Abstract:
A tire pressure management system for a tractor/trailer system establishes allowable bounds for tire pressure compensated for temperature and determines leaks after determining that internal tire temperature measurements are not available. The system is based on valve-mounted external tire pressure sensors, wireless connections to uplink tire pressure data, keyed by location, an ambient temperature sensor and an onboard computer. Tires are assumed pressurized to a target pressure at a given temperature with minimum and maximum pressure bounds set around the target pressure. Ambient temperature sensing, and determination that tires are likely to be at ambient temperature, allows adjustment of the minimum and maximum bounds, as well as the target pressure, compensated for temperature and thus a judgement made as to whether the cold pressure reading indicates deviation from acceptable limits. Determination of whether the vehicle is, or has recently been in movement, is made allowing internal temperature of the tires to be estimated based on changes in pressure. Inter-comparison of pressure readings from among the tires is used to determine the occurrence of leaks during vehicle operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The invention relates to tire pressure monitoring systems. 
         [0003]    2. Description of the Problem: 
         [0004]    Tire pressure monitoring systems (TPMS) provide for alerting drivers to variation in measured tire pressure from desired norms particularly while the vehicle is in operation. As is well known from Boyle&#39;s law, the pressure of a fixed quantity and volume of air is proportional to its temperature. Absent compensation for the effects of temperature changes, TPMS report actual tire pressure. For example, a truck tire inflated to 100 psi when cold could easily be reported as pressurized to 115-125 psi when the vehicle is running at highway speeds. Conversely, if ambient temperature were to drop after filling a tire, a low pressure warning might be generated. Without temperature compensation it is difficult for TPMS to determine if a pressure change is due to a leak or to temperature changes. 
         [0005]    Accordingly, TPMS commonly utilize pressure and temperature sensors mounted on the wheel rim and within the tire. This allows reported tire pressure values to be adjusted for temperature changes that occur in the course of normal driving and for taking into account changes in ambient temperature occurring when the vehicle is parked. Not only does this help keep TPMS from issuing false warnings, but it allows tires to be filled when hot since the user can be directed to add air to a pressure which will match the recommended pressure after cooling. However, not all vehicles are equipped with the temperature sensors and even if they are, there still exists the difficulty of supplying such sensors with power. 
         [0006]    Some TPMS have used externally mounted pressure sensors which attach to the valve stem in place of a valve cap. Externally mounted systems are easily retrofitted to existing commercial vehicles. This allows easy replacement upon depletion of the batteries used to energize the transmitter portion of the sensor. However, external sensors cannot conveniently provide a measurement of internal tire temperature. Vehicle operators therefore must understand the effects changes in temperature can have on tire pressure and make decisions in light of that understanding. This is not to say that no action should be taken on account of pressure changes due to ambient temperature changes, especially where consistent with movement of the vehicle from a warmer to a colder climate or to seasonal changes. 
         [0007]    TPMS intended for use on commercial vehicles are developed with awareness of the National Highway Traffic Safety Administration Vehicle in Use Inspection Standard 570.57. TPMS allow automation of some of the inspections required of drivers of commercial vehicles. 
       SUMMARY OF THE INVENTION 
       [0008]    According to the invention a tire pressure management system for a tractor/trailer system operates to provide some features of temperature compensation when direct measurement of internal tire temperature is not available. The system first determines availability of tire temperature sources, and initializes itself for indirect compensation when such measurements are determined to be unavailable. The system is based on valve-mounted external tire pressure sensors, wireless connections to uplink tire pressure data, keyed by location, an ambient temperature sensor and an onboard computer. A target pressure at a given temperature with minimum and maximum pressure bounds set around the target pressure are set for the tires. Ambient temperature sensing, and determination that the tires are likely to be at ambient temperature, allow adjustment of the minimum and maximum bounds, as well as the target pressure, compensated for temperature and thus a judgement made as to whether the cold pressure reading indicates deviation from acceptable limits. Determination of whether the vehicle is, or has recently been, in movement allows internal temperature of the tires to be estimated based on changes in pressure. Inter-comparison of pressure readings from among the tires is used to determine the occurrence of leaks during vehicle operation. 
         [0009]    Additional effects, features and advantages will be apparent in the written description that follows. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
           [0011]      FIG. 1  is a schematic illustration of the major components of the present tire pressure monitoring system. 
           [0012]      FIG. 2  is a block diagram of a tire pressure monitoring system receiver. 
