Abstract:
A microprocessor controlled vehicle relay for high capacity HVAC systems in large high occupancy vehicles is provided that can be configured in different ways to provide a number of operational functions for improved safety as well as greater motor and/or compressor longevity.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention is related to the field of microprocessor controlled relays for motors and compressors, and more particularly, to a microprocessor controlled vehicle relay for HVAC systems installed in buses and other high occupancy vehicles that can be configured in different ways to provide a number of operational functions for improved safety as well as greater motor and/or compressor longevity. 
     Description of the Related Art 
     Climate control systems such as air conditioning systems in vehicles draw significant current upon initial start up. In large high occupancy vehicles such as buses which require heavy duty systems to provide the necessary cooling, the initial in-rush of current to the DC fan motor upon start-up of the air conditioning system can be on the order of 60 amps or more. This large inrush current could create a safety hazard under certain circumstances and also requires that the system include a large fuse able to withstand the surge in current. Should the motor fail, the high amount of current can create a dangerous condition and may overheat the fan motor to the point of catastrophic failure. 
     Accordingly, a need exists for a vehicle relay that reduces the inrush current to the DC fan motor upon climate control system start-up in buses and other high occupancy commercial vehicles to improve safety while also eliminating the need for large fuses and increasing motor longevity. 
     In addition, it would be advantageous to have a relay for a commercial high occupancy vehicle that is installed and able to protect against HVAC compressor overspeed, undervoltage to the compressor clutch in an HVAC system, and excessive cycling of the HVAC system compressor for extended motor life. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention is directed to an microprocessor controlled vehicle relay for high occupancy vehicles, such as commercial buses and the like, that have a large heavy duty HVAC or climate control system. The terms “climate control system” and “HVAC” are used interchangeably herein, with both referring to heating, ventilating and air conditioning systems installed, with respect to the present invention, in large vehicles. Large vehicles are those vehicles having a high occupancy capacity such as buses, whether commercial or private, including charter buses, airport shuttle buses and the like. 
     The vehicle relay includes a printed circuit board (PCB) having an input terminal, an output terminal, a plurality of transistors and a bypass relay circuit having an inductor and a switch. The bypass relay circuit is controlled by a microprocessor. The input terminal is connected to a power source and the output terminal supplies voltage to a fan motor or the like in a functional operating system such as the climate control system or the like. As used herein, the foregoing structural components are referred to as “the core vehicle relay components” or “the core components”. The core vehicle relay components may be configured to include one or more additional components as will be described hereinafter according to the function that the relay is to perform. 
     According to a first embodiment of the present invention, the core vehicle relay components further include a negative temperature coefficient (NTC) resistor downstream of the input terminal. When the climate control system is off, the switch in the bypass relay is open. The microprocessor is programmed with an algorithm that controls a start up sequence for the fan motor that is connected to the output terminal of the PCB. Upon climate control system start-up, power is applied to the PCB input terminal. Because the switch in the bypass relay is open, current flows to the output terminal through the NTC resistor which reduces the amount of current that is able to initially rush into the fan motor. After a predetermined delay, the microprocessor energizes one or more transistors which energize the bypass relay circuit to close the switch. Once the switch is closed, the NTC resistor is bypassed by the bypass relay circuit and maximum operating voltage is then applied to the fan motor(s) of the HVAC system. 
     The core vehicle relay components according to the present invention may also be configured to perform other operational functions that reduce wear on the motor and/or compressor clutch of the climate control system. These functions include protection against compressor overspeed, undervoltage to the compressor clutch, and excessive cycling of the compressor. 
