Fan Clutches 20.02 General Information
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Fan Clutches 20.02 General Information General Information switch’s set point. The switch then signals the solenoid valve to engage the fan drive. The fan pulls cooling air through the air conditioner The fan clutch engages and disengages the fan condenser coils, to cool the condenser/ clutch hub with the engine fan. When they are en- refrigerant and reduce refrigerant pressure. gaged, the fan turns, to pull cooling air through the radiator, the charge air cooler, and the air condition- • An optional manual override switch, located on ing condenser. When the fan clutch is disengaged, the vehicle dashboard. the engine does not turn the fan. This maintains The solenoid valve is the heart of the control system, proper cooling, and enables the air conditioning to opening and closing to regulate the air flow to the fan work. drive. It is a 3-way valve, having two inlet/exhaust A large engine-cooling fan may require 70 to 100 en- ports and one outlet port. Air pressure from the vehi- gine horsepower to spin it. That reduces power avail- cle’s air system feeds into one of the inlet ports (de- able to move the vehicle, adds to engine noise, and pending whether the system is normally open or nor- uses more fuel. The fan clutch disengages when the mally closed) and the outlet port is connected to the fan is not needed, so the vehicle can operate as effi- fan drive. The plunger connects the solenoid outlet ciently and quietly as possible. port to the normally-open port, on Freightliner ve- hicles. Turning the electric current to the solenoid Most Freightliner fan clutches are actuated by com- valve on and off causes a plunger inside the solenoid pressed air. A solenoid-controlled valve regulates the valve to move up and down. supply of compressed air to the fan clutch to engage and disengage the fan. Freightliner fan clutches nor- In the normally-open control system, when all mally disengage the fan when air is routed to them, switches are below their set points, they are open and engage the fan when the air supply is blocked. and no electrical current reaches the solenoid valve. For the specific conditions that should trigger the en- The solenoid valve has a 3/64-inch (1.2 mm) orifice gine fan, refer to your engine manufacturer’s specifi- to regulate the volume of air and ensure smooth en- cations. gagement and disengagement of the fan drive. In older vehicles, the fan clutch is controlled by an The normally-open valve outlet port is always con- electrical circuit containing three switches. The nected to the fan drive. switches are wired in parallel so that any one of Mounting the solenoid valve in a location away from them can trigger the closing of the solenoid valve, to the engine minimizes the valve’s exposure to exces- cut air pressure to the fan clutch and cause the cool- sive heat, vibration and contaminants, and prolongs ing fan to spin. See Fig. 1 the life of the solenoid valve. The three switches are: Newer vehicles monitor and control all engine func- • A temperature switch, sensing the engine cool- tions, including the fan drive, with the Electronic Con- ant temperature. In a normally-open tempera- trol Module (ECM). An ECM consists of a digital mi- ture switch, the electrical contacts are open croprocessor, random access memory (RAM), and when the temperature is below the set point, read only memory (ROM). The ROM contains the and closed when the temperature is above the computer’s program. Programmers can change a set point. vehicle’s control systems and performance, simply by • A refrigerant pressure switch is installed in the changing the ROM’s programming. vehicle’s air conditioning system. When the air conditioner is running, the refrigerant absorbs NOTE: Minimum fan-on time for air conditioner heat from the air in the cab, through the engagements should be no shorter than 120 evaporator. The heated refrigerant goes seconds, and the fan clutch should be set to through tubing to the air conditioning con- engage 10°F (12°C) above the temperature at denser, which is in front of the engine radiator. which the thermostat is fully open. Reducing the As the refrigerant heats, the pressure rises in number of times the fan clutch engages and the air conditioner high pressure line, until the disengages extends the life of the fan clutch. refrigerant pressure reaches the pressure Century Class Trucks Workshop Manual, Supplement 6, September 2004 050/1 20.02 Fan Clutches General Information 4 1 2 3 5 6 7 11/22/2005 f200690 1. Manual Override Switch 2. Coolant Temperature Switch 3. A/C Refrigerant Pressure Switch 4. Air Filter 5. Air Supply to Solenoid Valve 6. Solenoid Valve 7. Fan Clutch Fig. 1 ECM-equipped vehicles have no direct connection ECM systems are more reliable and simpler than in- between the sensors and the solenoid valve. Instead, dependent hard-wired systems. The computer pro- information from sensors and switches feeds into the gram sets the conditions when the fan will be en- ECM, and the ECM then controls the solenoid valve. gaged, and all sensors and actuators are wired to The solenoid valve controls the compressed air sup- the ECM, instead of to each other. This provides ply, to engage or disengage the fan clutch. See more efficient and appropriate component operation. Fig. 1. 