789B Off-Highway Truck Hydraulic System Caterpillar

Illustration 1	g00321238
(1) Two-section gear pump for the hoist and rear brake cooling

Illustration 2	g00397683
(2) Four-section gear pump for the torque converter, parking brake release, and brake cooling

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

Problem : The hoist and rear brake cooling gear pump makes noise and the cylinder rods do not move evenly. Also, there are air bubbles in the oil.

Probable Cause

• At least one hoist relief valve opens at low oil pressure.

• There is a loose connection in the oil line of the inlet side of two-section gear pump (1) .

• The two-section gear pump (1) is too worn.

Problem : The temperature of the oil is too hot.

Probable Causes

• The viscosity of the oil is incorrect.

• At least one hoist relief valve opens at low oil pressure.

• The two-section gear pump (1) is too worn.

• The brake cooling section on the four-section gear pump (2) is too worn.

• There is a restriction in an oil passage.

• The load on the system is too high.

Problem : The output of the hoist and rear brake cooling gear pump is too low.

Probable Cause

• The hydraulic tank oil level is too low.

• The viscosity of the oil is incorrect.

• The two-section gear pump (1) is too worn.

• The brake cooling section on the four-section gear pump (2) is too worn.

Problem : The pressure of the oil is too low.

Probable Cause

• At least one hoist relief valve opens at low oil pressure.

• The two-section gear pump (1) is too worn.

• The four-section gear pump (2) is too worn.

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