Abstract:
A system and process for controlling vehicle loading on a multi-axle vehicle and improving maneuverability is disclosed. The system includes a reservoir of pressurized air and associated leveling valve that uses a relay valve to improve refill times when the vehicle returns from a dump mode operation to normal operation. The process reduces pressure in one or more of the axles when maneuvering the vehicle at predetermined slow speeds and maintaining that first predetermined pressure while the other axle(s) is at a greater pressure than the first axle. By preventing complete exhaustion of pressure from the suspension system, restoring air pressure is attained more quickly. Also, use of a relay valve enables higher flow rates to improve the refill time of the pneumatic suspension system after the dump operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This disclosure relates to a suspension system for a multi-axle vehicle, and more particularly to providing a system, method, and control logic for implementing air suspension control for at least one axle on a trailer. More particularly, the disclosure is directed to a method that alters the axle air suspension pressure partially exhausting or removing air from air suspension bags to reduce tire wear when maneuvering at low speeds. The disclosure precludes a driver of the vehicle from reducing air pressure from the axle suspension above a predetermined speed, and thereby prevents overloaded conditions. 
         [0002]    Federal regulation, namely Title 49 of the Code of Federal Regulations, Section 393.207, states that “the air suspension exhaust controls must not have the capability to exhaust air from the suspension system of one axle of a two-axle air suspension trailer unless the controls are either located on the trailer, or the power unit and trailer combination are not capable of traveling at a speed greater than ten miles per hour while the air is exhausted from the suspension system.” 
         [0003]    One proposed solution is outlined in U.S. Pat. No. 5,052,713, the disclosure of which is incorporated herein by reference. The &#39;713 patent is directed to a vehicle suspension system such as used in multi-axle tractor-trailers and other multi-axle vehicles. When maneuvering a trailer in a confined area such as a loading dock, torque is exerted on the trailer frame. One solution to the torque issue is to remove the load from all but one axle. The &#39;713 patent teaches that air should be exhausted from the air bags on all but one axle in order to improve maneuverability at low speeds or during tight turns. Moreover, and per the federal regulation, the load must be redistributed to the multiple axles once the tight turn maneuvering is complete. Otherwise, the potential exists that a single axle may be overloaded, since the load has not been shifted to multiple axles. The overloading could result in potential damage to the trailer frame or dynamic loading encountered by the vehicle. 
         [0004]    Although the &#39;713 patent provides one solution, there are some downsides to this methodology and system. First, there is risk of damage to the air bags when all air is exhausted therefrom. The air bags may be pinched. 
         [0005]    It is also important to note that the trailer will always be pivoting off the front axle, whether loaded or unloaded. It becomes important, therefore, that the system maintains the front axle with more pressure than the rear axle. That is, it is not just a question of unloading the rear axle, but assuring that the front axle has greater pressure than the rear axle. 
         [0006]    There is also an issue of tire chattering that occurs when all of the pneumatic pressure is exhausted from one axle of the suspension system. Thus, there is a desire to prevent full exhaustion of the air suspension so that the tires associated with the axle are pushed to the ground. 
         [0007]    Still another consideration relates to refilling the air bag. As will be appreciated, once maneuvering at low speeds is complete it becomes important to quickly re-distribute the load over the multiple axles. Known arrangements take as long as thirty (30) to forty (40) seconds to refill the exhausted air suspension. Unfortunately, in that time frame, the vehicle can be up to speed and the load has not been adequately re-distributed. 
         [0008]    Yet another issue is that the air that supplies this system is obtained from the same reservoirs that are associated with the brake system. 
         [0009]    There is also a potential advantage of using existing systems and components. 
         [0010]    Thus, a need exists for an improved system that adds additional benefits in an economical, efficient manner. 
       SUMMARY OF THE INVENTION 
       [0011]    An improved system and process for controlling vehicle loading on a multi-axle vehicle is provided. 
         [0012]    The process for controlling the vehicle includes reducing pressure in a first axle suspension system when maneuvering the vehicle at predetermined slow speeds. Pressure is kept in the first axle pneumatic suspension system at the predetermined slow speed, and likewise a different pressure is kept in a second axle pneumatic suspension system at the predetermined slow speed. In one embodiment, keeping the pressure includes maintaining a first predetermined pressure at the slow speed, and a second predetermined pressure is maintained in a second axle of the pneumatic suspension system. 
