Throttle Modulator: Testing
- With engine stopped, inspect visually to see if throttle modulator is fully retracted. There should be a slight clearance between end of modulator unit and throttle lever. If no clearance exists, modulator is either stuck or improperly adjusted. Modulator plastic housing should have about 1/4" of bind free movement. Adjust position of modulator or replace if necessary to obtain smooth movement of unit.NOTE: It will be necessary to determine engine RPM which is equal to 24 MPH on light duty vehicles. This will be cut-in RPM used in following tests. On light duty vehicles, place rear axles on floor stands so wheels may be free to spin. Place transmission in high gear.
- With engine at normal operating temperature, install a tachometer and operate engine at idle speed. Observe throttle modulator, it should remain in retracted position when engine is at curb idle. If modulator extends when engine is idling (causing excessive idle speed), either solenoid vacuum valve (heavy duty) or engine speed sensor is faulty.
- Accelerate engine slowly and observe throttle modulator and tachometer. Throttle modulator should extend when engine reaches cut-in speed and remain extended at engine speeds over cut-in speed. Decelerate engine slowly while observing throttle modulator and tachometer. Throttle modulator should retract when engine speed drops below cut-in speed. If throttle modulator does not respond to engine speed changes, either throttle modulator, solenoid vacuum valve (heavy duty) or speed sensor unit is faulty.
- Check high idle speed with throttle lever resting against extended throttle modulator as follows; disconnect vacuum hoses from carburetor or intake manifold vacuum port and throttle modulator. Connect a vacuum hose directly between throttle modulator and vacuum port. Start engine and accelerate to 1500-2000 RPM. Throttle modulator should be extended. Release throttle allowing engine to decelerate. Extended modulator should hold engine speed at specified high idle speed. If high idle speed is not within specified limits, loosen lock nut and adjust position of throttle modulator to obtain correct high idle speed. Disconnect temporary vacuum hose from throttle modulator and hold finger over end of hose. Throttle modulator should retract and engine speed return to normal curb idle. Stop engine and reconnect hoses to their proper connections.