solenoid vacuum control valve-to-speed switch wire terminal at the switch. This should cause the throttle lever to extend with the engine running.
    1. If the actuator does not extend, the wire between the speed switch and the solenoid is open and must be repaired.
    2.  
    3. If the actuator does extend, check the speed switch ground wire for an open. If it is OK, replace the engine speed switch.
    4.  

  12.  
  13. 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.  

  14.  

Throttle Lever Actuator Test
  1. Disconnect the valve-to-actuator hose at the valve and connect it to an external vacuum source.
  2.  
  3. Apply 20 in. Hg vacuum to the actuator and seal the vacuum source. If the vacuum gauge reading drops, the valve is leaking and should be replaced.
  4.  
  5. Check the throttle lever, shaft, and linkage for freedom of operation.
  6.  
  7. Start the engine and warm it to operating temperature.
  8.  
  9. Note the idle rpm.
  10.  
  11. Apply 20 in. Hg vacuum to the actuator and manually operate the throttle. Allow it to close against the extended actuator plunger. Note the engine rpm.
  12.  
  13. Release and reapply 20 in. Hg vacuum to the actuator and note the rpm at which the engine speed increases (do not assist the actuator).
  14.  
  15. If the engine speed obtained in Step 7 is not within 150 rpm of that obtained in Step 6, then the actuator may be binding. If the binding cannot be corrected, replace the actuator.
  16.  
  17. Release the vacuum from the actuator. The engine speed should return to within 50 rpm of the speed noted in Steps 5 and 6.
  18.  

 
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