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| Illustration 1 | g03498796 |
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At front of the engine compartment (1) Air To Air Aftercooler (ATAAC) fan system | |
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| Illustration 2 | g03498981 |
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(3) Air to air cooler
(4) Fuel cooler | |
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| Illustration 3 | g03498863 |
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Located in the cooling compartment at the rear of the tractor, behind the cab. (2) Hydraulic demand fan system | |
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| Illustration 4 | g03498996 |
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(5) Radiator
(6) Output transfer gear oil cooler (7) Hydraulic oil cooler | |
There are two hydraulic cooling fan systems installed on the 725C, 730C and 730C EJ Articulated Trucks. The two systems are:
- Air To Air Aftercooler (ATAAC) fan system, which provides air flow through the ATAAC, and the fuel cooler.
- Hydraulic demand fan system, which provides air flow through the radiator, the Output Transfer Gear (OTG) oil cooler, and the hydraulic oil cooler.
The hydraulic demand fan is located in the cooling compartment, at the right rear of the tractor. The hydraulic demand fan motor is driven by hydraulic oil flow from the "Fan/Brake/Hoist" pump.
The speed of the hydraulic demand fan motor is regulated by the fan speed solenoid. The fan speed solenoid is controlled by the transmission ECM.
The transmission ECM controls the fan speed based on the coolant temperature, OTG oil temperature, transmission oil temperature, and torque converter oil temperature. The coolant temperature sensor is an input component to the engine ECM. The engine ECM shares the coolant temperature data via the "CAN A Data Link". The remaining temperature inputs are local to the Transmission ECM. If the demand fan speed needs to be adjusted, the transmission ECM increases or decreases the current to the demand fan speed solenoid. The demand fan speed solenoid is installed in the combination valve.
The return oil flow from the hydraulic demand fan motor flows to the hydraulic oil cooler. A pressure and thermal operated cooler bypass valve is installed in the large diameter horizontal tube at the top of the hydraulic oil cooler core. The bypass valve directs varying amounts of return oil through the oil cooler, depending on oil temperature and pressure. In general, the flow rate of oil through the oil cooler increases proportionally to the rise in oil temperature.
If the oil is cold, little oil will flow through the small diameter vertical tubes in the cooler core. This situation creates a back pressure ahead of the core, which could negatively impact the operation of the fan motor. This back pressure will OPEN the normally closed bypass valve. Cracking pressure or "opening pressure" for the bypass valve is approximately
The thermal bypass valve starts to OPEN at approximately
The "Fan/Brake/Hoist" pump is a load sensing pressure compensated piston-type hydraulic pump. The pump is driven by a gear in the right side of the pump drive housing. This pump supplies the oil necessary for operation of the hydraulic demand fan, and oil for the brake and hoist hydraulic systems.
Oil from the hydraulic tank is drawn into the pump inlet through hard steel line. Pump discharge oil is supplied to the combination valve via the top high-pressure hydraulic hose. The load sensing signal from the combination valve is returned to the pump control valve, through a load sensing signal line.
The highest load signal from the brake, hoist, or demand fan systems will cause the pump to "UPSTROKE". This process is to provide sufficient oil for the system requiring the most oil flow. The current to the demand fan speed solenoid from the transmission ECM will be increased or decreased to regulate the oil flow to the fan motor, regardless of the pump condition and flow rate.
Case drain oil from the fan/brake/hoist pump is directed to the case drain oil system. The oil is directed via the hydraulic hose on the outboard side of the pump.
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| Illustration 5 | g03499242 |
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(1) Pump (Fan, Brake, Hoist)
(2) Solenoid (demand fan speed) (3) Valve (Diverter) (4) Pressure port (fan drive) (5) Motor (Fan drive) (6) Valve (anti-cavitation) (7) Cooler (hydraulic oil) (8) Valve (thermal bypass) | |
Oil flow from pump (1) enters the Combination Valve at the P1 port. The oil for the demand fan circuit then flows to diverter valve (2) and out of the "F" port to fan drive motor (5).
