See Figure 1
Two different throttle return control 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 idle position.
The second TRC system is used in 1979. 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 preset 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 6-cylinder, or on the thermostat housing mounting stud on the 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.
See Figures 2, 3, 4 and 5
- 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 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 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 plus or minus 1 / 2 in. Hg of vacuum.
See Figures 6 and 7
- 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 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 valve of the valve.
- If the gauge reading is not within 0.5 in. Hg of the specified value (see the chart), the valve must be adjusted. If the trapped vacuum drops off faster than 0.1 in. Hg 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 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 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 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.