A construction site reported that a Shantui SL50W wheel loader had a 3-second delay when shifting from Neutral to Forward. Additionally, once in gear, the machine would "hunt" (speed up and slow down rhythmically) when climbing a grade.
The technician connected pressure gauges to the 1st and 2nd speed clutch ports. The pressure built up slowly (taking 5 seconds to reach 1.5 MPa). When holding the speed on a hill, the pressure gauge needle oscillated wildly between 1.0 MPa and 1.5 MPa. This pressure fluctuation caused the clutch plates to slip and grab repeatedly, creating the "hunting" sensation.
He checked the "Transmission Modulating Valve" (also called the accumulator valve). This valve smoothens the shift. Upon removal, he found the "Modulating Spool" had worn grooves in the valve bore due to hydraulic abrasion. The wear allowed oil to leak past the spool, preventing it from building steady pressure.
Furthermore, he inspected the "Transmission Suction Hose." The hose clamp was loose. When the engine revved, the vibration caused the hose to suck air through the loose connection. The air bubbles compressed in the pump, causing the pressure to bounce erratically.
The repair involved honing the valve body and installing an oversized spool kit to restore the clearance. He also tightened the suction hose clamp and replaced the crushed hose. The hydraulic filter was changed to remove any metallic particles. After bleeding the air, the transmission engaged instantly, and the "hunting" disappeared, restoring smooth uphill performance.