Oil flow from the pump (1) enters the Combination Valve at the P1 port. The oil for the demand fan circuit then flows to the diverter valve (2) and out of the F port to the fan drive motor (5).
The normally open diverter valve senses pump pressure on the left. An orifice in the diverter valve directs system pressure to the spring side of the valve. This pressure becomes load sensing (signal) pressure that enters the resolver network in the Combination Valve. The load sensing signal causes the pump to UPSTROKE or DESTROKE. The diverter valve senses system pressure, signal pressure, and spring force at the same time and therefore, maintains the correct oil flow and pressure to the Fan Drive Motor, regardless of the system pressure and oil flow rate.
The fan speed solenoid (2) acts as a signal limiting valve and proportionally restricts or drains load sensing signal pressure to tank. This load sensing signal pressure also works with the spring in the Diverter Valve to maintain the correct oil flow to the fan motor. When no current is sent to the solenoid by the Transmission ECM, the full load sensing signal from the fan circuit is sent into the resolver network to the pump compensator. At the same time, the full load sensing signal pressure helps the spring keep the diverter valve open. When the current to the solenoid is increased, load sensing signal pressure to the pump is decreased. The decrease in signal pressure also causes the Diverter Valve to close down, DECREASING the oil flow to the fan motor, reducing the fan speed.
In the schematic above, no other circuit in the hydraulic system is creating a load sensing signal to the Fan/Brake/Hoist Pump control valve. The signal from the Diverter Valve, therefore, causes the pump to UPSTROKE to provide the correct oil flow for the desired fan speed. Because the oil is WARM, the Thermal Bypass Valve (8) is OPEN. This process causes the return oil from the fan motor to flow through the Hydraulic Oil Cooler (7).
In the condition shown above, the Transmission ECM is sending minimal current to the Fan Speed Solenoid and the load sensing signal is not being reduced. The speed of the fan is therefore regulated by the natural load signal from the fan circuit to the pump.
If the fan needs to turn slower, the Transmission ECM INCREASES the current to the solenoid, based on the temperature sensor inputs. When the solenoid is ENERGIZED, load sensing signal pressure is drained away. This process reduces the signal to the pump and allowing the Diverter Valve to restrict the flow rate to the Fan Drive Motor.
An Anti-Cavitation Valve (6), located inside the fan drive motor case, prevents fan motor cavitation when the engine stops and momentum causes the fan motor to continue rotating.
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| Illustration 6 | g03501403 |
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(1) Pump (Fan, Brake, Hoist)
(2) Solenoid (demand fan speed) (3) Valve (Diverter) (4) Pressure port (fan drive) (5) Motor (Fan drive) (6) Valve (anti-cavitation) (7) Cooler (hydraulic oil) (8) Valve (thermal bypass) | |
The schematic above shows the hydraulic system with the hoist being RAISED and the demand fan in the LOW SPEED condition, due to hydraulic oil and engine coolant being COLD.
The HOIST RAISE command is sending a high load sensing signal to Fan/Brake/Hoist Pump (1). This command causes the pump to UPSTROKE, to provide the pressure and flow required to RAISE the dump body on the truck. But because the hydraulic oil and engine coolant temperatures are COLD, Fan Drive Motor (6) needs to turn at LOW SPEED. The high signal pressure from the Hoist Control Valve to the pump control valve causes the pressure and flow in the hydraulic system to INCREASE. This condition would cause the fan to speed up beyond the cooling demand, given the temperature inputs.
To regulate the hydraulic demand fan to the correct speed, the Transmission ECM INCREASES the current to Fan Speed Solenoid (3). This process drains away some of the load sensing signal pressure from Diverter Valve (4). The lower pressure on the spring side of the diverter valve causes the valve to restrict the flow rate to the fan motor, caused by the higher load sensing signal from the hoist circuit. The restricted (reduced) flow rate to Fan Drive Motor (6) causes the fan to rotate at the correct speed. The higher signal pressure from the Hoist Control Valve is sent on to the pump control valve at the Resolver (9) in the Combination Valve.
