RELATED APPLICATION AND CLAIM OF PRIORITY 
       [0001]    The present application is a continuation-in-part under 35 USC §120 of U.S. patent application Ser. No. 13/907,407 filed May 31, 2013 and claims further priority to U.S. Provisional Patent application 61/704,220 filed on Sep. 21, 2012, under 35 USC §119(e) both of which are hereby incorporated by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to operation of heat, ventilation, and air conditioning (HVAC) equipment. More particularly, the disclosure pertains to power saving during HVAC operation and identification of fault conditions as applied to earthmoving equipment. 
       BACKGROUND 
       [0003]    A piece of heavy equipment, such as earthmoving equipment, is often operated in very adverse conditions, including dust or mud, extreme temperatures, high winds, etc. A heating, ventilating, and air conditioning (HVAC) unit must both operate in these conditions and protect itself from the hazards posed by these conditions. 
         [0004]    However, diagnosis of problems such as clogged fans or low levels of refrigerant is difficult and the inability to diagnose such problems creates a risk of severe damage to expensive HVAC components such as the compressor. 
         [0005]    U.S. Pat. No. 5,289,692 (the &#39;692 patent) describes a heat transfer system that controls operation of a compressor, expansion valve, and whether a compressor fan is off or on based on the working fluid quality and high side fluid pressure. The &#39;692 patent does not contemplate continuous control of the compressor fan speed while in a normal operating range or reversing the fan when the pressure increases beyond a limit. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    In a first embodiment, a method of operating a heat, ventilation, and air conditioning (HVAC) unit operated in an earthmoving machine senses an ambient temperate at the earthmoving machine, senses a compressor output pressure, and senses a compressor input pressure for use in determining operating and fault modes. During a time period when the compressor output pressure is in an operating pressure range, a speed of a condenser fan speed is controlled responsive to a combination of the ambient temperature and the compressor output pressure; and further operates the condenser fan in a reverse direction at maximum reverse speed responsive to the compressor output pressure being above the operating pressure range. 
         [0007]    In a second embodiment, a controller for use in HVAC equipment operated in an earthmoving machine includes a compressor control signal output, a condenser fan control signal output, an alarm output, a first pressure sensor input that receives an output pressure of a compressor, a second pressure sensor input that receives a suction pressure between the evaporator and the compressor, and a temperature input that receives an ambient air temperature. The controller further includes a processor coupled to the compressor control signal output, the condenser fan control signal output, the alarm output, the first pressure sensor input, a second pressure sensor, and the temperature input, and a computer-readable memory that stores i) operating characteristics including condenser fan control data and ii) instructions. When executed by the processor, the instructions cause the controller to turn on the compressor via the compressor control signal output based on a request for air conditioning and an ambient temperature that is above a first threshold temperature, select a condenser fan speed from the condenser fan control data responsive to the ambient air temperature and the output pressure of the compressor, and set the condenser fan speed to the selected condenser fan speed via the condenser fan control signal. Subsequent to setting the condenser fan speed, the controller may send an alarm signal via the alarm output when the suction pressure is less than a threshold suction pressure level and the ambient temperature is above a second threshold temperature. 