           [0013]      FIG. 3  includes block diagrams for a valve-mounted sensor and a system repeater. 
           [0014]      FIG. 4  is a flow chart of a detection routine which determines if direct measurement of tire internal temperature is available. 
           [0015]      FIG. 5  is a flow chart of an initialization routine executed upon determination that direct internal tire temperature measurements are not available. 
           [0016]      FIG. 6  is a flow chart for determining tire pressure when the tires are cold. 
           [0017]      FIG. 7  is a flow chart for a routine determining if a leak has developed during running. 
           [0018]      FIG. 8  is a flow chart for a routine continuing with checking for leaks on a running vehicle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring now to the figures, a tractor/trailer combination  10  is illustrated on which a tire pressure monitoring system  30  has been installed. Tractor/trailer combination  10  comprises a tractor  12  and a trailer  14 . Tractor  12  is equipped with tires  18  installed on wheels  15 . Trailer  14  is equipped with tires  20  installed on wheels  17 . The internal pressure of tires  18 ,  20  is monitored by externally mounted sensor/transmitters  38 . A low pressure warning light  16  may be installed on the forward exterior wall of trailer  14  from where it can be seen from the cab of tractor  12 . Antennae  22 ,  26  are strategically located on trailer  14  and tractor  12  for enabling radio frequency data links between the tractor and trailer to extend the tractor control system to the trailer. 
         [0020]    A tire pressure monitoring system (TPMS)  30  includes components installed on both the trailer  14  as well as on the tractor  12  and is topologically illustrated in  FIG. 1 . The TPMS  30  illustrated is a preferred embodiment suited for an OEM installation where tractor  12  is equipped with an SAE J1939 compliant controller area network (CAN)  56 . Simplified systems for aftermarket installation on vehicles not equipped with a CAN are certainly possible as will be clear to those skilled in the art. Trailer  14  components are a subset  29  of the TPMS  30  and are capable of limited, stand-alone operation. A trailer  14  may be attached to a tractor  12  which does not have components required to complete TPMS system. Thus, a trailer warning light  16  is installed on the exterior of the forward wall of trailer  14  to afford subset  29  limited stand alone functionality. Trailer warning light  16  should be installed so as to be readily visible in a rear view mirror from the cab of tractor  12  and the trailer based subset  29  of TPMS  30  should be configured so as to support activation of the warning light  16  even in the absence of additional functionality of tractor-based portion  31  when low pressure is detected in any of tires  20 . Trailer base portion  29  is also configured to switch automatically from battery to vehicle power if tractor power is available. As illustrated, trailer warning light  16  is used only to indicate that a tire is low on pressure, but does not indicate which tire. 
         [0021]    Tire pressure information is made available in the cab of tractor  12  on a display  60 . A receiver having the service functionality of TPMS receiver  44 , less (or not using) its CAN interface, could serve as such a receiver. Where a vehicle is equipped with a controller area network  56  TPMS receiver  44  is connected to CAN  56  for transfer of data to drive display  60 . A body computer  58  is programmed to compensate pressure readings for temperature-based changes in pressure notwithstanding the lack of direct temperature measurements from inside the tires if required. Data from an ambient temperature sensor  36  and a vehicle speed sensor  35  are connected to the body computer  58  to implement tire pressure management using alternatives to direct tire temperature data in implementing temperature compensation of pressure readings. 
         [0022]    A representative TPMS  30  includes a valve pressure sensor  38  for all of tires  18 ,  20  on both tractor  12  and trailer  14 . The valve pressure sensors  38  are typically installed on the valve stem for each wheel. The considerations involved in such installations are the usual ones of weight, balance, stem vibration, visual appeal, environmental resistance, ease of installation, clearance from the wheel and theft deterrence. Integral batteries are used to supply power to valve pressure sensors  38 . The batteries are generally not replaceable and efforts are taken to maximize battery life to avoid the need for frequent replacement of the valve pressure sensors  38 . This may be achieved in part using a sleep mode when the vehicle is off. Transmission frequency may be varied depending upon circumstances, for example, it may be reduced when pressure levels are acceptable. Transmission frequency can be increased in response to a variance of pressure from desired norms and upon request of the host system. Typically the transmission rate is elevated when the vehicle is moving. 