     Particularly, according to a second embodiment of the present invention, the vehicle relay is configured to prevent overspeed of the HVAC compressor due to overly high engine RPM. The core vehicle relay components are provided with a fourth terminal that is electrically connected to the microprocessor through a resistor. The output terminal is connected to the compressor clutch of the climate control system. While the climate control system is working, the switch in the bypass relay circuit is closed, allowing current to flow to the compressor and keep the clutch engaged. The fourth terminal provides an engine RPM input signal to the microprocessor. When the input signal received by the microprocessor indicates that the engine RPM has exceeded the compressor rating, the microprocessor de-energizes one or more transistors and the bypass relay circuit, opening the switch and interrupting current flow to disengage the clutch from the compressor. The high engine RPM is thus prevented from overloading and damaging the compressor. Once the engine RPM falls below the compressor rating, the microprocessor re-energizes the transistors and bypass relay circuit to close the switch and allow current to once again flow to and engage the clutch to activate the compressor. 
     According to a third embodiment of the present invention, the vehicle relay is configured to prevent undervoltage to the compressor clutch which can result in the clutch not being pulled in tightly and therefore slipping, causing unwanted wear. The core vehicle relay components are provided with an electrical connection between the input voltage and the microprocessor that runs through a resistor. Through this connection, the microprocessor monitors the input voltage going to the compressor clutch of the HVAC system to ensure the voltage is sufficient for proper clutch operation. If the voltage is detected as having fallen below a threshold value, the microprocessor de-energizes one or more transistors and the bypass relay circuit, opening the switch and interrupting current flow to the clutch to disengage the same. Once sufficient input voltage is restored, the microprocessor re-energizes the transistors and the bypass relay circuit to close the switch and allow current to once again flow to and engage the clutch for compressor operation. 
     According to a fourth embodiment of the present invention, the core components of the vehicle relay are configured to prevent excessive cycling of the HVAC system compressor which causes undue wear and reduces the life of the compressor. With the vehicle relay of the present invention in the anti-cycling configuration, the microprocessor is programmed with a timing function. With the climate control system off, the switch in the bypass relay circuit is open. When the climate control system is started, the microprocessor delays activation of one or more transistors and bypass relay for a predetermined interval, such as 20-30 seconds. By delaying closure of the switch and engagement of the compressor clutch, the number of times that the compressor is made to start up every minute is significantly reduced, extending compressor life. 
     Accordingly, it is an object of the present invention to provide a vehicle relay that reduces the initial surge of inrush current to the fan motor upon start up of a large capacity climate control system such as the HVAC systems installed on buses and the like, improving system safety and reducing the size requirements of fuses used in the system. 
     Another object of the present invention is to provide a vehicle relay in accordance with the preceding object that is easily installed and adaptable to virtually any vehicle HVAC system that could benefit from an initial reduction in current to a motor or compressor of the climate control system installed in the vehicle. 
     A further object of the present invention is to provide a vehicle relay in accordance with the preceding objects that, by reducing the initial current surge, reduces arcing and resulting wear on the motor brushes, increasing the longevity of the motor. 
     Yet another object of the present invention is to provide a vehicle relay that can be configured to prevent overspeed of the compressor in a climate control system due to overly high engine RPM. 
     A still object of the present invention is to provide a vehicle relay that can be configured to prevent undervoltage to the compressor clutch in a climate control system which can result in clutch slippage and unwanted wear. 
     Yet another object of the present invention is to provide a vehicle relay that can be configured to prevent excessive cycling of the compressor in a climate control system which causes undue wear and reduces the life of the compressor. 
     These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the vehicle relay according to the present invention. 
         FIG. 1A  is an enlarged plan view of the printed circuit assembly of the relay shown in  FIG. 1 . 
         FIG. 2  shows the relay, printed circuit board, and connector of  FIG. 1  as assembled. 
         FIG. 3  shows an exploded view of the components shown in  FIG. 2  according to a first embodiment of the present invention in which the relay is configured for provision of a “soft start” of the fan motor(s) of a vehicle climate control system upon activation thereof. 
         FIG. 4  is a schematic of the printed circuit board shown in  FIG. 3 . 