         [0008]    In another embodiment, an HVAC unit operated in an earthmoving machine may include a compressor that is selectively operable responsive to a compressor control signal, a condenser fluidly coupled to the compressor, an expansion valve fluidly coupled to the condenser, and an evaporator fluidly coupled to the expansion valve and the compressor. The HVAC unit may also include a first pressure sensor that measures output pressure of the compressor, a second pressure sensor that measures suction pressure between the evaporator and the compressor, an ambient temperature sensor, a condenser fan that provides cooling to the condenser, the condenser fan having variable speed and reversible direction responsive to a condenser fan control signal, and a controller electrically coupled to the first pressure sensor, the second pressure sensor, the ambient temperature sensor, and the fan, the controller configured to: i) provide the compressor control signal to selectively operate the compressor, and ii) provide the fan control signal. The controller may include a processor, a compressor control signal output, a condenser fan control signal output, a first pressure sensor input, a second pressure sensor input, an ambient temperature input, a computer-readable memory that stores instructions. When executed by the processor the instructions may cause the controller to turn on the compressor via the compressor control signal output based on a request for air conditioning and an ambient temperature is above a first threshold temperature, select a condenser fan speed from the condenser fan control data responsive to the ambient air temperature and the output pressure of the compressor, set the condenser fan speed to the selected condenser fan speed via the condenser fan control signal, and send an alarm signal via the alarm output when the suction pressure is less than a threshold suction pressure level and the current operating ambient temperature is above a second threshold temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a view of a representative earthmoving machine including an HVAC unit; 
           [0010]      FIG. 2  a block diagram of an exemplary HVAC unit; 
           [0011]      FIG. 3  is a block diagram of an exemplary HVAC controller; 
           [0012]      FIG. 4  is a flow chart of an exemplary method of high level operation of the HVAC unit; 
           [0013]      FIG. 5  is a flow chart of an exemplary method of determining a low suction fault; 
           [0014]      FIG. 6  is a flow chart of an exemplary method of determining condenser fan operation; 
           [0015]      FIG. 7  is an exemplary map of fan speed and direction to compressor discharge pressure; 
           [0016]      FIG. 8  is another exemplary map of fan speed and direction to discharge pressure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  illustrates a representative earthmoving machine  10 . The earthmoving machine  10  may include a hydraulically operated arm  12  and a bucket  14  for scooping and discharging payload. The earthmoving machine  10  main include tracks  16  for propelling the unit and an engine  18  that provides both electrical and mechanical power, a cab  20  for an operator, and an HVAC unit  22 . 
         [0018]    The earthmoving machine  10  may be any of a number of fixed or mobile machines, including, but not limited to, hydraulic shovels, dozers, mining, agriculture, off-road trucks, on-road trucks, etc. The HVAC unit  22 , while presented in a mobile machine environment, may be used in virtually any HVAC environment including industrial, commercial, and residential units. 
         [0019]    In operation, the HVAC unit  22  may provide cooling for operator comfort but may also provide low humidity air for use in removing or preventing fogging on the windows of the cab  20 . In such cases, an air conditioning function of the HVAC unit may be operated even when ambient air temperatures are lower than those typically associated with air conditioning usage. 
         [0020]      FIG. 2  illustrates an exemplary HVAC unit  22 . HVAC unit  22  may include a compressor  52 , a condenser  54 , an expansion valve  56 , and evaporator  58 . Connecting these components are respective lines  60 ,  62 ,  64 , and  66 . These lines fluidly couple the compressor  52 , condenser  54 , expansion valve  56 , and evaporator  58  and allow refrigerant in the form of liquid, gas, or a combination of liquid and gas, to flow between the elements as shown. A clutch  68  or similar device controls operation of the compressor  52 . A condenser fan  70  provides cooling to the condenser  54  and, as described below, may be used to affect discharge pressure of the compressor  52 . In an embodiment, a brushless direct current (BLDC) fan may be used instead of a more common alternating current (AC) fan. A BLDC fan allows continuous speed settings from full reverse to full forward. This provides a full range of control as discussed below with respect to  FIG. 6  and  FIG. 7 . A DC brush-type fan embodiment may use a resistor block to provide a few discrete fan speeds. 
         [0021]    A controller  72  receives inputs from a discharge pressure sensor  74  and a suction pressure sensor  76  as well having outputs that control the clutch  68  and the condenser fan  70 . 
         [0022]    Because in a standard refrigeration cycle the input and output of the condenser  54  are at a constant pressure, the pressure in lines  60  and  62  are generally equivalent. A pressure sensor  74 , also referred to as a discharge pressure sensor, is shown coupled to line  62  but in an alternative embodiment, a pressure sensor  74   a  coupled to line  60  may be used instead of or in addition to the pressure sensor  74 . Similarly, the inputs and outputs of the evaporator  58  are generally at a constant pressure and a pressure sensor  76 , also referred to as a suction pressure sensor, may be used instead to read suction pressure. Similarly, in an alternative embodiment, a pressure sensor  76   a  on line  66  may be used instead of or in addition to pressure sensor  76 . 
         [0023]    A temperature sensor  78  may provide ambient temperature readings to the controller  72 . In some embodiments, the condenser fan  70  may also provide feedback to the controller  72  as to the operational status of the fan, such as direction and speed. A network connection  80  may be used to connect the controller  72  to an external device such as an engine computer, a control panel, a dashboard, etc., and may be used to communicate inbound information such as requests for air conditioning and/or outbound information such as alarms. 