         [0013]    In a preferred arrangement, the second predetermined pressure is greater than the first predetermined pressure, and more preferably, the first predetermined pressure is on the order of 10 psi. 
         [0014]    A pressure reducing step is dependent on the vehicle speed being ten miles per hour or less, and the process includes restoring air pressure upon vehicle speed exceeding the predetermined value. 
         [0015]    The process advantageously uses an anti-lock brake system controller that is modified to include these control functions. 
         [0016]    The air pressure restoring step uses a valve, such as a relay valve, to enable high flow rates to refill the pneumatic suspension system. 
         [0017]    The system includes a reservoir of pressurized air, a leveling valve receiving pressurized air from the reservoir, a valve selectively delivering air to the air suspension assembly, and a controller monitors speed of the vehicle and permits air to be selectively reduced to a predetermined pressure once a predetermined speed is reached. 
         [0018]    A primary benefit of the invention is the improved maneuverability of the vehicle at low speeds. 
         [0019]    Another benefit relates to less torque being imposed on the frame, as well as reduced wear on the tires. 
         [0020]    Still another benefit resides in less pinching of the air bags and the desired need to allow the air bags to keep their shape. 
         [0021]    Yet another benefit relates to the improved re-inflation or quicker restoration of air pressure in the suspension system. 
         [0022]    A further benefit is associated with conserving air in the overall system. 
         [0023]    A still further benefit resides in the ability to integrate the process and system with an existing ABS system and controller. 
         [0024]    Still other benefits and advantages of the disclosure will become apparent to those skilled in the art upon reading and understanding the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a schematic of the preferred logic associated with the present disclosure. 
           [0026]      FIG. 2  is a schematic of a first preferred embodiment of the present disclosure. 
           [0027]      FIG. 3  is a schematic representation of a second preferred embodiment. 
           [0028]      FIG. 4  is a schematic representation of a third preferred embodiment. 
           [0029]      FIG. 5  is a schematic illustrating a fourth preferred arrangement. 
           [0030]      FIG. 6  shows a schematic of a suspension circuit for a fifth preferred embodiment. 
           [0031]      FIG. 7  schematically illustrates a sixth preferred embodiment. 
           [0032]      FIG. 8  is yet another schematic of a seventh preferred embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    Turning first to  FIG. 1 , a trailer suspension dump valve (TSDV) is referenced for a semi-trailer (not shown) equipped with axle air suspensions in which air can be partially exhausted from air bags to reduce tire wear and improve vehicle maneuvering at slow speeds. This disclosure prevents the driver of the vehicle from removing air from the suspension of an axle above a predetermined speed, e.g., ten (10) miles per hour, using modified software incorporated into a conventionally available ABS brake controller. The system and method also reduce air bag pressure while preventing exhaustion of all air to atmosphere. The disclosure also advantageously improves the fill rate of the air bags once the slow speed maneuvering is completed and the vehicle then exceeds the predetermined speed. 
         [0034]    Thus, as schematically illustrated, either original or restored air bag pressure is provided in the suspension system as noted by reference numeral  100 . The operator actuates a switch, for example on the dashboard of the vehicle, the vehicle speed is monitored through a signal provided to an electronic control unit (ECU) of the vehicle, and the same information is input to the trailer antilock brake system (ABS or TABS) controller as typically found on a vehicle. In addition, a warning lamp may be provided to indicate whether or not the switch for activating the trailer suspension dump has been actuated. Thus, with the switch either activated to reduce air pressure in the rear axle air bags, or alternatively switched to a position set to restore air bag pressure, it is evident that the switch must be set to a desired position, and the speed of the vehicle monitored. The modified software incorporated into an existing ABS controller then indicates if the vehicle speed is greater than ten (10) miles per hour, whereby air must be automatically restored to the rear axle air bags. Only when the vehicle speed reaches some predetermined threshold, e.g., less than eight (8) to ten (10) miles per hour, will the software permit air bag pressure to be reduced. Thus, wheel speed is already provided to the ABS controller and can interact with the modified software to achieve these functions. 
         [0035]      FIG. 2  shows a first preferred arrangement, in which front air bags  110 ,  112  are associated with a first or front axle  114 . Similarly, the suspension system includes air bags  116 ,  118  are associated with a second or rear axle  120 . Pressurized air is provided from reservoir  130  which is maintained at a desired pressure by a compressor (not shown), as is well known in the art. All of the details of a conventional trailer antilock brake system (TABS) are not shown in order to reduce complexity, simplify the drawings, and for purposes of brevity. However, TABS controller  132  is represented as being in communication with the air reservoir  130  to provide rapid pulsed or controlled brake application in an anti-lock braking event, again, as is well known in the industry. As will also be appreciated, suitable signals are provided from the wheels to the controller  132  so that vehicle speed can be monitored. 
