MAINTENANCE MATTERS
Kneeling know-how:

 

A closer look at the ups and downs


The kneeling feature on a J4500 is an electrically controlled air-operated system that reduces the ground-to-first-step height, allowing the passengers to get on and off the bus more easily. It accomplishes this by removing air from the front axle air springs to lower the front end of the vehicle.

The operation is simple: A switch in the driver's instrument panel controls the system. The driver presses the switch to the "down" position to lower the front end of the coach. When the switch is pressed to the "up" position, the front end rises so it is ready to drive.

The following is an overview of how the kneeling system works. A more technically detailed explanation follows.

What it looks like

Under the front end of the coach is a set of air valves which are mounted on the vehicle body that connect to the front axle/suspension via two rods (a single rod before unit 62806). The right-hand valve (leveling valve) controls the front-end ride height by controlling the air to the front air-suspension air springs during normal operation.

The left-hand valve (kneeling valve) is used by the kneeling system to sense the height as the vehicle rises after the front end was kneeled. Once the front end has kneeled, air is required to be induced into the front air springs to raise the vehicle. This valve senses the raised height of the front end and, by controlling the air pressure to an air switch, electrically disconnects the raise control of the kneeling system when the proper height has been reached.

The kneeling system is controlled by a switch located in the right-hand driver's instrument panel. The system is inhibited from operating unless the park brake is applied, the transmission is in neutral and the front door is closed. By pressing the down position of the switch half, the front end lowers (kneels) to the kneeled position and is ready to allow passengers to get on and off more easily. To return the front end to normal operating height, press down the up (recover) position of the kneel switch half or release the park brake.

The kneeling switch actuates an electro-pneumatic valve (kneeling module) that is located in the back wall of the spare-tire compartment. This valve is made up of an electric air switch and electro-pneumatic air valves mounted in a manifold that are connected to the air system as follows:

There are three air supply lines through the kneeling module, two of which go to the front air springs. One comes from the auxiliary air tank and supplies air for the kneeling recovery mode, while the second runs from the leveling valve to obtain the operating front-ride height or driving mode. The last line is the control line from the park brake valve. When the park brake is not applied, the air pressure on this line causes the kneeler to be inhibited from lowering coach front end. It also actuates the raising of the front end if the vehicle is kneeled and the park brake is then released. Thus it acts as a backup to the recovery part of the instrument panel kneel switch should the driver not adequately raise the front end prior to releasing the park brake. There is also an exhaust port on the valve to evacuate air from the front air springs when kneeling.

To actuate kneeling, the kneeling switch on the instrument panel is pushed into the down position.  This causes the spool valve in the kneeling module to slide and open the air spring bellows ports to the exhaust ports allowing the air to leave the air springs. At the same time, it blocks the air from the leveling valve and the supply air from the auxiliary air system. To retain a minimum air pressure in the front air springs, the air exhaust is interrupted when the air pressure preset switch on the kneeling module sensing the air spring pressure is actuated. To hold this kneeled position, the exhaust port is then closed and the air is held in the blocked position from the leveling valve and the auxiliary air supply port.

The recovery mode is initiated by either the kneel switch in the instrument panel or releasing the park brake. The kneeling module spool valve shifts allowing the auxiliary supply air to connect to the front air springs which causes the coach front end to rise until the kneel valve actuates and disconnects the quick recovery system.

Front Ride Height Control System

Under driving conditions, air movement to and from the front axle bellows is controlled by the front leveling valve connected to the front axle as discussed previously. If the ride height on the front axle is below the set point of 11½ inches measured as indicated in the maintenance manual, this leveling valve will provide air through the kneeling module via the V3 air valve and check valves. If the ride height on the front axle is above the set point, air in the front air springs will be exhausted through the kneeling module exhaust port.

Kneeling in detail

When the down kneel switch is actuated on the instrument panel, solenoid SV2 in the kneeling module is energized and allows air to flow to spool valves V2 and V3, causing the valves to shift. This will cause V3 to stop air flow from the main coach leveling valve to the air springs and V2 to exhaust the air out of the air springs, causing the vehicle to kneel.

When the air pressure switch in the kneeling module senses less than 15 psi, it closes, sending a ground signal to the electrical system that de-energizes solenoid SV2. This stops the air from exhausting and maintains a positive air pressure in the air springs. The vehicle is now knelt and holding at that position.

Electrically, when the kneel switch is depressed, a ground signal is sent to input #25 on VMM 1 module (Front Junction Box upper module) which will activate output #14 on VMM 1. This energizes the solenoid SV 2 on the electro-pneumatic valve. Output #17 of module VMM 1 is also activated to supply power to the audible alarm and warning light on the exterior of the coach.

When the coach has knelt as low as it can go, air pressure in the front axle air springs drops below 15 psi. The air pressure switch closes, sending a ground signal to input #7 on VMM 1 module, and output #14 on the VMM1 module is de-energized. This removes power from solenoid SV2 and output #17, turning off the audible alarm and exterior kneel lamp.

Kneel Recovery in detail

Actuating the kneeling switch recovery mode located in the driver's instrument panel energizes the SV1 solenoid actuating the V1 valve that directly connects supply air to the front air springs for quick recovery. This causes the vehicle to rise rapidly until a ride height reaches approximately 10-10½ inches when the kneeling valve/air pressure switch de-energizes the circuit.

When the park brake is released, air pressure is applied via the V3 valve and the front leveling valve to the front air springs. The electrical system is programmed to energize the SV1 valve when the park-brake-applied signal is removed.

Electrically, the kneeling system operates the same whether the park brake is released or the recover switch in the driver instrument panel is actuated.  A ground signal is supplied to input #6 on the VMM-1 module, which energizes the output #15 on VMM-1 module and actuates the SV1 solenoid. It also turns on output #17 on VMM 1 module to power the audible alarm and the exterior warning light. When air pressure in the front air springs increases above 15 psi, the pressure switch opens removing the ground signal to input #7 on the VMM-1 module. When ride height reaches 10 – 10 ½ inches when measured as described in the maintenance manual, the pressure switch on the kneeling valve provides a ground signal to input #6 on VMM-1 module, which de-energizes output #15 on this module. Output #17 VMM-1 module is also de-energized, deactivating the audible alarm and exterior kneel lamp.

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