         [0024]    In operation, the well-known refrigeration cycle involves the compressor  52  compressing a vapor phase refrigerant and the condenser  54  cooling the vapor to a liquid or vapor/liquid mix. The pressure at the output of the compressor  52  is a function of the amount of cooling provided by the condenser  54 . In turn, the amount of cooling provided by the condenser  54  is a function of the condenser fan  70  and the ambient temperature. When the condenser fan  70  operates at maximum speed, heat transfer from the condenser  54  to the outside air is increased, which generally lowers the output pressure of the compressor  52 . A condenser  54  typically has fins surrounding cooling coils (not depicted) that provide a large surface area for heat transfer. When the coils and/or fins are clean and unobstructed, airflow is optimized and improves cooling performance. 
         [0025]    Several conditions may affect performance and efficiency of the HVAC unit  22 . First, the condenser  54 , particularly fin structures, may become obstructed with dust or other debris that both reduce airflow through the condenser  54  and provide an unwanted insulator that negatively affects the fin-to-air heat transfer. By sensing compressor output pressure, it is possible to adjust the fan speed to compensate for partially obstructed fins. That is, as the compressor output pressure builds, the fan speed can be increased to attempt to maintain a lower compressor output pressure. When fan speed is at a maximum and the compressor output pressure rises above a threshold limit, the compressor  52  may be stopped by disengaging the clutch  68  and the condenser fan  70  may be reversed in an attempt to blow out accumulated dust or debris. 
         [0026]    Second, when operating in relatively low ambient temperatures, or other conditions when the cooling load is low, the condenser  54  may provide too much cooling that can cause icing of the evaporator  58  causing in rapid compressor cycling and increased wear resulting ultimately in premature failure. Another effect of icing is difficulty in effective control of the system. Further, running the condenser fan  70  at full speed regardless of cooling requirements wastes engine horsepower and increases operating costs. Third, in order for the compressor to operate correctly, the amount of refrigerant must be maintained above a minimum level. Operating with low refrigerant levels for even a relatively short period of time can cause a mechanical failure of the compressor  52  that is typically extremely expensive to repair. However, sensing low refrigerant levels is very difficult because the system pressures may be maintained even as refrigerant is lost. Similarly, sensing cooling efficiency changes is difficult because lower cooling efficiency due to loss of refrigerant may be masked by other conditions such as condenser dirt or debris. However, to overcome the difficulty of sensing refrigerant levels, evaluating a combination of ambient temperature and suction pressure at line  64  or  66  allows identification of low refrigerant levels, as discussed further below. 
         [0027]    Also described further below, when the condenser fan  70  is an adjustable speed fan, ambient temperature measurements, and monitoring system pressures allows a control strategy that addresses reduced condenser efficiency due to dirt or debris, adjusts the compressor pressure to environmental requirements, and identifies low refrigerant conditions. The use of an adjustable speed fan allows continuous adjustment of the fan speed over its full operating range to meet the cooling requirements of the system. Such use may result in energy savings over systems where the condenser fan runs at full speed in all conditions. Not only does this save energy but may also help prevent conditions such as evaporator icing due to overcooling at low ambient temperatures. 
         [0028]      FIG. 3  illustrates an exemplary HVAC controller  72  that may be the same as or similar to the controller  72  of  FIG. 2 . HVAC controller  72  may include a processor  102 , a memory  104 , a communication port  106 , a processor bus  108 , and an external network connection  110  coupled to the communication port  106 . The external network connection  110  may be the same as or similar to the network connection  80  of  FIG. 2 . 
         [0029]    The HVAC controller  72  may also include pressure sensor inputs  112 ,  114 , an ambient temperature sensor input  116 , and a fan status input  118 . Pressure sensor input  112  may provide data to the HVAC controller  72  from a discharge pressure sensor  74  and pressure sensor input  114  may provide data from a suction pressure sensor  76 . The HVAC controller  72  may also include one or more outputs, such as an output  120  that may be used to control the compressor clutch  68 , another output  122  that may be used to control the fan  70 , and an alarm output. In a brushless DC fan embodiment, the output  122  may be an analog output. The analog output may drive a separate DC voltage controller (not depicted) that physically drives the condenser fan  70 . Other embodiments of providing variable level outputs to a BLDC fan may be supported, such as a register driven digital to analog converter, etc. 