         [0036]    The air suspension system, and particularly, the individual air bags  110 ,  112 ,  116 ,  118  of the multiple axles are supplied with pressurized air from the reservoir  130 . That is, a pressure protection valve  134  is located downstream of the reservoir and protects system air pressure and maintains a constant specified pre-set pressure below that of the reservoir  130  if a downstream failure in the suspension system occurs. The pressure protection valve  134  would then prevent system pressure loss for the remaining pneumatic systems of the vehicle. The reduced pressure air is directly supplied to a pressure limiting valve assembly which includes a normally closed, three-way solenoid valve  140 , and particularly a supply or inlet port  142  thereof. Delivery port  144  of the solenoid valve provides a control signal to an inversion valve  150 , and particularly control port  152  thereof. Supply pressure from the protection valve  134  is also provided to a leveling valve  160 , and specifically to the supply port  162  of the inversion valve. There are two delivery ports on the leveling valve. The first delivery port  164  supplying pressure to the air bags  110 ,  112  associated with the front axle. The second delivery port  166  communicates with a supply port  154  of the inversion valve  150  so that when a control signal is provided by the solenoid valve, pressure continues downstream to delivery port  156  that communicates with relay valve  170 . More particularly, the delivered pressure from the inversion valve  150  communicates with a control port  172  of the relay valve so that supply port  174 , that receives pressurized air from the pressure protection valve  134 , is selectively delivered to first and second delivery ports  176 ,  178  and thus delivers pressurized air to the air bags  116 ,  118  associated with the rear axle. 
         [0037]    Line  200  is representative of a signal received from a switch mounted on a dash of a vehicle cab. Line  200  communicates with a relay  202  which is also adapted to receive a signal from the TABS controller  132  indicative of a predetermined speed. When the TABS controller sees a velocity of less than ten (10) miles per hour, for example, and where a signal is received through line  200 , then an appropriate signal is sent along line  204  that communicates with the solenoid valve  140 . The solenoid is then energized and allows the solenoid valve to deliver air from port  144  to a control port  152  of the inversion valve and thereby cause the inversion valve to exhaust its delivery through a pressure protection valve  210  associated with the inversion valve. The pressure protection valve  210  is designed to only partially exhaust the pressurized air, i.e., so that the air suspension reaches a predetermined level, for example 10 psi, and causes relay valve  170  to exhaust the suspension associated with the rear axle to this same level. Thus, the pressure protection valve does not allow the suspension associated with the rear axle to fully or substantially exhaust and instead keeps pressure in the rear suspension system. When the signal through line  204  is removed, such as if the dash switch is de-actuated, or if the velocity goes above the predetermined speed (e.g. ten (10) miles per hour), the system then reverts back to a non-dump operation. 
         [0038]    In the non-dump operation, leveling valve  160  delivers air to the supply port of the inversion valve. Since line  204  is not actuated, and thus the solenoid is not energized, control port  152  does not receive air pressure from the solenoid valve and thus the inversion valve delivers air to the control port of the relay valve  170 , which, in turn, causes the relay valve to deliver air pressure to the suspension air bags  116 ,  118 . 
         [0039]    A second preferred embodiment is shown in  FIG. 3  and for consistency and ease of illustration, like components will be identified by like reference numerals, while new components are identified by new reference numerals. In this arrangement, the solenoid valve  140  still communicates with the pressure protection valve  134  via supply port  142 . As in the first embodiment, the leveling valve  160  still supplies the inversion valve  150  and particularly at the supply port  154  thereof. Here, however, a pressure reducing valve  211  (not a pressure protection valve as in  FIG. 2 ) also receives pressure from the protection valve  134  at port  212 . During a non-dump operation, the pressure from protection valve  134  proceeds through the reducing valve  211  to its outlet port  214 , where it communicates with one side of a double-check valve  220 , and namely port  222 . However, higher pressure provided from the inversion valve  150 , and namely from port  156  during a non-dump operation, communicates with port  224  of the two-way valve. The double-check valve  220 , thus delivers the higher pressure from the inversion valve to the control port of relay valve  170 . This causes the relay valve to deliver air to the air bags  116 ,  118  of the suspension associated with the second axle. Thus, the leveling valve  160  delivers air to the supply port of the inversion valve  150 . Since there is no delivery from the solenoid valve  140 , since no signal is present on line  204 , the inversion valve  150  delivers air to the double-check valve. This port  224  of the double-check valve sees higher pressure than the air that has proceeded through the reducing valve  211  and which communicates with the other port  222  of the double-check valve. Accordingly, the double-check valve will deliver the higher pressure from the inversion valve to the relay valve and bring the air bags up to pressure. 