         FIG. 4A  is a graph of the typical inrush current to fan motor(s) upon start-up of a conventionally configured vehicle climate control system. 
         FIG. 4B  is a graph of the greatly reduced initial current to the fan motor(s) upon start-up of the vehicle climate control system when the “soft start” relay has been inserted into the system wiring according to the present invention. 
         FIG. 5  shows an exploded view of the components shown in  FIG. 2  according to a second embodiment of the present invention in which the relay is configured for overspeed prevention. 
         FIG. 6  is a schematic of the printed circuit board shown in  FIG. 5 . 
         FIG. 7  shows an exploded view of the components shown in  FIG. 2  according to either a third embodiment in which the relay is configured to prevent undervoltage to the compressor clutch or a fourth embodiment in which the relay is configured to prevent over-cycling of the compressor in accordance with the present invention. 
         FIG. 8  is a schematic of the printed circuit board shown in  FIG. 7  according to the third embodiment when the relay is configured for undervoltage protection prevention. 
         FIG. 9  is a schematic of the printed circuit board shown in  FIG. 7  according to the fourth embodiment when the relay is configured for over-cycling prevention. 
         FIG. 10A  shows a prior art vehicle wiring arrangement between three fans of a climate control system and a power source. 
         FIG. 10B  shows the components of  FIG. 10A  with the vehicle relay inserted into the existing vehicle wiring in the “soft start” configuration according to the first embodiment of the present invention. 
         FIG. 11A  shows a prior art vehicle wiring arrangement between a compressor and control panel of a climate control system. 
         FIG. 11B  shows the components of  FIG. 11A  with the vehicle relay inserted into the existing vehicle wiring in the overspeed protection configuration according to the second embodiment of the present invention. 
         FIG. 11C  shows the components of  FIG. 11A  with the vehicle relay inserted into the existing vehicle wiring for use in either the undervoltage protection configuration or the over-cycling prevention configuration according to the third and fourth embodiments of the present invention, respectively. 
         FIG. 12  is a flowchart of the vehicle relay operating in the “soft start” configuration according to the first embodiment of the present invention. 
         FIG. 13  is a flowchart of the vehicle relay operating in the overspeed prevention configuration according to the second embodiment of the present invention. 
         FIG. 14  is a flowchart of the vehicle relay operating in the undervoltage protection configuration according to the third embodiment of the present invention. 
         FIG. 15  is a flowchart of the vehicle relay operating in the anti-cycling configuration according to the fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While several preferred embodiments of the invention are disclosed herein, it is to be understood that these embodiments are given by way of illustration only. It is not intended that the invention be limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. Also, in describing the preferred embodiments, specific terminology will be resorted to for the sake of clarity. It is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
     As shown in  FIGS. 1-2 , the present invention is directed to a relay generally designated by reference numeral  10 . The relay has at least four embodiments including a “soft start” relay  10   a , an overspeed protection relay  10   b , an undervoltage protection relay  10   c  and an over-cycling prevention relay  10   d.    
     The relay  10  includes a printed circuit board (PCB) assembly  12 . The PCB assembly  12  has electrical relay terminals including a first relay terminal  14 , a second relay terminal  16 , and a third relay terminal  18 . The relay is configured to connect with a connector  20  having a wiring harness, generally designated by reference numeral  22 , and splice terminals including a first splice terminal  24 , a second splice terminal  26  and a third splice terminal  28 . When the connector and the relay are assembled as shown in  FIG. 2 , the first relay terminal  14  is electrically connected with the first splice terminal  24 , the second relay terminal  16  is electrically connected with the second splice terminal  26 , and the third relay terminal  18  is electrically connected with the third splice terminal  28 . The relay may also have a mounting hole  11 . 
