Charge pump
The charge pump is necessary to supply cool fluid to the system, to maintain positive pressure in the
main system loop, to provide pressure to operate the control system, and to make up for internal leakage.
Charge pressure must be at its specified pressure under all conditions to prevent damage to the
transmission.
The charge pump is a fixed-displacement, gerotor type pump driven off the main pump shaft. The charge
relief valve limits charge pressure.
Pump charge system
Case
drain
line
Charge relief valve
Charge
pump
System
check
valves
Input
Inlet filter
Tank
PV PF
P104 121E
Charge relief valve
The charge relief valve on the pump maintains charge pressure at a designated level. A direct-acting
poppet valve relieves fluid when charge pressure reaches a certain level. This level is nominally set
referencing case pressure at 1500 rpm. This nominal setting assumes the pump is in neutral (zero flow). In
forward or reverse, charge pressure is lower. The pump model code specifies the charge relief valve
setting.
Charge pressure relief valve
Multi-function valves
The multi-function valve incorporates the system check valve, the pressure limiter valve, the high
pressure relief valve, and the bypass valve in a replaceable cartridge. These functions are described
separately. There are two multi-function valve cartridges in each Series 90 pump to handle functions in
either direction. See corresponding sections for adjustments and repairs.
Service Manual
Series 90 Pumps
Operation
520L0818 • Rev 0101 • August 2015 11
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