         [0023]    A full TPMS  30  may require two or more antennae per vehicle. Provided are a tractor antenna  26  and a trailer antenna  22  for establishing a data link  43  between the vehicle sections. Trailer antenna  22  serves for repeater station  42 . Tractor  12  is preferably equipped with a controller area network (CAN)  56  conforming to the SAE J1939 standard for transferring data to computers implementing higher level functions of the TPMS  30 . CAN  56  will include a body computer  58  which executes management programs, including the routines described below, and which passes data to a cab display  60  where warnings and indications of tire condition are imaged. 
         [0024]    Referring to  FIG. 2  a block diagram schematic of a TPMS receiver  44  is illustrated. TPMS receiver  44  includes pressure monitoring and fault detection functionality through a programmed microprocessor  76  for use in case of installation on a vehicle not having a CAN. TPMS receiver  44  typically receives wireless reports of data from valve pressure sensors  38 ,  46  over an antenna connected to a radio frequency transceiver  74 , and reports the data to microprocessor  76 . Microprocessor  76  can receive data over other channels as well including CAN  56  through J1939 interface  70  and, optionally, J1587/J1708 interface  86 . A reprogramming interface may be connected to CAN  56 . All interfaces are connected to supply data directly to microprocessor  76 . The network interfaces  70 ,  86  are further connected to exchange data with diagnostics block  78 . Memory  80  is available to microprocessor  76  and diagnostics block  78 . Microprocessor  76  also receives inputs over a digital input/output port  82 . Inputs potentially relate to axle positions specifically identified with tires. Outputs including a drive LED warning light output can be generated over digital I/O  82 . 
         [0025]    Referring to  FIG. 3 , functional block diagrams for valve pressure sensors  38  and trailer repeater  42  are provided. Valve pressure sensor  38  includes a battery  86 , a low frequency transceiver  88  for communication with TPMS receiver  44  or trailer repeater  42 , a microprocessor  90 , an RFID tag reader  92  (though RFID tags are assumed not present here), a sensor package transceiver  94  and a sensor package  96  including a stem pressure sensor  98 , a temperature sensor  100  providing ambient temperature at start up and a motion sensor  102 . Start up ambient temperature readings may be used for temperature compensation instead of a tractor mounted, ambient temperature sensor. Trailer repeater  42  includes a local processor  106  and memory  108  and, in case no tractor TPMS is available, can function as a stand alone system performing low pressure detection. Trailer repeater  42  further includes a rechargeable battery  110 , recharge circuit  112  and an LED driver  114 . In order to save power repeater  42  has a power down or sleep mode and a wake up circuit  118  is provided. An RFID interrogator  116  is provided as well as a TPMS transceiver  120  for the exchange of data with transceiver  44 . 
         [0026]    Valve sensors  38  transmit data to a TPMS receiver  44  directly or by trailer repeater  42 . Microcontroller  90  is programmed with a pressure threshold. In response to detection of pressure falling below the threshold, the pressure reading transmission rate increases. Normally the pressure transmission rate is quite slow to prolong battery life. A motion sensor  102  enables sensor transmission rate increases if the vehicle is moving. This allows TPMS  30  to determine fast leakage rates and other warning conditions. The programmable threshold can be reprogrammed by TPMS  30  if the system determines that operational parameters for the truck have changed. The parameters can include average climate, average load and other factors. Default parameters are selected to be universally applicable, but are preferably optimized for specific vehicles in order to improve fuel economy and prolong battery life. LF receiver  88  allows sensor  38  to be awakened and communicated with by a hand tool  24  or by repeater  42 . 
         [0027]    Repeater  42  is used as a bridge from tire pressure sensors  38  to TPMS receiver  44  which is mounted on tractor  12 . RF retransmission is provided. In order to extend battery service life, repeater  42  is not always in a listening mode. An embedded RFID transceiver  116  detects when a tractor  12  has backed to the trailer  14  and a handshake signal is transmitted to TPMS receiver  44 . The handshake provides the unique tire ID numbers for tires installed on the trailer  14 , along with axle locations and may be used to activate repeater  42 . 
         [0028]    Wheel valve sensors  38  transmit at a slow rate when the vehicle is not moving. Repeater  42  includes a rechargeable battery  86 , which provides power allowing the repeater to receive these signals and to store the most recent data. If a leak is detected, an LED, which is mounted on trailer  14  where easily seen, is set to flashing. This serves to alert yard mechanics to attend to the tires when a tractor is not present. If a tire pressure problem exists when the tractor comes into position to connect to the trailer  14 , repeater  42  alerts the TPMS  30  upon activation. When a tractor  12  connects to a trailer  14  the repeater&#39;s battery  86  goes into recharge mode and the repeater begins to use the tractor&#39;s power supply. 