     When the relay  10  and connector  20  are coupled to one another for use, either the first splice terminal  24  or the second splice terminal  26  may be connected to a power source such as a 12V battery  30 . If the first splice terminal  24  is connected to the power source, the printed circuit board is powered through the first relay terminal  14 . Conversely, if the second splice terminal  26  is connected to the power source, the printed circuit board is powered through the second relay terminal  16 . The third relay terminal  18  and the third splice terminal  28  are connected to ground  32  (see  FIGS. 4, 6, 8 and 9 ). 
     The printed circuit board assembly  12  of the relay  10  may be configured in different ways according to the function that it is to perform. As described herein, there are at least four different embodiments of the relay including a “soft start” relay  10   a , an overspeed protection relay  10   b , an undervoltage protection relay  10   c  and an over-cycling prevention relay  10   d . As described herein, use of “relay  10 ” is intended to include all four embodiments, while use of the specific reference numerals  10   a - 10   d  indicates the corresponding specific embodiment that is in view. 
     In all four embodiments described herein, the vehicle relay  10  includes a common set of components comprising the core components of the relay  10 . These core components include a microprocessor (U 1 )  40 , a voltage regulator (U 2 )  42 , transistors (Q 1 , Q 2 )  44 , a plurality of diodes (D 1 , D 2 , D 3 )  48   a ,  48   b ,  48   c , and a bypass relay circuit X 1  relay generally designated by reference numeral  50 . The bypass relay circuit X 1   50  includes a switch  52  and a relay coil  54 . The core components are mounted in the PCB  12 . 
     The voltage regulator  42  provides a constant voltage level to the microprocessor  40  and the transistors  44  isolate the unregulated voltage from the microprocessor. When the relay  50  has been energized and then subsequently de-energized, the D 3  diode  48   c  clamps the inductive energy created by the relay inductance  54  in the X 1  relay  50  to prevent a voltage spike. In all of the embodiments shown herein, the power source is a 12V battery  30  although other power sources could be used. 
     As noted above, the core vehicle relay components may be configured with one or more additional components as will be described hereinafter according to the function that the relay is to perform. 
     In the first embodiment shown in  FIGS. 3 and 4 , referred to herein as the “soft start” configuration, the printed circuit assembly  12  of the relay  10   a  is configured to provide a ramped start-up voltage, or “soft start”, for a motor, compressor or the like, shown in  FIG. 4  as a fan motor  56 . In addition to the core components of the U 1  microprocessor  40 , the U 2  voltage regulator  42 , the Q 1  and Q 2  transistors  44 , the plurality of diodes  48   a ,  48   b ,  48   c , and the X 1  bypass relay circuit  50 , the vehicle relay  10   a  is provided with a negative temperature coefficient (NTC) R 1  resistor  46  downstream of the input terminal. The microprocessor  40  is programmed with an algorithm that controls the “soft start” start-up sequence for the fan motor  56 . 
     In the embodiments shown schematically in  FIGS. 4, 6, 8 and 9 , the relay  10  is powered through the first relay terminal  14  with voltage generated by the relay  10   a  being applied to the load, such as fan  56  or compressor  57  through compressor clutch  55 , through the second relay terminal  16 . However, by including the D 2  diode  48   b  as well as the D 1  diode  48   a , the heavy gauge wiring polarity can be reversed because the microprocessor can be powered through either the first relay terminal or the second relay terminal, with voltage being applied to the compressor clutch or fan through the other of the two terminals, i.e., the terminal not connected to the power source. This ability to power the relay through either the first or the second relay terminal makes installation easier since the installer cannot “go wrong” regardless of which terminal he or she connects to the power source. 
