Front Kneeling System (Coaches Only); Principle Of Operation; Maintenance; Air Spring Control Solenoid Valves - PREVOST H3-41 Maintenance Manual

SECTION 16 : SUSPENSION
9 FRONT KNEELING SYSTEM
(COACHES ONLY)
The kneeling system is used to lower front of
vehicle. This allows passengers to board the
vehicle with greater ease. The kneeling action is
achieved by exhausting air from the front air
springs (bellows). This system bypasses the
height control valve to provide a fast up and
down movement of the front suspension. Only
seven seconds are required to lower vehicle
from normal level to the lowered position, and
approximately the same time to raise the vehicle
back to normal level. The quick response is
achieved by the kneeling air tank installed beside
the secondary air reservoir (for exact position,
refer to Section 12, "Brake and Air System").
This tank provides sufficient air supply to the
kneeling system for
operations. The system is provided with two
safety features; first, a speed switch will enable
the kneeling system to work only below 5 mph (8
km/h). Secondly, the
automatically applied, and a limit switch will keep
it applied as long as the vehicle has not returned
to a certain height where the driver will be able to
manually remove the parking brake.
The purpose of the front axle hi-buoy function in
this system is to raise the front end of the vehicle
to allow an extra ground clearance for particular
situations. In driving condition, the height control
valve is in operation and only the hi-buoy can be
operated.
9.1

PRINCIPLE OF OPERATION

Refer to the air system schematic diagram
annexed at the end of Section 12, "Brake and Air
System".
DOWN (FRONT KNEELING):
Both the air spring control and air spring exhaust
solenoid valves are energized, so the air control
valves release air from front air springs. The
height control valve is bypassed to ensure no air
is forwarded to air springs while lowering the
front suspension.
UP (FRONT HIGH-BUOY):
Only the air spring control solenoid valve is
energized, so the air coming from the kneeling
air tank is routed through air control valves, and
up to front air springs.
50 PA1621 Maintenance Manual All Series | Section revised October 2018
some successive
parking brake is
The height control valve is bypassed until the
kneeling proximity switch signals the kneeling
module to cut off the air spring control solenoid
valve, about 1" (25 mm) below normal ride
height. The final height adjustment is achieved
by the height control valve.
9.2

MAINTENANCE

Since the kneeling action is issued from both the
air system and electrical system, refer to
Section: 12, "Brake and Air System" and Section
06, "Electrical System".
For diagnosis and understanding of the system,
refer to wiring diagrams, and to the appropriate
air system schematic diagram annexed to
Section 12, "Brake and Air System".
9.3 AIR SPRING CONTROL SOLENOID
VALVES
9.3.1

Removal and installation

1. On the rear side of steering compartment,
locate both the air spring control and air
spring exhaust solenoid valves.
2. Identify hoses
reinstallation. Disconnect solenoid wires and
the three flexible black hoses from solenoid
valves.
3. Unscrew and remove the control solenoid
valve and exhaust solenoid valve assembly.
Place on a clean working place.
Reverse removal procedure to reinstall.
CAUTION
Always adjust on "fill cycle". If it is necessary to
lower vehicle height, release sufficient air to be
well below height, and adjust to height or fill
cycle.
10 HIGH-BUOY SYSTEM (COACHES
ONLY)
The purpose of the full high-buoy system is to
raise the entire vehicle body about 4" (100 mm)
in order to increase ground clearance to board a
ferryboat, to jump a curb, etc. This system can
and wires to ease