         [0030]    The memory  104  may include one or more kinds of physical volatile or nonvolatile computer-readable memory such as ROM, RAM, rotating media, flash memory, or other physical structures capable storing computer data readable by the processor  102 , but does not include propagated media such as carrier waves. The memory  104  may include modules or functions that when executed by the processor  102  cause various software or hardware operations to be performed. For example, the memory  104  may include an operating system  124  and utilities  126 , such as diagnostics or communication protocols. The memory  104  may also include program code  128  that may include one or more modules including an air-conditioning control module  130  as well as operating data and/or lookup tables  132  used by the air-conditioning control strategy as described in more detail below. 
         [0031]      FIG. 4  is a flow chart of a method  150  of operating an HVAC unit  22 , particularly with respect to detection of and response to certain error conditions. At a block  152 , an ambient temperature sensor  78  may provide an air temperature of the air surrounding the earthmoving machine  10  in which the HVAC unit  22  is installed. At a block  154 , pressure at an output of the compressor  52  may be measured by a pressure sensor  74  and reported to the controller  72 . As discussed above, even though pressure sensor  74  is coupled to line  62 , a pressure sensor coupled to line  60  would be expected to perform the same function as pressure sensor  74 . Similarly, at block  156 , a pressure sensor  76  may report pressure at the input of the evaporator  58  and return that pressure to the controller  72 . An equivalent pressure reading to that measured at line  64  would be expected at line  66  and may be used in some embodiments. 
         [0032]    At a block  158 , a determination of low suction may be performed both prior to starting the compressor  52  and during the operation of the compressor  52 . Low suction, e.g., on line  64  or line  66 , in combination with ambient temperature may be indicative of low refrigerant levels. Additionally, when the ambient temperature is low, for example, around freezing, the compressor  52  may be disabled to avoid competition with a heating unit (not depicted). The discussion below with respect to  FIG. 5  discusses the low suction determination. If there is no indication of low suction at block  158  the “no” branch may be taken from block  158  to block  160 . At block  160 , the ambient temperature and compressor output pressure measured at pressure sensor  74  may be used to determine a condenser fan speed and direction. Condenser fan speed and direction are discussed in more detail with respect to  FIG. 6  and  FIG. 7  below. Following execution at block  160 , an HVAC control strategy may continue at block  152  and repeat the above process. If, at block  158 , a low suction determination is made the “yes” branch from block  158  may be taken to block  162 . 
         [0033]    At block  162 , an alarm condition may be raised and an appropriate response taken. The response may include shutting off the compressor, lighting an indicator on a control panel, sending a message to an external service monitoring facility, etc. As discussed above, low refrigerant level is a significant source of damage in compressors and shutting down the compressor after such a fault is indicated may result in substantial cost savings. 
         [0034]      FIG. 5  is a flow chart of an exemplary method  170  of determining a low suction fault. At system initialization, execution may move from a block  172  to a block  174 . At block  174 , a determination of the presence and operating status of the ambient temperature sensor  78 , the discharge pressure sensor  74 , and the suction pressure sensor  76  may be made. If any of the sensors are not installed, or are not operating properly, the “no” branch from block  174  may be taken to block  182  and additional HVAC operations may continue, bypassing the low suction pressure function. Otherwise, the “yes” branch from block  174  may be taken to block  175 . At block  175 , if the compressor is already running, the ‘yes’ branch may be taken to block  178  where suction pressure as a function of ambient temperature are checked as described below. If, at block  175  the compressor is not running, the ‘no’ branch may be followed to block  176 . 
         [0035]    At block  176 , two checks may be performed prior to allowing the compressor to start. First, if the ambient temperature is near or below freezing (˜0° C.), compressor operation may be blocked to avoid competition with a heater (not depicted). Second, with respect to refrigerant charge level, the output of pressure sensor  74  may be checked to determine whether it is less than approximately  28  pounds per square inch gauge (PSIG). An ambient temperature as measured at temperature sensor  78  may be checked to determine whether an air temperature is greater than about 5° Celsius (C). If either condition fails, that is, if the ambient temperature is &lt;5° C. or the discharge pressure is greater than about 28 PSIG, the “no” branch from block  176  may be taken to block  182  and the HVAC unit may be operated normally. If both conditions are true, that is, discharge pressure is less than 28 PSIG and the ambient temperature is greater than 5° C. the “yes” branch may be taken from block  176  to block  180 . 
