Hydraulic System 119
When the dual vane pump is turned on, the transmission electronic control unit (ECU) starts
monitoring vehicle and engine speed, and allows the vane pump to engage (or not). If the vehicle is
going faster than 15 mph (25 km/h) or if engine speed exceeds 900 rpm, the vane pump will not
engage. After the pump is engaged, it will stay engaged at any engine speed under 2300 rpm (for more
information, see Allison Transmission Parameters on page 205).
The first section of the vane pump, known as the body vane pump, located closer to the pump shaft,
powers all body functions (tailgate, body hoist, and packer) through the directional control valve (see
below). It is capable of delivering a flow of 20 gallons per minute (gpm) at 700 rpm. A dump valve
located on the chassis (see Figure 5-1) limits the flow to the valve to 45 gpm. All excess flow is sent
back to the hydraulic tank. When the pump is turned off, the oil returns to the hydraulic tank.
The second section of the vane pump, known as the arm vane pump, powers lifting arm functions and
other options through the proportional valve (see Proportional Valve on page 121). It is capable of
delivering a flow of 16 gpm at 700 rpm. A dump valve located at the pump outlet limits the flow
(see Figure 5-1) to the valve to 20 gpm. All excess flow is sent back directly to the pump inlet. When
the pump is turned off, all the oil returns to the inlet.
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