See Figures 1, 2 and 3
Two different Throttle Return Control (TRC) systems are used. The first is used from 1975 to 1978. It consists of a control valve and a throttle lever actuator. When the truck is coasting against the engine, the control valve is open to allow vacuum to operate the throttle lever actuator. The throttle lever actuator then pushes the throttle lever slightly open reducing the HC (hydrocarbon) emission level during coasting. When manifold vacuum drops below a predetermined level, the control valve closes, the throttle lever retracts, and the throttle lever closes to the idler position.
The second TRC system is used in 1979 and later. It 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 present amount, above normal engine idle speed, in response to a signal from the solenoid vacuum control valve. The valve, mounted at the left rear of the engine above the intake manifold on the six cylinder, or on the thermostat housing mounting stud on the V6 or V8, 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.
- Disconnect the valve-to-carburetor hose and connect it to an external vacuum source with a vacuum gauge.
- Disconnect the valve-to-actuator hose at the connector and connect it to a vacuum gauge.
- Place a finger firmly over the end of the bleed fitting.
- Apply a minimum of 23 in.Hg (158 kPa) vacuum to the control valve and seal off the vacuum source. The gauge on the actuator side should read the same as the gauge on the source side. If not, the valve needs adjustment. If vacuum drops off on either side (with the finger still on the bleed fitting), the valve is defective and should be replaced.
- With a minimum of 23 in.Hg (158 kPa) vacuum in the valve, remove the finger from the bleed fitting. The vacuum level in the actuator side will drop to zero and the reading on the source side will drop to a value that will be the value set point. If the value is not within 1 / 2 in.Hg vacuum of the specified valve set point, adjust the valve.
- Gently pry off the plastic cover.
- Turn the adjusting screw in (clockwise) to raise the set point or out (counterclockwise) to lower the set point.
- Recheck the valve set point.
- If necessary, repeat the adjustment until the valve set point is attained between 22.5 in.Hg and 23.5 in.Hg vacuum.
- Disconnect the valve-to-carburetor hose at the carburetor. Connect the hose to an external vacuum source, with an accurate vacuum gauge connected into the line near the valve.
- Apply a minimum of 25 in.Hg (172 kPa) of vacuum to the control valve vacuum supply fitting while sealing off the vacuum supply between the gauge and the vacuum source. The vacuum gauge will indicate the set point value of the valve.
- If the gauge reading is not within 0.5 in.Hg (3.4 kPa) of the specified value (see the chart), the valve must be adjusted. If the trapped vacuum drops off faster than 0.1 in.Hg (0.68 kPa) per second, the valve is leaking and must be replaced.
- To adjust the valve set point, follow Steps 6-9 of the 1975-76 adjustment procedure.
- Disconnect the valve-to-actuator hose at the valve and connect it to an external vacuum source.
- Apply 20 in.Hg (137 kPa) vacuum to the actuator and seal the vacuum source. If the vacuum gauge reading drops, the valve is leaking and should be replaced.
- Check the throttle lever, shaft, and linkage for freedom of operation.
- Start the engine and warm it to operating temperature.
- Note the idle rpm.
- Apply 20 in.Hg (137 kPa) vacuum to the actuator and manually operate the throttle. Allow it to close against the extended actuator plunger. Note the engine rpm.
- Release and reapply 20 in.Hg (137 kPa) vacuum to the actuator and note the rpm at which the engine speed increases (do not assist the actuator).
- 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.
- Release the vacuum from the actuator and the engine speed should return to within 50 rpm of the speed noted in Steps 4 and 5.
- To adjust the actuator, turn the screw on the actuator plunger until the specified TRC speed range is obtained.
See Figures 4 and 5
This procedure applies to 1979 and later models only.
- 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.
- Reduce the engine speed to 1700 rpm. The lever actuator should retract.
- If the actuator operates outside of the speed limits, the speed switch is faulty and must be replaced. It cannot be adjusted.
If the actuator does not operate at all:
- 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.
- If the correct voltage is present at one component but not the other, the engine wiring harness is faulty.
- If the voltage is not present at all, check the engine harness connections at the distributor and the bulkhead connector and repair as necessary.
- 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.
- 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.
- If the actuator extends in Step d, 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 1890 rpm, replace the speed switch.
If the actuator is extended at all speeds:
- Remove the connector from the vacuum solenoid.
- 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.
- 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.