The length sensor carries part number 140529000009A and model designation HG-WY-3000-V. It is a component found on SANY truck crane boom systems. This sensor is part of the machine's load moment indicator (LMI) system. The LMI system monitors multiple parameters to help the operator stay within safe working limits.

Here is a breakdown of what this sensor does and how it fits into the crane.

1. Physical Design
The sensor is a cable-extension transducer, sometimes called a string potentiometer. It consists of a small housing that contains a spring-loaded spool, a stainless steel cable, and a precision potentiometer. The housing is sealed to resist dust and moisture. The cable extends from the housing through a guide tube or directly to the boom tip. The entire unit is compact enough to mount inside the boom structure.

2. How It Works
When the telescopic boom extends, the cable pays out from the spool. The spool rotates, turning the potentiometer shaft. When the boom retracts, the spring rewinds the cable, returning the spool to its original position. As the potentiometer rotates, its electrical resistance changes. This change produces a voltage signal. The signal increases as the boom extends and decreases as the boom retracts. The crane control unit reads this voltage and calculates the actual boom length based on calibration settings stored in its memory.

3. Role in the Load Moment Indicator System
The LMI system on a SANY truck crane uses boom length data along with boom angle, load weight, and hydraulic pressure to calculate safe working capacity. The length signal is one of several inputs. The control unit compares the measured length against the crane's load chart data. If the combination of length and load approaches the crane's limit, the system sounds an audible alarm and may cut off certain functions to prevent overloading or tipping. Without a working length sensor, the LMI cannot perform its full safety function.

4. Installation Location
The sensor is mounted inside the boom section. The cable runs from the sensor housing to an anchor point near the boom tip. The housing is typically mounted at the base of the first boom section, secured with bolts. The cable passes through the boom interior. The electrical connector is routed to the machine harness and carries the signal back to the control unit in the cab or chassis. Installation requires access to the boom base and cable routing path.

5. Installation and Replacement Procedure
Replacing a failed sensor involves a few steps. First, the boom should be fully retracted and rested on a stable support. The electrical connector is unplugged from the harness. The mounting bolts are removed and the old unit is taken out. The new sensor is installed using the original bolt holes. The cable is routed to the boom tip anchor point. The electrical plug is reconnected to the harness. After installation, the LMI system requires a length calibration to match the sensor output to actual boom extension. This calibration is performed using the crane's diagnostic interface or service tool.

6. Common Failure Signs
Operators may notice an inconsistent length reading on the cab display. The LMI may trigger a fault code related to boom length. The reading may jump erratically or remain fixed at zero regardless of boom movement. These symptoms usually point to a failing sensor, a broken cable, or a damaged connector. A physical inspection can reveal cable damage or loose mounting.

7. Conclusion
The HG-WY-3000-V length sensor is a straightforward but necessary component on SANY truck cranes. It provides reliable boom length data to the LMI system. When the sensor functions correctly, the LMI can perform its safety calculations accurately. Regular inspection and prompt replacement of faulty units help keep the crane's safety systems in proper working order.