System Operation

Connect a tachometer (accurate to within 10 RPM). Start engine and open throttle until tachometer reads 1890 RPM. Throttle lever actuator should be extended at this speed. Decrease engine speed to 1700 RPM. Throttle actuator should be retracted at this speed.
If throttle actuator operates at specified engine speeds, system is functioning. If actuator operates outside of RPM limits, replace speed sensor. If actuator does not operate at any speed, proceed with the following steps:
Using a voltmeter, check for battery voltage at voltage wire terminal on solenoid valve and speed sensor. If voltage is present at one component only, repair wiring harness as required. If no voltage at both components, check engine harness connections at distributor and bulkhead connector and repair as required.

Fig 1: Schematic of Throttle Return Control (TRC) System
Courtesy of GENERAL MOTORS COMPANY

Throttle return control (TRC) system is used on all Heavy Duty Emission models.
If battery voltage is present at solenoid valve and speed sensor, start engine and use a jumper wire to ground solenoid-to-speed sensor connecting wire terminal at speed sensor. Throttle actuator should extend.
- If actuator did not extend, remove throttle actuator hose from solenoid and check solenoid orifice for blockage. If orifice is plugged, clean as required. If orifice is clear, replace solenoid.
- If actuator did extend, ground solenoid-to-switch wire terminal at speed switch. If actuator does not extend, repair speed switch-to-solenoid wire. If it extends, ensure speed switch ground wire reads ground with engine running and check speed switch-to-distributor wire connections. If actuator still does not extend with ail wires properly connected and engine speed above 1890 RPM, replace speed sensor.
If throttle actuator remains extended at all speeds, remove electrical connector from solenoid.
- If actuator remains extended, check actuator vacuum orifice on solenoid valve for blockage. Clean orifice, and reconnect system. If actuator again remains extended, remove solenoid connector. If actuator does not retract, replace solenoid valve.
- If actuator retracts with connector removed, reconnect and then remove speed switch connector. If actuator retracts, replace speed switch. If actuator does not retract, solenoid-to-switch wire is shorted to ground in harness. Repair wire.