         [0029]    Tractor  12  is also equipped with an RFID interrogator  25 , which maybe UHF or LF-BASED depending upon the required transmission range. RFID interrogator  25  is located in an area where it will activate a trailer mounted RFID tag  116  when the tractor comes within five feet of the trailer which can in turn supply a wake up signal to repeater  42 . Interrogator  25  may be triggered by the driver, automatically or when a particular state is true, for example, if the tractor is in reverse. Tractor RFID interrogator  25  listens for a response identifying the trailer  14  and for trailer conditions requiring attention. Again the present invention assumes that the response to interrogator  25  inquiries directed to determine if RFID tags  116  are present is negative. Such RFID tags would provide tire temperature measurements and their absence could be one, among several, causes for the absence of tire temperature readings. 
         [0030]    The present invention provides some of the attributes of temperature compensation for detected pressure measurements absent availability of temperature readings from the tires. Readings from an ambient temperature sensor, such as may be located in a vehicle&#39;s air intake or on the valve stems are available to on board vehicle computer, as are tire pressure readings. As is well know, tire pressure and temperature are positively correlated. If a pressure drop occurs due to a leak then temperature will also decrease, provided all other factors remaining unchanged. However, if a vehicle is in motion a pressure drop will result in more sidewall flexing of the tire, resulting in the generation of heat and a rise in the temperature inside the tire (with a consequent increase in pressure and, often, the leak rate). Leaks may be difficult to detect quickly when the only variable monitored is pressure. Programming for the vehicle computer is adapted to determine initially whether tire temperature readings are available. 
         [0031]    Referring to  FIG. 4 , a routine  400  executed on vehicle start sets flags indicating whether tire temperature is available. Step  402  indicates whether use of a hand tool has been requested. If so a tool mode flag is set at step  404  following the YES branch from step  402  and the program ends. The pertinent path here is the NO path from step  402  and a determination if tire temperature is available at step  406 . If not, following the NO branch advances processing to step  408  where the Temp_No flag is set. Otherwise processing advances along the YES branch to step  410  where the Temp_Yes flag is set. After flags are set in any of steps  404 ,  408 , or  410  the process ends. As should be clear from the character of tests employed, the subsequent series of routines are executed repeatedly after the vehicle has been turned on. The balance of the discussion assumes that the Temp_No flag was set in routine  400 . 
         [0032]    Referring to  FIG. 5 , an initialization routine  500  executed upon determination that tire temperature signals are not available is illustrated. Step  502  indicates the report of pressure signals from the valve pressure sensors  38 . The next decision step  504  reflects that when tires are filled to a recommended pressure, a temperature at which that pressure occurs is implicit. If done casually, the temperature will be the prevailing temperature at the time the tire is filled. Commercial operators are however more likely to be explicit about a temperature choice, depending upon time of year and the prevailing geographic location of operation of a vehicle. Consider an operator of vehicles which are usually located in Texas confronted with transfer of a vehicle for sustained operations in Manitoba in January. Absent adjustment of the quantity of air in the tires of a vehicle the measured pressure in the tires can be expected to drop substantially, even though no leak has occurred. While an operator may not change inflation of the tires it may be necessary to adjust warning levels to avoid generating false leak indications. Accordingly a shift in ambient temperature shift levels may be indicated at step  504 . This may occur automatically, if the vehicle records temperature readings and notes a sustained change over a period of days, as would occur upon relocations of a vehicle from a warm to a cold climate. Or, an operator, anticipating a shift in operations, may program a change in the shift level. It is even possible that a vehicle equipped with a global positioning sensor could use its position and date information to access meteorological data bases or weather forecasts and generate an anticipated or expected ambient temperature for the purposes of setting shift levels. In any event, an indication that the shift level requires change results in revision of pressure warning levels at step  506  following the YES branch from step  504 . Absent a change in expected ambient temperature the NO branch is taken. 