         [0040]    When a vehicle operator actuates the rear axle air dump switch, a signal is provided on line  200 . In addition, once the TABS controller  132  detects a velocity of less than ten (10) miles per hour, for example, a signal is then provided on line  204  to the solenoid valve  140 . This signal causes the solenoid to energize and, in response, delivers air from port  144  to the control port  152  of the inversion valve, thereby causing the inversion valve to exhaust to atmosphere. As a result, pressurized air does not reach port  224  of the double-check valve. Consequently, the double-check valve still receives a reduced level of air pressure from reducing valve  211  (on the order of 10 psi) which proceeds through the double-check valve and is delivered to control port  172  of the relay valve. This causes the relay valve to exhaust the suspension to the same pressure level, i.e., on the order of 10 psi. Once the signal from the dash switch is removed so there is no signal on line  200 , or once the velocity exceeds ten (10) miles per hour so that no signal is present to relay  202 , the solenoid valve  144  is no longer energized due to the absence of a signal on line  204  and the system reverts to the non-dump operation described above. 
         [0041]    A double-check valve arrangement is maintained in the third embodiment of  FIG. 4 , however, a slightly different schematic is used to supply port  222  thereof. More specifically, a synchronization valve  240  is added to the circuit. The synchronization valve includes a supply port  242  that is delivered pressure at a reduced level from delivery port  214  of the pressure reducing valve  211 . In the non-dump operation, when no control signal is present on line  204  to energize the solenoid valve  140 , there is no delivery from port  144  to the inversion valve. An additional branch line communicates between the delivery port  144  of the solenoid valve and port  244  of the synchronization valve. Thus, if there is no delivery from port  144  of the solenoid valve, the synchronization valve likewise will not deliver air through port  246  to port  222  of the double-check valve. Consequently, the double-check valve will deliver the higher pressure from port  156  of the inversion valve to the control port  172  of the relay valve. This causes the relay valve to deliver air to the air bags of the suspension. 
         [0042]    The operation of the system of  FIG. 4  will now be described with reference to the dump operation. As noted previously, the vehicle operator actuates a switch to provide a signal along line  200  indicating that the vehicle operator would like to remove some air from one of the axles in order to improve maneuverability. If the TABS controller recognizes a velocity of less than ten ( 10 ) mile per hour, for example, then a signal from the auxiliary connector  202  is provided along line  204  to the solenoid valve. The solenoid is then energized and delivers air to inversion valve  150 , namely port  152 . This control air causes the inversion valve to exhaust to atmosphere. In addition, the solenoid valve will deliver air to the control port  244  when the solenoid is energized. This causes the synchronization valve to deliver 10 psi pressurized air, or another desired pressure level, from the reducing valve  211  to supply port  242 , which then communicates with port  246  of the synchronization valve. In this manner, port  222  of the double-check valve  220  delivers the 10 psi air pressure to the relay valve control port  172 . This, in turn, causes the relay valve to exhaust the air suspension associated with the rear axle to 10 psi. As will be appreciated, when the signal from the auxiliary connector  202  is removed, or the velocity goes above ten (10) miles per hour, the system then automatically reverts back to the non-dump operation described above. 
         [0043]    Still another solution for selectively dumping a portion of the air from one of the axles to improve maneuverability is shown in  FIG. 5 . This embodiment still includes the solenoid valve and inversion valve of the pressure limiting valve assembly, and also the relay valve arrangement for quickly refilling the air bags, but additionally employs a limiting valve  250 . Particularly, a supply port  252  of the limiting valve communicates with the delivery port of the pressure protection valve  134 . In addition, the delivery port  156  of inversion valve  150  does not directly communicate with the relay valve  70 . Instead, the inversion valve delivers air pressure to control port  254  of the limiting valve. Delivery port  256  of the limiting valve communicates with the control port of the relay valve  170 . 
