Kneeling System/Air Flow - D Series Coaches

Coach kneeling is a basic feature available on MCI® D series coaches. This feature allows the driver to lower the front end of the coach while the coach is parked, creating a lower step height enabling easier access into and out of the coach. The components for this feature are located in the #1 left hand baggage compartment and the front electrical junction box below the driver's window. There are two switches; one on the dash and the second on the coach exterior beside the exterior switch for the entrance door.

The basic function of the system is to exhaust air pressure out of the front axle bellows, dropping the front end down onto the rubber suspension stops. When the recover switch is pressed, the system then brings the suspension back up to normal ride height. This sounds very simple in theory, but in actuality requires four relays, four pressure switches, two solenoids, two double check valves, one relay valve, and one leveling valve. Also included with this feature is an audible alarm and exterior flashing light.

To minimize confusion while explaining the operation of the kneeling system, only the air flow will be discussed. The electrical portion of the system will be discussed in the December service tip.

There have been different versions of the kneeling system used over the years. This service tip will be addressing the system used on all D series built after 1996 with the exception of the new models featuring the Dinex I/O multiplexed electrical system. As a quick reference, remove the cover from the kneeling box in the #1 left hand baggage compartment and inspect the contents. The various air and electrical components should be arranged as illustrated in the picture below. See Figure 1.

Figure 1

In the figure above there are several abbreviations used. Below is an explanation:

The air flow during the operation of the kneeling system will be explained in four separate steps, normal ride, bump up, kneel, and recovery.

Kneeling System Air Flow - Normal Operation

Located in the front axle area are two leveling valves. When the coach is driving, air pressure from the auxiliary air tank supplies the LH leveling valve, which controls the air pressure in the front axle air bellows through the normally-open solenoid (SOL 2) and the double check valves. See Figure 2.

Figure 2

Kneeling System Air Flow - Bump Up Stage

With the ignition on, air system up to pressure, park brake applied, and the entrance door closed, pressing the kneel switch will cause the coach front end to briefly rise, or bump up. The purpose for this is to shift the double check valves so the front axle bellows are no longer connected to the LH leveling valve, but instead to the R12 relay valve. See Figure 3.

Figure 3

Kneeling System Air Flow - Kneeling Stage

When the kneel switch is pressed, SOL2 is energized separating the LH leveling valve from the double check valves. SOL2 also exhausts the air pressure from the air line routed to the double check valves. When this air pressure drops below 3 psi the PS1 switch, which is also connected to this air line, opens disconnecting the power to SOL1.

When SOL1 is de-energized, the air pressure on the control port of the R12 relay valve is allowed to exhaust through SOL1 and out the pressure protection valve (PPV). As the air pressure to the R12 relay valve decreases, air is exhausted out of the front axle bellows, lowering the front end. The PPV is preset to close at 20 psi, maintaining this pressure on the R12 relay valve control port and thus maintaining 20 psi through the relay valve, through the double check valves, and to the front axle air bellows. This pressure allows the front suspension to lightly rest on the front suspension stops. See Figure 4.

Figure 4

Kneeling System Air Flow - Recover Stage

When the Recover switch is pressed, SOL1 is energized sending coach air pressure to the control port on the R12 relay valve. This opens up the R12 relay valve to send coach air pressure through the double check valves to the front axle air bellows, causing the coach rapidly raise. See Figure 5.

Figure 5

When the coach ride height is approximately 9.5 - 10 inches, the SOL1 solenoid is de-energized, and coach air pressure is removed from the R12 relay valve control port. At the same time, power and ground are reversed to SOL2 reconnecting the LH leveling valve to the double check valves. The air now supplied through the LH leveling valve shifts the double check valves and flows into the front axle bellows, slowly raising the ride height to the factory setting of 11 inches. See Figure 2.

For further assistance, please contact the MCI Customer Support Center at (800) 241-2947.

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