LINK AS 2045659 - Caterpillar

Boom
Turn the engine start switch key to the OFF position.
Raise the left console.
Turn the engine start switch key to the ON position.
Place the hydraulic lockout control in the LOCKED position.
Place the joystick into the BOOM DOWN position.If the accumulator still has a charge, the boom will lower.If the boom fails to lower, use the boom lowering control valve procedure below to lower the boom.Boom Lowering Control Valve
If the engine is stopped or the hydraulic system is disabled, the operator can still lower the boom. Use the following procedure if the machine is equipped with a boom lowering control device:
Boom load may cause cylinder oil pressure of the boom lowering control device when the boom is supported by one cylinder. Boom can lower suddenly.Be sure no one is under or near the front equipment before manually lowering the boom.Keep all personnel away from the boom drop area when lowering the boom with the engine stopped.
Illustration 1 g06189901Note: The boom lowering control device is at the base of the boom cylinder. The boom lowering control device allows the operator to manually lower the boom if the engine is stopped. The boom lowering control device also prevents a sudden drop of the boom if there is an oil leak in the hydraulic line of the boom.
Illustration 2 g02490136
Loosen the locknut.
Slowly, turn the screw clockwise.The boom will lower to the ground when the boom lowering control device is opened.
After the work tool has lowered completely onto the ground, turn the screw back to the original position. Tighten the locknut.
Make any necessary repairs before placing the excavator back into service.Note: Consult your Caterpillar dealer for further information.