     According to the first embodiment shown in  FIGS. 3, 4 and 10B , when the climate control system  300  (see  FIGS. 11A-11C ) of the vehicle (not shown) is off, the switch  52  in the X 1  bypass relay  50  of relay  10   a  is open. Upon climate control system start-up, power is applied to the input terminal  14 . With the switch  52  in the bypass relay  50  being open, current flows to the output terminal of the relay  10   a  through the NTC resistor  46 , with the NTC resistor  46  acting to reduce the amount of current that is able to initially rush into the fan motor(s)  56  (see also three fan motors  80  in  FIG. 10B ). After a predetermined time delay programmed into the microprocessor, the microprocessor energizes the transistors  44  which, in turn, energize the bypass relay circuit  50  to close the switch  52 . Once the switch is closed, the NTC resistor  46  is bypassed by the relay circuit  50  and maximum operating voltage is applied to the fan motor by the relay  10   a . With the “soft start” sequence, reduction in the initial inrush of current to the fan motor has been realized from on the order of 60 amps as occurs without the vehicle relay, shown graphically in  FIG. 4A , to on the order of 6.7 amps, shown graphically in  FIG. 4B . This current reduction allows the 25 amp fuse used in the  FIG. 4A  scenario to be reduced in size to a 10 amp fuse as used with the “soft start” as shown in  FIG. 4B , and also provides for longer motor life. 
     The second embodiment of the present invention, in which the vehicle relay  10   b  is configured to protect the compressor  57  from operating above the manufacturer&#39;s recommended RPM, is shown in  FIGS. 5, 6 and 11B . Overspeed of the compressor  57  can occur when overly high engine RPM is applied to the compressor through the compressor clutch  55 . Excessive engine RPM can be a problem with large after-market-manufactured vehicles such as airport shuttle buses which are often built with larger engines which can overdrive the compressor. Overspeed can also occur when the vehicle is accelerated, as in a passing situation. With the overspeed protection function of the second embodiment of the present invention, the compressor clutch is disengaged in response to this rapid acceleration which not only protects the compressor but, by removing the load of the compressor on the engine, gives the vehicle engine more power and speed to complete the passing maneuver. 
     According to the second embodiment of the relay  10   b  shown schematically in  FIG. 6 , a fourth terminal  60  is added to the core vehicle relay components. The fourth terminal  60  is electrically connected to the microprocessor through a resistor (R 3 )  62 . The output terminal of the vehicle relay  10   b  is connected to the compressor clutch of the climate control system and the switch in the bypass relay circuit is normally closed, allowing current to flow to the compressor clutch  55  to start the compressor  57 . The microprocessor  40  monitors the engine RPM signal received through the fourth terminal  60  and, in the event that the signal indicates that the engine RPM has exceeded the compressor rating, the microprocessor  40  de-energizes the transistors  44  and the bypass relay circuit  50  which opens the switch  52  and interrupts current flow to disengage the clutch. The high engine RPM is thus prevented from overloading and damaging the compressor  57 . When the microprocessor detects that the engine RPM has fallen back below the rating of the compressor, the microprocessor re-energizes the transistors and the bypass relay circuit to close the switch and allow current to once again flow to engage the compressor clutch and activate the compressor. 
     The third embodiment of the present invention, in which the vehicle relay  10   c  is configured to prevent undervoltage to the compressor clutch, is shown in  FIGS. 7, 8 and 11C . When voltage to the compressor clutch  55  is too low, the clutch is not pulled in tightly enough and therefore slips, causing unwanted wear. Typical causes of inadequate voltage include installation of electrical systems in the vehicle without heavy enough wire, or the addition of extra accessories, such as a wheelchair power lift, to a stock vehicle, such that there is insufficient voltage to power all of the vehicle systems, particularly when the vehicle engine is idling, such as at a stop light. 
     In the third embodiment as shown schematically in  FIG. 8 , the core vehicle relay components are further provided with an electrical connection  65  between the input voltage on terminal  14  and the microprocessor  40  that runs through a resistor (R 1 )  66  to prevent possible damage to the microprocessor. Through this connection  65 , the relay microprocessor monitors the input voltage going to the clutch  55  to ensure that the voltage is sufficient for proper clutch operation. If the voltage is detected as having fallen below a threshold value, the microprocessor de-energizes the transistors and the bypass relay circuit which opens the switch and interrupts current flow to the clutch for disengagement thereof. Once the microprocessor detects that sufficient input voltage has been restored, the microprocessor re-energizes the transistors and the bypass relay circuit to close the switch and allow current to once again flow to and re-engage the clutch. By disengaging the clutch when the voltage is too low for proper clutch operation, needless wear on the clutch is prevented, extending the life thereof. 
