797B Off-Highway Truck Fan Drive Hydraulic System: Fan Drive Hydraulic System

Hydraulic System

Fan drive hydraulic system (1) Fan drive motor (2) Makeup valve (3) Fan drive pump (4) Pump drive (5) Pressure reducing valve (fan drive) (6) Oil filter (case drain) (7) Solenoid and relief valve (8) Oil filter (steering and fan drive) (9) Oil cooler (steering and fan drive)(10) Hydraulic tank

Location of Components

JSM1-168 Left side view of fan drive lines (2) Makeup valve (3) Fan drive pump (4) Pump drive (5) Pressure reducing valve (fan drive) (6) Oil filters (case drain) (7) Solenoid and relief valve(10) Hydraulic tank

JSM169-UP Left side view of fan drive lines (2) Makeup valve (3) Fan drive pump (4) Pump drive (5) Pressure reducing valve (fan drive) (6) Oil filters (case drain) (7) Solenoid and relief valve(10) Hydraulic tank

Rear view of fan drive lines (1) Fan drive motor (2) Makeup valve(11) Radiator

Operation

Oil flows from fan drive pump (3) through makeup valve (2) to fan drive motor (1). Oil flows from the motor through makeup valve (2). The oil flows from makeup valve (2) through oil filters (8) and oil cooler (9). The oil flows from oil cooler (9) to hydraulic tank (10) for the steering and fan drive.
Fan drive motor (1) is a fixed displacement motor. The fan speed is determined by the amount of flow from fan drive pump (3). Fan drive pump (3) is a variable displacement piston pump that is controlled by the Brake/Cooling ECM.
Case drain oil flows from fan drive motor (1) and fan drive pump (3) through oil filter (6) to the steering and fan drive hydraulic tank.
If the supply oil to the fan stops suddenly, the fan and the motor may continue to rotate because of the mass of the fan. Makeup valve (2) allows the oil in the circuit to flow from the return side to the supply side. This prevents a vacuum in the lines on the supply side.
Pressure reducing valve (5) utilizes steering system pressure in order to keep the fan speed below 10 rpm in cold weather. The fan speed should never be 0 rpm.
Fan speed is controlled by the Brake/Cooling ECM. Fan speed will vary depending on many inputs.
The minimum fan speed will be below 10 rpm. The minimum fan speed will occur when all of the temperature inputs are cold.
The following inputs determine the fan speed:

• Jacket water coolant temperature
• Aftercooler coolant temperature
• Transmission lube temperature
• Torque converter outlet oil temperature
• Brake oil temperature
• Brake status
• Ground speed
• Output status of the hoist system

Table 1

Standard Fan Speed Control Limits (RPM) 106 Inch Fan
Engine Speed	Non-Retarding Fan Speed Limit	Retarding Fan Speed Limit
1250 or Less	339	525
1300	353	525
1450	394	525
1500	407	525
1600	434	525
1700	461	525
1750 or more	475	525

Refer to Table 1 for the fan speed limits that are based on the temperature sensor inputs, the ground speed and the retarding/braking inputs.

Table 2

Engine Fan Control, Non-Retarding Or Retarding Below 4 MPH 106 Inch Fan (1)
Aftercooler Temperature	Jacket Water Coolant Temperature	Transmission Lube Temperature	TC Out Temperature	Brake Oil Temperature	Fan Control
Priority 1	2	3	4	5
< 65 °C (149 °F)	< 88 °C (190 °F)	< 88 °C (190 °F)	< 88 °C (190 °F)	< 102 °C (215 °F)	Off
< 65 °C (149 °F)	< 99 °C (210 °F) > 88 °C (190 °F)	< 96 °C (204 °F) > 88 °C (190 °F)	< 102 °C (216 °F) > 88 °C (190 °F)	< 107 °C (225 °F) > 102 °C (215 °F)	Modulate By Priority
> 65 °C (149 °F)	> 99 °C (210 °F)	> 96 °C (204 °F)	> 102 °C (215 °F)	> 107 °C (225 °F)	"Limit Chart"

( 1 )

When you use Retarding/Braking 4 mph and above 4 mph will set the fan speed to the "limit chart".

Refer to Table 2. If all of the temperature sensors are below the lower limits, then the fan control is turned off and the fan speed could be near 0 rpm. If the temperatures are between the sensor's lower limits and the sensor's upper limits, then the fan speed will be modulated by priority. If any of the temperatures are above the upper limits, the fan speed will be set to the limits that are shown in Table 1.
Note: When the body is being raised or lowered, the fan speed will be 200 rpm in order to reduce the load on the pump drive.

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