         [0033]    The balance of routine  500  is directed to determining if the tires may be considered to be cold, that is, at ambient temperature and setting appropriate flags indicative of this state or for use in determining if this state holds. Following step  506  or the NO branch from step  504  processing leads to a decision step  508  to determine if the vehicle is moving from the vehicle speed sensor  35 . If the vehicle is moving the YES branch is followed to step  510  which sets, or confirms, that the Running_NoTemp flag is set, which in effect indicates that the tires are not at ambient temperature, and the process ends. If the vehicle speed sensor  35  indicates that the vehicle is stopped additional tests are required before it can be assumed that the tires are at or have returned to ambient temperature. First an inquiry (step  512 ) is made to see if the vehicle has been started for the first time that day. If not, step  514  is executed to determine if the vehicle has been halted for a minimum of three hours, the time period required to allow cooling of the tires to ambient. If the vehicle has been stopped for three hours step  518  is executed to set the “Cold_NoTemp” flag indicating that the tires may be assumed to be at ambient temperature and processing ends. If the vehicle has not been stopped for a minimum of three hours as determined at step  514 , the NO branch is taken to step  510  and the “Running_NoTemp” flag is confirmed. 
         [0034]    Returning to step  512 , if it is determined that the vehicle start if the first of the day, then the YES branch is followed to step  516  where a “First Power_On” flag is set followed by execution of step  518  where the “Cold_NoTemp” flag is set. 
         [0035]    Referring to  FIG. 6  a routine  600  provides for estimating temperature compensated tire pressure is illustrated. It is assumed that a fleet operator has supplied a target fleet air pressure at a predetermined temperature T, here assumed to be about 70° F. (294 Kelvins). At step  602  tire pressure readings are taken from the tire pressure management system. Next, for each tire, the pressure reading P is normalized to the first measured temperature T BASE  for the day. Normalization requires a pressure adjustment: 
         [0000]      Δ P =( T   BASE   −T )×( P   SLOPE ) 
         [0000]      then, 
         [0000]    
       
      
       P 
       NORM 
       =P+ΔP  
      
     
         [0036]    where,
       P NORM =105 psi (or as set by operator)   P SLOPE =0.35 (ΔP/ΔT over T)   P, P NORM  are measured pressure.       
 
         [0040]    Next, at step  606  the variable T AMBIENT  is initialized to the first measured temperature T BASE . Next, at step  608  an expected cold pressure for each tire is calculated; 
         [0000]    
       
      
       P 
       COLD 
       =P 
       BASE 
       +ΔP  
      
     
         [0041]    Next, at step  610 , the expected pressure P NORM  for each tire is displayed. Next, at step  612 , the cold average pressure (P avgCold ) for each group of tires is determined. Typically one group of tires is taken to be the tires  18  installed on the tractor  12  and the second group of tires is taken to be the tires  20  installed on the trailer  14 . Next, at step  614 , a new ambient temperature average is calculated, if actual readings are used for updating whether a change in shift level is required. 
         [0042]    The routine must, of course, provide basic warnings of pressurization problems with the tires. At step  616  it is determined if one or more compensated pressures violate one of the pressure thresholds WarnLow or WarnHigh: 
         [0000]      P&lt;(WarnLow×P COLD ), or 
         [0000]      P&gt;(WarnHigh×P COLD ). 
         [0043]    If one of the pressure thresholds was found violated at step  616  processing advances along the YES branch to step  620  where the appropriate warning flags are stored, the required fill pressure determined (FillPress=P COLD −P NORM ), and the positions of the tires determined to be over or under filled stored. Step  622  indicates display of the low pressure tire positions before processing returns to step  618 . Where no violation has occurred then the NO branch from decision step  616  advances processing directly to decision step  618 . At step  618  it is determined whether P NORM  is more than 3 psi lower than that determined at the time of the last cold read. If so, a possible leak is present and a cold leak check  624  is done to determine whether a slow or fast leak is present. Successive tests  626  and  628  are used for locating slow and fast leaks, respectively, which are equated to the speed of pressure loss. Positive results of the tests lead to suspect wheel positions being displayed and storage of warning flags for the tires suspected of leaking (steps  627 ,  629 ). If the pressure loss is determined not to be leak related, and after storage of results when a leak is found, processing returns to the main path following the No result from step  618  to a step  630  where the values ColdThreshLow and ColdThreshHigh are set before the process is terminated: 
         [0000]      ColdThreshLow=P COLD −(WarmLow×P COLD ) 
         [0000]      ColdThreshHigh=P COLD +(WarmHigh×P COLD ). 