         [0044]    In a non-dump operation, the leveling valve delivers air to the supply port  154  of the inversion valve. Since there is no delivery from port  144  of the solenoid valve, the inversion valve delivers air to the control port  254  of the limiting valve  250 . With the limiting valve  250  being supplied with pressure from the pressure protection valve  134 , and being controlled by the inversion valve  150 , the limiting valve  250  will deliver full pressure to control port  172  of the relay valve. In this manner, the relay valve will deliver pressurized air from the protection valve  134  via ports  174  to ports  176 , 178  that communicate with the air bags. 
         [0045]    If the vehicle operator desires to improve maneuverability of the vehicle at low speeds, a signal is provided to line  200 , for example by actuating the switch on the dash. Only when the relay  202  also receives a signal from the TABS controller acknowledging that the vehicle velocity is below a predetermined threshold, for example ten (10) miles per hour, is a signal then provided along line  204  to energize the solenoid. Energizing the solenoid of valve  140  delivers air from port  144  to the control port  152  of the inversion valve. This causes the inversion valve to exhaust air to atmosphere. Thus, no pressure signal is provided to control port  254  of the limiting valve. Without such a control signal, the limiting valve will only deliver 10 psi to the control port  172  of the relay valve. As will be appreciated, this causes the relay valve to partially exhaust the suspension air bags  116 ,  118  to a predetermined level, on the order of 10 psi. When the signal  200  to the relay (by de-actuating the switch) is removed or if the vehicle velocity increases above ten (10) miles per hour (as monitored by the TABS controller), then the system solution of  FIG. 5  reverts to the non-dump operation. 
         [0046]    The use of single check valves in a suspension circuit in conjunction with the solenoid valve, inversion valve, and relay valve, is illustrated in  FIG. 6  as another potential solution. Here, single check valves  260 ,  262  are provided with interconnecting lines  264 ,  266  to provide selective one-way communication between the first/front and second/rear sets of suspension air bags. Thus, check valve  260  and line  264  interconnect the air bags on one side of the vehicle, while line  266  in conjunction with check valve  262  interconnects the air bags on the other side of the vehicle between the rear and front suspension assemblies. The check valves are arranged to permit communication from the rear suspension air bags to the front suspension air bags. The single check valves will not, however, transfer from the front suspension to the rear suspension. In this latter direction, the check valves act as a buffer against pressure spikes, for example chuck holes, bumps, etc., and preclude an air transfer from the front suspension air bags  110 ,  112  to the respective rear suspension air bags  116 ,  118  when in the dump mode. 
         [0047]    Thus, typical non-dump operation of the schematic solution of  FIG. 6  is as follows. The leveling valve  160  delivers air to the supply port  154  of the inversion valve  150 . Since the solenoid is not energized in valve  140 , there is no signal provided to control port  152  of the inversion valve. Consequently, the inversion valve delivers air from port  156  to the control port  172  of the relay valve  170 . This, in turn, delivers air through ports  176 ,  178  to the air bags  116 ,  118  of the rear axle of the suspension arrangement. 
         [0048]    When the vehicle operator would like to convert to the dump operation, the switch is activated to provide a signal along line  200  to relay  202 . Once the TABS controller  132  indicates that vehicle speed has been reduced to a velocity under ten (10) miles per hour, a signal is then provided on line  204  to energize the solenoid. This, in turn, provides a control signal from port  144  of the solenoid valve to the control port  152  of the inversion valve and the inversion valve then exhausts air that it would have otherwise delivered to the relay valve through the pressure protection valve  210 . The pressure protection valve is designed to only partially exhaust the air to a certain predetermined level, for example 10 psi, and thereby cause the relay valve to exhaust the rear suspension  116 ,  118  to the same level. In this manner, the rear suspension air bags are maintained at a reduced level relative to the air bags of the front suspension, and the check valves maintain this pressure differential. 
         [0049]    An alternate solution using a commercially available R-12 relay valve available from Bendix Commercial Vehicle Systems LLC, the assignee of the present application, is shown in  FIG. 7 . More particularly, the leveling valve  160  delivers air directly to supply port  174  of the relay valve. In addition, the pressure protection valve  134  delivers pressurized air directly to the supply ports of the solenoid valve  140  and the inversion valve  150 . The solenoid valve delivery port  144  still communicates with the control port  152  of the inversion valve. Thus, in the non-dump operation when the solenoid is not energized, the inversion valve delivers air from its supply port  154  to its delivery port  156  and thereby provides a control pressure signal to control port  172  of the relay valve. This causes the relay valve to deliver air from its supply port  174  to the dual delivery ports  176 ,  178  associated with air bags  116 ,  118 , respectively, of the rear suspension. 