050/2 Century Class Trucks Workshop Manual, Supplement 6, September 2004 Fan Clutches 20.02 General Information In an ECM-controlled system, a given sensor may NOTE: Kysor K22RA and Horton DriveMaster® affect several actuators and an actuator may be af- fan clutches engage and disengage differently fected by several sensors, depending on how the from Horton Advantage™ fan clutches: ECM is programmed. • Kysor K22RA and Horton DriveMaster fan The ECM computer monitors the data from the sen- clutches engage with a spring, and disengage sors. Then, it sends the appropriate signals to the with compressed air. When the fan clutch sole- controls and actuators based on programming logic. noid vents air pressure, the spring pushes the The ECM also sends status information to the opera- fan clutch into engagement, to spin the fan. tor’s warning lights and gauges. With this system, even though the compressed Sensors used in ECM systems are different from air supply may be interrupted, the fan contin- those used in non-ECM systems. In ECM systems, ues to turn. sensors only send digital measurements. In non- • Horton Advantage fan clutches use com- ECM systems, the sensors control switches, to open pressed air to engage, and a spring to disen- or close individual circuits. gage. When the fan clutch solenoid vents air Instead of the simple open/close type of sensor, pressure, the internal spring pushes the fan ECM systems use thermistors and sending units to clutch out of engagement. send signals to the ECM (temperature, pressure, On non-ECM vehicles with air conditioning, the fan speed, or whatever function is being sensed). For clutch solenoid valve is also connected to a fan- example, instead of a thermal switch opening or clos- cycling switch at the receiver-drier. When refrigerant ing at a preset temperature, ECMs use a tempera- pressure reaches the setting on the fan-cycling ture sensor to set exact voltage, which the logic pro- switch, the switch tells the solenoid valve to gram converts into an precise temperature power-up the fan. See Fig. 3. measurement. Instead of simply knowing if the cool- ant temperature is above or below the set point, for example, hotter than 190°F (88°C), the program Horton Advantage Fan Clutch knows the exact temperature; for example, 196.4°F (91.3°C). This makes control of the cooling system In a cold engine start with the Horton Advantage fan more precise and efficient. clutch, the solenoid valve restricts air pressure to the fan clutch, so it does not spin the fan. In an ECM-controlled system, the fan drive solenoid is not wired to the sensors, as it is in an older sys- When the coolant warms to the hot end of the oper- tem. Instead, it is wired to a relay controlled by the ating temperature range, the ECM triggers the sole- ECM. The ECM computer program compares the noid valve. The solenoid valve then supplies com- data from several sensors, and decides when to en- pressed air to engage the fan clutch. gage and disengage the fan drive. The program con- The vehicle compressed air supply should be clean, siders engine coolant temperature, air-conditioner’s dry, and between 90 and 120 psi (620 and 830 kPa) refrigerant pressure, intake-manifold air temperature, for proper operation. engine speed, and the engine brake status. When the engine temperature drops to the cut-off NOTE: Some vehicles have a manual override point, the fan clutch solenoid valve vents air pres- switch for the fan clutch on the dashboard. sure, and the clutch disengages. Air conditioned vehicles not equipped with an ECM On non-ECM vehicles with air conditioning, the fan usually have two sensors to control the fan automati- clutch solenoid valve is also controlled by a fan- cally: cycling switch at the receiver-drier. When the refriger- ant pressure reaches the setting on the fan-cycling • The coolant temperature switch (see Fig. 3). switch, the switch triggers the solenoid valve, engage • The air conditioning refrigerant pressure switch the fan clutch and spin the fan. at the receiver-drier (see Fig. 3). Horton Advantage fan clutches have System Sentry™ fuses, to protect the fan clutch bearings from over- heating.