     The fourth embodiment of the present invention, in which the vehicle relay  10   d  is configured to prevent excessive or over-cycling of the compressor, is shown in  FIGS. 7, 9 and 11C .  FIGS. 7 and 11C  are common to both the third and fourth configurations because the relay is shown therein only as a component without illustrating the specific layout of the PCB which is where the differences between the third and fourth embodiments are configured. 
     Excessive cycling of the compressor can result due to low ambient temperature, low refrigerant charge and/or improper system design. For example, commercial buses are often provided with a large climate control system compressor. Large compressors are able to lower the temperature in the bus within a few seconds, after which they turn off. In a very short time, however, the temperature rises above the desired setting and must again be lowered, causing the compressor to start back up. As a result, the compressor may start and stop an excessive number of times every minute which shortens the life of the compressor. 
     With the vehicle relay  10   d  in the over-cycling prevention configuration according to the fourth embodiment shown schematically in  FIG. 9 , the microprocessor has a timing function  69 . With the climate control system off, the switch in the relay is open. When the climate control system is started, it sends a start-up signal to the compressor via the microprocessor. The microprocessor, however, in response to the timing function  69 , waits for a predetermined interval before energizing the transistors and the relay circuit  50  to close the switch and allow current to flow to the compressor. The delay time interval is programmed into the microprocessor and is preferably on the order of about 20-30 seconds. This time delay before the relay is energized to activate the compressor significantly reduces the number of times that the compressor is made to start up every minute, extending compressor life. 
     The vehicle relay as described herein can be easily inserted into existing vehicle wiring such as that shown in  FIG. 10A . As shown, the wiring between the battery  30  and the three fans  80  includes a condenser control relay  82  and a 20 amp fuse  84  for each of the fans  80 . In the case of the “soft start” configuration, the relay  10   a  is inserted between the battery  30  and the fan motors  80  as shown in  FIG. 10B  to control the initial rush of current to the fan motors. With the “soft start” relay  10   a , smaller fuses can be used, shown in  FIG. 10B  as 15 amp fuses  86 . The fan motors  80  are represented in the schematic of  FIG. 4  by fan  55 . 
     The vehicle relay as described herein can also be easily inserted into existing vehicle wiring such as that shown in  FIG. 11A . In the overspeed protection configuration shown in  FIG. 11B , the relay  10   b  is inserted between the refrigeration compressor  57  and the PCB  92  of a climate control system  300 . Similarly, in either the over-cycling prevention configuration or the undervoltage protection configuration shown in  FIG. 11C , the relay  10   c  or  10   d , respectively, is inserted between the refrigeration compressor  57  and the climate control system PCB  92 . 
     A method of operation of the vehicle relay  10   a  in the first “soft start” configuration is summarized in the flowchart of  FIG. 12 . Initially, the climate control system is off and the switch in the bypass relay is open, step  100 . When the climate control system is started, power is applied to the input terminal of the PCB, step  102 . Current flows to the output terminal through the NTC resistor, step  104 , which reduces the initial inrush of current to the fan motor. After a predetermined time delay, the microprocessor energizes the transistors and the bypass relay circuit to close the switch  52 , step  106 . Once the switch is closed, the NTC resistor is bypassed by the relay circuit  50  and maximum operating voltage is applied to the fan motor while the transistors isolate the microprocessor from unregulated voltage, step  108 . As long as the relay circuit remains energized, step  110 , maximum operating voltage continues to be applied to the fan motor and the NTC resistor is bypassed, step  108 . If the relay circuit is de-energized, step  110 , a diode downstream of the transistors clamps the relay&#39;s inductive energy, step  112 , allowing for gradual dissipation of the inductive energy and preventing a voltage spike. 
