CHANNEL 4532536 - Caterpillar

current Caterpillar machines are equipped with hydraulic systems for the operation of implements and other circuits. These hydraulic systems have different designs for different applications, and some systems have characteristics which can cause a service problem. When a machine is in the shop and the engine can not be run, but the blade, bucket or other implement must be lifted, special procedures must be used with some hydraulic systems. To be sure safe service procedures are followed, the serviceman must understand the basic design and characteristics of the system and use caution during hoist operation.
For example, in pilot operated hydraulic systems, pilot pressure is used to operate the valve spools in the main control valve. Since there is no pilot pressure when the engine is stopped, movement of the operator's control lever will not move the control valve spool. If the control lever is moved from "HOLD" to "FLOAT" position, the valve spool will still be in the "HOLD" position. The effect of this characteristic is to cause a hydraulic lock in the circuit when the engine is stopped, and the implement can not be lifted unless the hydraulic pressure is released.
Some other hydraulic systems have similar characteristics for other reasons. For example, on series G motor graders, the lock valves will cause a hydraulic lock in the circuit.
On any machine that has this characteristic of a hydraulic lock in the circuit when the engine is stopped, the hydraulic pressure must be released before the circuit can be mechanically activated. If the blade, bucket, or other implement is to be lifted with a hoist or other method, and the hydraulic lock is not released, the front (or rear) of the complete machine will be lifted. This can cause an overload of the hoist or other equipment.
To release the hydraulic lock in a circuit, different procedures are necessary for different hydraulic systems. On some pilot operated systems, a pilot line can be disconnected. On others, it will be necessary to disconnect cylinder lines. On series G motor graders, the lines can be disconnected at the lock valves. The schematic diagram and other hydraulic system information in the Service Manual for the machine can be used as a reference.