A beverage distributor faced a major operational headache with a Clark GEX25 internal combustion pneumatic tire forklift. Whenever the operator shifted from neutral into forward gear, the truck would slam into engagement with a violent jerk, jarring the operator and threatening to destabilize the load. Shifting into reverse was smooth, and the truck drove perfectly once in gear. This harsh engagement was not only uncomfortable but was rapidly destroying the U-joints on the drive axle.
The Clark GEX25 utilizes a power-shift transmission with a modulating valve specifically designed to cushion the shift. The modulating valve meters the hydraulic pressure to the forward clutch pack, causing it to ramp up over a fraction of a second rather than applying instantly. A diagnostic gauge was plumbed into the forward clutch apply port. When the shift was made, the pressure spiked instantly from 0 to 180 psi, with zero ramp-up time, confirming a modulating valve failure.
The transmission control valve was removed from the side of the transmission. The modulating valve spool was extracted and found to be heavily varnished and stuck in the open position. The varnish was a result of overheated, degraded automatic transmission fluid (ATF) that had not been changed in over 2,000 hours. Because the spool couldn't move to restrict the flow, the clutch received full line pressure instantly, causing the harsh engagement.
However, the failure didn't stop at the valve. The sudden, shock-loading of the forward clutch over months had taken its toll. Dropping the transmission oil pan revealed a thick layer of friction material and aluminum shavings on the magnetic drain plug. The forward clutch pack was slipping slightly upon initial engagement before biting hard, a symptom of glazed friction discs. The sharp pressure spikes had also cracked the forward clutch piston seals, allowing a slight internal leak that forced the modulating valve to compensate by dumping more pressure quickly.
The fix required a complete transmission overhaul. The forward clutch pack was rebuilt with new friction discs, separator plates, and piston seals. The control valve body was chemically cleaned to remove all varnish, and the modulating valve spool was polished and tested for free movement in its bore. A new transmission oil cooler was installed, as the old cooler's internal fins were found clogged with debris, which had contributed to the initial fluid overheating. After refilling with fresh Mercon V ATF and purging the torque converter, the forklift shifted into forward with a smooth, progressive engagement, restoring drivability and preventing further drivetrain damage.
