777F Off-Highway Truck Hydraulic System Caterpillar

Illustration 1	g01256745
(1) Hoist pump (2) Outlet side (3) Inlet side

Performance tests can be used for diagnosing poor performance and for finding the source of oil leakage inside the hydraulic system.

Problem: The hoist pump makes noise and the cylinder rods do not move evenly. Also, there is air bubbles in the oil.

Probable Cause

• The low pressure relief valve in the hoist control valve opens at low oil pressure.

• The high pressure relief valve in the hoist control valve opens at low oil pressure.

• There is a loose connection in the oil line on inlet side (3) of hoist pump (1).

• The hoist pump (1) is excessively worn.

Problem: The temperature of the oil is too hot.

Probable Causes

• The viscosity of the oil is incorrect.

• The low pressure relief valve in the hoist control valve opens at low oil pressure.

• The high pressure relief valve in the hoist control valve opens at low oil pressure.

• The hoist pump (1) is excessively worn.

• There is a restriction in an oil passage.

• The load on the system is too high.

Problem: The output of the hoist pump is too low.

Probable Cause

• The oil level of the hydraulic and transmission tank (torque converter, hoist, and brake section) is too low.

• The viscosity of the oil is incorrect.

• The hoist pump (1) is excessively worn.

Problem: The pressure of the oil is too low.

Probable Cause

• The low pressure relief valve in the hoist control valve opens at low oil pressure.

• The high pressure relief valve in the hoist control valve opens at low oil pressure.

• The hoist pump (1) is excessively worn.

• An O-ring seal failed in the hydraulic system.