     A method of operation of the vehicle relay  10   b  in the second overspeed prevention configuration is summarized in the flowchart of  FIG. 13 . Initially, the engine is running and the switch in the bypass relay circuit is closed, step  120 , allowing current to flow from the output terminal of the relay to the compressor clutch. The microprocessor monitors the engine RPM signal, step  122 , with the RPM signal being received through a fourth terminal that is electrically connected to the microprocessor through a resistor. In the event that the signal indicates that the engine RPM has exceeded the clutch compressor rating, step  124 , the microprocessor de-energizes the transistors and the bypass relay circuit which opens the switch  52  and interrupts current flow to the clutch resulting in disengagement thereof, step  126 . The microprocessor continues to monitor engine RPM, step  128 . When the microprocessor detects that the engine RPM has fallen back below the rating of the compressor, step  130 , the microprocessor re-energizes the transistors and the bypass relay circuit, step  132 , to close the switch and allow current to once again flow to the compressor clutch for re-engagement thereof. 
     A method of operation of the vehicle relay  10   c  in the third undervoltage protection configuration is summarized in the flowchart of  FIG. 14 . Initially the engine is running and the switch in the bypass relay circuit is closed, allowing current to flow from the output terminal of the relay to the compressor clutch, step  140 . The microprocessor monitors the input voltage going to the clutch, step  142 , through an electrical connection between the input voltage and the microprocessor that runs through a resistor. In the event that the microprocessor detects that the input voltage going to the clutch is insufficient for proper clutch operation, step  144 , the microprocessor de-energizes the transistors and the bypass relay circuit which opens the switch and interrupts current flow to disengage the clutch, step  146 . The microprocessor continues to monitor the input voltage to the clutch, step  148 . Once the microprocessor detects that sufficient input voltage has been restored, step  150 , the microprocessor re-energizes the transistors and the bypass relay circuit to close the switch and re-engage the clutch, step  152 . 
     A method of operation of the vehicle relay  10   d  in the fourth over-cycling prevention configuration is summarized in the flowchart of  FIG. 15 . Initially, the climate control system is turned off and the switch  52  in the X 1  bypass relay  50  is open, step  160 . When the climate control system is started, step  162 , a signal is sent to activate the compressor, step  164 . The signal goes through the relay  10   d  where the microprocessor delays energizing the transistors and the relay circuit for a predetermined interval, step  166 . At the conclusion of the time delay interval, the microprocessor energizes the transistors and the bypass relay circuit to close the switch and allow current to flow to and start the compressor, step  168 . The compressor operates, step  170 , until the desired temperature has been reached. Once the temperature has been reached, step  172 , the compressor shuts down, step  174 , while the climate control system continues to operate a circulation fan providing cool air into the vehicle, step  176 . When the temperature in the vehicle rises above a predetermined threshold, step  178 , steps  164 - 178  are cyclically repeated for as long as the climate control system is turned on in the vehicle. 
     The present invention is also directed to the combination of a large high occupancy vehicle, a vehicle relay  10  and a climate control system in the high occupancy vehicle having at least one fan motor electrically connected to an output terminal of the relay  10 , representatively shown in  FIG. 10B , as well as to the combination of a vehicle relay and a vehicle climate control system having a refrigeration compressor and a compressor clutch, representatively shown in  FIGS. 11B and 11C , as installed in a large high occupancy vehicle. 
     The foregoing descriptions and drawings should be considered as illustrative only of the principles of the invention. The invention may be configured in a variety of shapes and sizes and is not limited by the dimensions of the preferred embodiment. Numerous applications of the present invention will readily occur to those skilled in the art. Therefore, it is not desired to limit the invention to the specific examples disclosed or the exact construction and operation shown and described. Rather, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.