         [0050]    Presuming that the vehicle operator actuates a switch for improved maneuverability at low speeds, a signal is then present at line  200 . As discussed previously, it is also necessary for the TABS controller  132  to indicate to the relay  202  that the vehicle speed has dropped below a certain level, namely less than ten (10) miles per hour. Only then is a signal provided in line  204  to energize the solenoid and thereby deliver air from port  156  to the control port  152  of the inversion valve. This causes the inversion valve to exhaust its delivery through the single check valve  270  (not a pressure protection valve as in  FIG. 2  or a pressure reducing valve as in Figures and  4 ). Importantly, though, is the fact that the check valve will limit the exhaust when it reaches a predetermined level, for example approximately 10 psi, which causes the relay valve  170  to partially exhaust the suspension to the same level, again on the order of 10 psi. When the signal from the auxiliary connector is removed in line  200 , or if the TABS controller indicates that vehicle velocity has risen above ten (10) miles per hour, there is no signal at  204  thus de-energizing the solenoid and the system reverts to non-dump operation. 
         [0051]      FIG. 8  illustrates a modified SR valve in an arrangement that bears some similarities to that described with respect to  FIG. 7 . Here, however, there is no pressure protection or check valve associated with the inversion valve as was the case with the valve  270  in the  FIG. 7  solution. Instead, leveling valve  160  receives supply pressure at port  162  from the pressure protection valve  134 . It delivers air pressure from the leveling valve to the front suspension air bags  110 ,  112  and to a modified SR valve  280 , having a supply port  282 . In addition, the pressure protection valve  134  is plumbed directly to the supply ports  142  of the solenoid valve and  154  of the inversion valve. In the non-dump operation, i.e., when there is no signal in line  204  to energize the solenoid, there is consequently no delivery from port  144  to the control port  152  of the inversion valve. Consequently, the inversion valve  150  delivers air from delivery port  156  to control port  284  of the modified SR valve so that air at supply port  282  is delivered from ports  286 ,  288  to respective air bags  116 ,  118  of the rear air suspension. 
         [0052]    In the dump operation, the solenoid in solenoid valve  140  is energized and provides the air pressure signal to control port  152  of the inversion valve. This, of course, means that the vehicle operator has actuated the system switch, and that the vehicle is also operating at a velocity of less than ten (10) miles per hour. The control signal of port  152  causes the inversion valve to exhaust its delivery to atmosphere. This, in turn, means that there is no pressure signal at control port  284  and the modified SR valve then partially exhausts its delivery until an internal mechanism stops the exhaust at the predetermined threshold, for example on the order of 10 psi. Likewise, when the vehicle operator returns the switch to the non-dump position, or if the velocity goes above ten (10) miles per hour, the solenoid is de-energized and thus the system reverts to the non-dump operation described above. 
         [0053]    In summary, this disclosure provides semi-trailers equipped with axle suspensions to partially exhaust air from the air suspension or air bags to reduce tire wear when maneuvering at slow speeds. The disclosure prevents the driver of the vehicle from exhausting air from the suspension of an axle when the vehicle is operating at a speed above ten (10) miles per hour. This is accomplished by modifying the software capabilities of a conventional TABS brake controller. 
         [0054]    The disclosure also provides a method for reducing air bag pressure while importantly preventing exhausting of all of the air to the atmosphere, i.e., only partially exhausting the air. By not reducing the air suspension pressure to atmosphere, the fill time associated with refilling is reduced. 
         [0055]    Moreover, by using the relay valve, higher flow rates are enabled to the air bags, which also shortens the time to restore the pressure to the suspension. Stated another way, the relay valve improves system responsiveness since flow rate is not limited by the constraints of the leveling valve. 
         [0056]    The attendant risk of damaging air bags is also eliminated by preventing exhausting of the air to atmosphere. Moreover, these arrangements also assure that more pressure is provided to the front suspension assembly than the rear axle during the dump operation. 
         [0057]    The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon reading and understanding this specification. It is intended to include all such modifications and alterations in so far as they come within the scope of the appended claims or the equivalents thereof.