SHAFT BARREL PISTON PORT PLATE CASE DRAIN PORT RETURN PORT SUPPLY PORT SPEED SENSOR 48
Shown is a sectional view of the fixed displacement, bent-axis fan drive motor. The motor is rotated by flow from the fan drive pump. Oil flows through the supply port and the port plate and pushes the pistons out of the barrel. The pistons force the barrel and the output shaft to rotate. The output shaft turns the fan. As the barrel rotates and the pistons return, oil flows from the pistons through the port plate, the return port, and a makeup valve to the fan drive tank. Oil that leaks past the pistons into the motor housing provides lubrication for the rotating motor components. This oil leakage is referred to as case drain oil. Case drain oil flows through the case drain port and a case drain oil filter to the fan drive tank.
The fan drive motor speed sensor provides an input signal to the Brake ECM. The Brake ECM uses this input to maintain the desired fan speed. There are 67 teeth on the plate that the speed sensor uses to determine the motor speed
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Shown is the front of the 793C XQ truck engine with the radiator assembly removed. The fan drive hydraulic tank (1), the fan drive pump (2), and the fan drive case drain filter (3) can be seen.
A fan drive oil temperature sensor (4) is installed in the hydraulic tank and provides an input signal to the Brake ECM. The Brake ECM sends the signal to the VIMS, which informs the operator of the fan drive system oil temperature. If the fan drive system temperature increases above 107°C (225°F), the Brake ECM will log an oil temperature too high event.
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Shown is the fan drive hydraulic tank (1) located on the frame near the right front of the engine. The oil level is checked at the sight gauge (2). The oil level should first be checked with cold oil and the engine stopped. The level should again be checked with warm oil and the engine
running.
When filling the hydraulic tank after an oil change, fill the tank with oil to the FULL COLD mark on the sight gauge. Turn on the engine manual shutdown switch so the engine will not start. Crank the engine for approximately 15 seconds. The oil level will decrease as oil fills the fan drive hydraulic system. Add more oil to the tank to raise the oil level to the FULL COLD mark. Crank the engine for an additional 15 seconds. Repeat this step as required until the oil level stabilizes at the FULL COLD mark.
Ensure that the pump and motor cases are filled with oil and bleed the air out of the pump and motors before starting the engine.
Turn off the engine manual shutdown switch and start the engine. Warm the hydraulic oil. Add more oil to the tank as required to raise the oil level to the FULL WARM mark.
Use 30W Transmission Drive Train Oil (TDTO) oil in the fan drive hydraulic system. Change the oil every 2000 hours. If Scheduled Oil Sampling (S•O•S) is used, the oil change interval can maybe be extended to 4000 hours.
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Fan drive hydraulic system oil samples can be taken at the Scheduled Oil Sampling (S•O•S) tap (not shown) located in the case drain oil filter base.
The fan drive return oil filter (3) is located in the tank. The oil filter should be changed every 500 hours. A bypass valve is located in the filter housing. The oil filter bypass valve will open if the oil filter restriction is greater than 170 kPa (25 psi).
Before removing the cap to add oil to the fan drive hydraulic system, depress the pressure release button on the breather (4) to release any pressure from the tank. Inspect the hydraulic tank breather for plugging.
The fan drive pump and motors case drain filter (5) is located above the fan drive hydraulic tank. The case drain oil filter should be changed every 500 hours. An oil filter bypass switch is located on the oil filter base. If the filter restriction exceeds 140 kPa (20 psi), the oil filter bypass switch provides an input signal to the Brake ECM. The Brake ECM sends the signal to the VIMS, which informs the operator that the filter is restricted. A bypass valve is also located on the case drain oil filter base. The case drain oil filter bypass valve will open if the oil filter restriction is greater than 170 kPa (25 psi).
NOTICE
Failure to correctly fill the hydraulic tank and the pump and motor cases after an oil change may cause component damage.
