Return to Safe Position (Fail-Safe Position)
This is the predefined position the system will go to when the supply is cut or a fault is detected. On a cable-pull actuator this position is idle, because when the actuator stops producing force the engine's own return spring closes the throttle lever. This behaviour depends on the soundness of the mechanics; it is not an electronic safety function.
The concept of the safe position is the engineering question of which state a system will settle into in its most likely faults. In a cable-pull arrangement the answer is given mechanically: when the pulling force stops, the spring brings the lever to idle. The same logic exists in fuel shut-off solenoids too; an ETR type solenoid cuts the fuel when power is lost, that is, it falls to the safe side.
This behaviour has three conditions, and all three are met at installation:
- The return spring must be sound and of sufficient force.
- The throttle linkage joint must not be binding.
- The cable must be able to slide back freely inside the sheath.
If one of the three is not met, the lever can remain half open. For this reason the GS740's and GS740-3P's falling to idle is not declared as a safety function; stopping the engine must be provided by the fuel shut-off chain, and overspeed protection by the engine's own systems.
Context in which this term is used: Electronic speed and throttle control on a diesel engine