         [0044]    For a running vehicle it is not strictly necessary to know the pressure for a tire, but rather the problem sometimes is determining whether a leak is occurring or if a tire has become under inflated. The routine  700  described with reference to  FIG. 7  provides: determination of whether a warning threshold has been crossed during operation; leakage detection by comparing tire pressure among the tires of the vehicle; and display of warnings, but does not provide actual pressure readings after the first five minutes of vehicle movement. When measured pressure for a tire falls below a low pressure warming threshold, we can compare that tire&#39;s pressure against pressure measurements for the vehicle&#39;s other tires, or those for the axle on which the first tire is installed. An algorithm may be designed a number of ways to determine if the pressure drop stems from temperature change, rather than a leak, since we would expect all the tires on an axle to behave essentially the same, unless a leak has occurred. At step  702  it is determined if the “First Power_On” flag is set. If it is, step  704  is executed to determine if vehicle has been in motion for at least five minutes, and, if so, steps  706  and  708  are executed to discontinue display of tire pressures and to clear the “First Power_On” flag. Following the NO branches from steps  702  or  704 , or following execution of step  708 , step  710  is executed by interrogating the tire pressure management system for current pressure readings. Then, at step  714 , a running leak check is executed. This involves a comparison of each individual pressure reading against the average for all the tires on the vehicle, a variable termed P AVGHOT . Alternatively, a comparison may be made among the tires of a given axle. 
         [0045]    Next, step  714  is executed to determine if the threshold flags were set from step  620  indicating threshold violations during cold pressure measurements. If so, following the YES branch from decision step  714  to step  716 , a low tire warning indicating the position of the tire or tires in violation executed. After step  716  or following the NO branch from step  714  decision step  718  is executed to determine if a slow leak flag was previously set at step  627 . If YES, a slow leak caution warning is provided at step  720 . If no slow leak flag was set, step  722  is executed to determine if the fast leak flag has been set. If so a fast leak warning is displayed per step  724 . 
         [0046]    Actual compensated pressure is known when tires are at ambient (i.e. “cold”). These values serve as a baseline pressure when the tires heat up during use. The complication is to determine if a tire is leaking even whilst its pressure increases due to temperature increases. The routine  800  of  FIG. 8  tracks group average pressure from which an average expected temperature is calculated. From this a threshold is found, normalized to cold pressure and base temperature. Then it is determined if a particular tire has crossed below the newly established lower limit threshold indicating a leak. Any indicated leak is characterized as being fast or slow. 
         [0047]    Beginning at step  802 , the average running pressures P AVGH  are updated for each of the two groups of tires. At step  804  it is determined if these values have increased since the last determination. If yes, then execution is advanced to steps  810  and  812  where Boyle&#39;s Law is used to determine an expected temperature (normalized to P BASE  and T BASE ) assuming that the quantity of air and volume of the tires has remained unchanged. 
         [0000]    
       
      
       T 
       EXP 
       =T 
       AMB 
       ×P 
       AVGH 
       /P 
       AVGCOLD.M  
      
     
         [0048]    From this determination the values RunThreshHigh and RunThreshLow may be equated at step  812  to calculate thresholds for the expected (i.e. estimated) temperature of the tires. 
         [0000]      RunThreshLow=(WarnLow× P   COLD   ×T   EXP )/ T   BASE    
         [0000]      RunThreshHigh=(WarnHigh× P   COLD   ×T   EXP )/ T   BASE    
         [0049]    Then all tire pressures are compared (step  809 ) to the new RunThresh values and the positions of tires noted to be at low pressures are displayed, stored and flagged (step  816 ). 
         [0050]    Two other routes lead to step  809 , each initially following a determination at step  804  that average tire pressure has not increased. At step  806  it is determined if the average tire pressure has decreased in three of the last pressure reading evaluations. If not, then the comparison of the step  814  is executed as before. If yes, then all tires&#39; pressures are compared to cold threshold values and processing skips step  814  directly to step  809  and  816 , as already described. 
         [0051]    From step  816  processing is advanced to step  818  where it is determined if the pressure readings from one or more tires are more than 3 psi lower than that from its last reading. If not, process execution is completed and the program temporarily exited. If yes, than all of the tires pressures are compared to normative values to see if the drop is unique, confined to a few tires, or if it is widespread. If the drops are not widespread then leaks are indicated and the leaking tires are flagged by position (step  822 ) and the leaks are characterized as slow or fast (steps  824 ,  825 ) depending upon how quickly pressure is changing in the tires. Once the characterizations have been made, the appropriate flags are set at steps  826 ,  828 . 
         [0052]    Accordingly, the invention provides a back up for temperature compensation when direct indication of tire temperature is unavailable. 
         [0053]    While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.