This sequence of events allows the hoist to receive the pressure and flow from the pump that is needed to raise the hoist, according to the operator request, but still maintain the demand fan speed necessary for the given temperatures of the hydraulic oil and engine coolant.
A Fan Drive Pressure test port (4) is provided for calibrating the Fan Speed Solenoid. The test port is identified as "TPF," which is stamped in the valve body.
Pump pressure can be tested using the TP1 test port on the Combination Valve.
ATAAC Fan System Maximum Speed
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| Illustration 7 | g03501405 |
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(1) Fan pump (ATAAC)
(2) Engine (3) Fan speed solenoid valve (ATAAC) (4) Anti-cavitation valve (5) Fan motor (ATAAC) (6) Fan drive pressure (7) Hydraulic oil tank (8) Case drain port (9) Demand fan motor case drain | |
The illustration above shows the ATAAC fan hydraulic system at MAXIMUM SPEED, due to the intake air and fuel temperatures being HIGH.
ATAAC Fan Pump (1) draws oil from Hydraulic Oil Tank (7) and supplies oil to the inlet of ATAAC Fan Motor (5). Since the fan pump and motor are both a fixed displacement gear-type pump and motor, the oil flow rate is directly proportional to the speed of Engine (2). The Engine ECM monitors the fuel temperature sensor and the engine intake air temperature sensor to determine the demand for cooling. Given these two inputs, the Engine ECM sends a current to ATAAC Fan Speed Solenoid Valve (3). This process relieves oil pressure and flow from the ATAAC hydraulic circuit to control the fan speed.
In the schematic above, the temperatures are high, so the Engine ECM sends no current to the ATAAC Fan Speed Solenoid Valve. This process results in maximum fan speed. If the fan needs to turn slower, due to lower temperatures, the Engine ECM will send a current to ENERGIZE the proportional ATAAC Fan Speed Solenoid Valve. The next illustration shows the MINIMUM SPEED condition.
The fan motor has an Anti-Cavitation Valve (4) installed in the motor case. This valve prevents fan motor cavitation when the engine is shut off and momentum causes the fan to continue rotating.
Fan system pressure can be tested at the ATAAC Fan Drive Pressure test port (6).
Case Drain Port (8) can be used to prime the pump when needed and can also be used to test case drain oil pressure.
The case drain oil from the ATAAC fan motor joins with oil from Demand Fan Motor Case Drain (9) circuit before returning to the case drain oil filter and the hydraulic oil tank.
ATAAC Fan System Minimum Speed
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| Illustration 8 | g03501406 |
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(1) Fan pump (ATAAC)
(2) Engine (3) Fan speed solenoid valve (ATAAC) (4) Anti-cavitation valve (5) Fan motor (ATAAC) (6) Fan drive pressure (7) Hydraulic oil tank (8) Case drain port (9) Demand fan motor case drain | |
The illustration above shows the ATAAC fan hydraulic system at MINIMUM SPEED, due to the intake air and fuel temperatures being LOW.
Since ATAAC Fan Pump (1) is a fixed displacement gear-type pump, the oil flow rate to ATAAC Fan Motor (5) is directly proportional to the speed of Engine (2). If the speed of the engine, and therefore, the flow rate from the pump causes the fan to rotate faster than the demand for cooling, the Engine ECM will adjust to slow the fan. Since the Engine ECM monitors the fuel temperature sensor and the engine intake air temperature sensor to determine the demand for cooling, the ECM considers these temperatures and sends a current to ENERGIZE ATAAC Fan Speed Solenoid Valve (3). The amount of current sent to the solenoid is determined by the two temperatures, according to the temperature maps for this cooling system that are contained in the engine software (Flash File).
When current is applied to the Fan Speed Solenoid Valve, the valve relieves some of the oil pressure and flow to the ATAAC Fan Motor. This process slows the fan to meet the cooling demand. In the example above, the solenoid is fully ENERGIZED, slowing the ATAAC Fan Motor to MINIMUM SPEED. Since the solenoid is proportional, the speed of the ATAAC fan is infinitely variable between the MAXIMUM and MINIMUM speeds engineered into the system and software.