         [0036]    At block  180 , a low suction fault may be raised indicating a possible low refrigerant level and, as discussed above, an appropriate response may be taken, such as turning off the compressor  52 , sending an alarm, etc. 
         [0037]    At block  178 , suction pressure measured at pressure sensor  76  may be evaluated to determine whether it is less than a compressor minimum effective suction pressure, for example, in an embodiment this may be approximately 5 PSIG or 135 kiloPascals absolute (kPaA). Ambient temperature may be checked to determine whether the ambient temperature is greater than a system specific predetermined temperature, for example, about 18° C. Additionally, if both suction pressure and ambient temperature comparisons are true, a timer (not depicted) may be started to check for system stability. If the conditions of suction pressure being less than about five PSIG and the ambient temperature being greater than about 18° C. persists for 10 seconds or more, the “yes” branch may be taken from block  178  to block  180  and the low suction fault may be asserted. The 10 second time period may be more or less based on specific system and design criteria and current conditions. If at block  178  either condition is not true or if the fault conditions have not persisted for 10 seconds or more, the “no” branch from block  178  may be taken to block  182  and operations continued as normal. From block  182  execution of the control strategy may continue again at block  174 . It will be understood that the controller  72  may execute the method  170  in conjunction with other HVAC control strategies and fault detection techniques. 
         [0038]      FIG. 6  is a flow chart of an exemplary method  200  of determining condenser fan operation. At a block  202 , the condenser fan  70  may be set to a speed and direction according to an algorithm or predetermined map. Turning briefly to  FIG. 7 , an exemplary map of fan speed and direction  230  will be discussed in view of the method of  FIG. 6 . 
         [0039]    The map  230  of  FIG. 7  illustrates condenser fan speed and direction as a function of compressor discharge pressure. Fan speed is shown is going from −100%, or operation at full reverse, to +100% or operation at full forward. The map  230  further indicates fan speed and direction as a function of ambient temperature. A first plot  232  illustrates fan speed values at 32° F. while plot  234  illustrates fan speed values at 122° F. While only two temperature values are shown for ease of illustration, a plurality of fan speed value by temperature plots may be used in actual practice. 
         [0040]    To illustrate use of the map  230 , for a given operating condition, such as a discharge pressure of 150 PSIG and an ambient temperature of 32° F., a forward fan speed of ˜45% of maximum is indicated. For the same discharge pressure at a temperature of 122° F., a forward fan speed of approximately 55% of maximum is indicated. This follows logically in that a higher heat load is presented during the 122° F. operating condition than at the 32° F. operating condition. At low temperatures the sole function of the HVAC unit  22  may be defogging as opposed to heat removal. Other forward fan speeds are indicated for pressures up to, in this exemplary embodiment, about 310 PSIG where at either temperature the condenser fan  70  is operated at full forward capacity. When discharge pressure reaches approximately 325 PSIG, during operation at 32° F., the condenser fan  70  switches to full reverse operation. Similarly, when operating at 122° F. the full reverse operation of the condenser fan  70  occurs, in this exemplary embodiment, above about 350 PSIG. 
         [0041]    Note that in a condition such as progressive clogging of the condenser  70 , a continuous rise in compressor discharge pressure results in increasing fan speed until the maximum fan speed is reached. In an embodiment, the controller  72  may check that the fan has been operating at 100% forward capacity for a period of time before setting 100% reverse operation. This strategy may help ensure that a spike in discharge pressure does not prematurely trigger a reverse fan cycle. 
         [0042]    While the map  230  is depicted in  FIG. 7  as a chart, it is understood that in operation, the functional relationships illustrated between discharge pressure, temperature, and fan direction and speed may be expressed as equations, in a table, or in another known algorithmic sense. Also, the numbers and relationships expressed in the map  230  illustrate an exemplary embodiment and may vary by application based on the equipment, HVAC rating, compressor type, expected operating environment, etc., as would be understood by a person of ordinary skill in the art in view of this disclosure. 