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Shown is the fan drive pump located at the front of the engine. The fan drive pump is a variable displacement piston-type pump. The Brake ECM controls the flow of oil from the fan drive pump by energizing the displacement solenoid (1).
The Brake ECM analyzes the temperatures and the air conditioning status and sends between 0 and 640 milliamps to the solenoid. Between 0 to 220 milliamps, the pump is at maximum displacement and the fan speed is at maximum. Between 600 to 640 milliamps, the pump is at minimum displacement and the fan speed is at minimum.
The coil resistance through the solenoid is approximately 24 ohms.
The displacement solenoid moves a spool in the pressure and flow compensator valve (2) to control the flow of pump output pressure to the minimum angle actuator piston. The minimum angle actuator piston moves the swashplate to the minimum flow position. The current
adjustment screw (3) controls the minimum current required to start destroking the pump. The present setting is 220 milliamps. Do not adjust the current adjustment screw in chassis. This adjustment should only be done on a hydraulic test stand.
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The high pressure cut-off valve (4) controls the maximum pressure in the fan drive system. The high pressure cut-off valve controls maximum pressure by controlling the flow of pump output pressure to the minimum angle actuator piston. When system pressure is at maximum, the high pressure cut-off valve sends oil to the minimum angle actuator piston and moves the swashplate to destroke the pump only enough to maintain that maximum pressure. In the fan drive system, it does not go to minimum stroke because the fans are still turning. The pump will only
destroke a small percentage in order to hold the high pressure. At sea level the high pressure cut-off valve setting is 27000 ± 345 kPa (3920 ± 50 psi).
The high pressure cut-off valve setting must be set lower at higher altitudes. If the fan drive pump solenoid is disconnected and the engine is run at high idle, the fan speed would be too high if the high pressure cut-off valve setting is too high.
The fan drive pressure tap (5) is used to measure fan drive pump pressure.
The minimum angle stop screw (6) is located near the pressure and flow compensator valve. The maximum angle stop screw is located on the other side of the pump. Do not adjust the minimum or maximum angle stop screws in chassis. This adjustment should only be done on a hydraulic test stand.
Pump cut-off pressure should be set at least 2070 kPa (300 psi) greater than the pressure required to maintain the maximum fan speed (625 rpm) with the fan drive pump solenoid unplugged. This will vary with the elevation above sea level.
When accelerating from LOW IDLE to HIGH IDLE, the fan drive pressure will spike to start the fan rotation. The spike pressure may be the pump high pressure cut-off setting. When at low idle and you are requesting maximum fan speed, the pump will be at maximum stroke. When you go to high idle, the pump will remain at full stroke until the 625 rpm maximum fan speed is exceeded. After 625 rpm is exceeded, the control will start stroking the pump back to the proper displacement for this fan speed. This will take several seconds and fan speed may exceed 700 rpm. The pump/motor combination may result in fan speeds as high as 775 rpm if the pump cut-off pressure is set high enough.
To determine the correct high pressure cut-off setting at altitudes above sea level, use ET to override the hydraulic fan speed to 625 rpm, raise the engine speed to HIGH IDLE, and record the pump pressure and fan speeds. We now know what pressure is required to rotate the fans at 625 rpm at the current altitude. For example, at our current altitude, it required approximately 16675 kPa (2420 psi) to rotate the fans at 625 rpm. Pump cut-off pressure should be set a minimum of 2070 kPa (300 psi) above the pressure required to maintain the maximum fan speed (625 rpm) with the solenoid unplugged. This will vary with the elevation above sea level. So, at this altitude we should set the pump cut-off pressure to a minimum of 18740 kPa (2720 psi).
PRESSURE AND FLOW COMPENSATOR MINIMUM ANGLE STOP DRIVE SHAFT SWASHPLATE MAXIMUM ANGLE STOP ROTARY GROUP CHARGE PUMP IMPELLER MINIMUM ANGLE ACTUATOR PISTON MAXIMUM ANGLE ACTUATOR PISTON PISTON