         [0043]    To further illustrate an alternate operating strategy,  FIG. 8  illustrates another fan speed to discharge pressure map  250  for use at low temperatures, e.g., 32 degrees F. In this embodiment, the fan speed may remain constant at low discharge pressures in a situation where defogging may be the principal interest. By maintaining the fan speed over this low pressure range, the compressor is operated in a high efficiency region, with more continuous operation to reduce clutch operation cycling while preventing overcooling. This allows better matching of system operation to the load and improves reliability by reducing fan speed and cycling. 
         [0044]    Returning to  FIG. 6 , after initially setting the condenser fan speed and direction according to the map discussed above respect to  FIG. 6 , at block  204  a discharge pressure of the compressor  52  is checked to determine if it has arranged above a maximum pressure threshold. In an embodiment, the maximum pressure may be 425 PSIG. If the discharge pressure is below the maximum pressure threshold the “no” branch from block  204  may be taken to block  206 . At block  206 , a check is made to see whether the condenser fan  70  has been operating at maximum reverse for more than two minutes. If so, the “yes” branch from block  206  may be taken to block  208 . At block  208 , a determination may be made if the number of times the condenser fan  70  has been operated in reverse is less than a threshold number and if it has been greater than a predetermined time period since the last request for operation in reverse. The predetermined time may be set to allow for adequate cooling operation and to avoid such frequent reverse operation that it becomes a nuisance at the worksite. If both conditions are true the “yes” branch from block  208  may be taken to block  210  and the condenser fan  70  may be set to operate or continue to operate at 100% reverse. Typically, when the fan is operated at full reverse the compressor  52  is also turned off. If, at block  210 , the fan has been operating for two minutes at 100% reverse, the fan may be set to a forward speed, for example 100% forward and the compressor restarted before returning operation to block  202 . In the case where the reverse fan operation was successful and enough debris or dirt is removed to restore condenser functionality, at block  202  the condenser fan speed may be set according to the map value. If, however, the condenser remains blocked, the HVAC unit will eventually reach the fault condition at block  212  via repeated passes through block  208 . 
         [0045]    Returning to block  204 , if the compressor discharge pressure is less than a maximum setting for PSIG, for example, 425 PSIG, the “yes” branch from block  204  may be taken to block  210  and the fan operated according to the appropriate map. 
         [0046]    Returning to block  206 , if the request for condenser fan  70  operation has been in place for less than two minutes the “no” branch from block  206  may be taken to block  204  and operation in reverse may be continued. 
         [0047]    Returning to block  208 , if the number of times that a request has been made for operation of the condenser fan  70  in reverse exceeds a threshold, for example, 6 times in 24 hours, or if it has been less than 28 minutes since the last request for operation in reverse, the “no” branch from block  208  may be taken to block  212  and a fault may be set. As above, the fault may include stopping the compressor  52 , setting an alarm indicator, notifying a monitoring service, etc. 
       INDUSTRIAL APPLICABILITY 
       [0048]    In general, an HVAC unit in earthmoving or other large equipment fills requirements not only for operator comfort but, in the case of window defogging, also provides an essential safety feature. However, operation of the HVAC unit can unnecessarily divert energy from the main earthmoving task, increase operating cost, and expose the equipment owner to costly repairs when the HVAC unit is operated at high discharge pressures or with low refrigerant levels. The use of the disclosed system and methods allow optimizing condenser fan speed in view of heat load and ambient temperature to provide effective cooling while minimizing fan energy. The use of a brushless DC fan allows continuous speed adjustment over a full range of forward and reverse operation to reduce energy consumption by matching the fan speed to operating conditions. Additionally, the system and method allow identification of potential low refrigerant levels to help avoid compressor failure caused by low refrigerant levels. The use of this system and method can, over time, lead to improved equipment availability, lower cost operation, and ultimately, better profitability.
A heating, ventilation and air conditioning (HVAC) unit may both save energy and monitor for fault conditions by monitoring an ambient air temperature, compressor discharge pressure, and evaporator suction pressure and using these measurements to identify known conditions. Compressor discharge pressure and ambient temperature may be used as indices to a look up table or map to determine a speed and direction for a condenser fan. In some cases, the look up table may direct the fan to be reversed at full power in an attempt to remove a suspected buildup of dirt or debris at the condenser. Ambient temperature and evaporator suction pressure may be monitored for a combination of conditions that indicate low refrigerant levels, allowing the compressor to be turned off before damage to the compressor is sustained.