Suspension diagnostic
Chassis / Suspension

Air Suspension &
Steering Diagnostics

Complete diagnostic procedure for ECAS air suspension systems and power steering in heavy-duty coaches and buses. Covers ride height analysis, air spring testing, height sensor calibration, solenoid valve testing, and steering system inspection.

Critical Parameters

Air Pressure
8–12.5 bar
Front Height
320–380 mm
Rear Height
340–400 mm
Side Diff Max
15 mm

Contents

SymptomsECAS SchematicRide HeightDiagnostic StepsTroubleshootingFailure ModesQuick Reference
Vehicle sitting unevenly (one side low)
Vehicle dropped on suspension overnight
Kneeling function not working
Excessive body roll or bouncing
Steering wander or pull to one side
Compressor running continuously

The ECAS system is a closed-loop control: the ECU reads height sensors, compares to target ride height, and commands solenoid valves to inflate or deflate individual air springs. The system also manages kneeling (lowering entry side for passenger access), memory positions, and load-leveling.

ECAS (Electronically Controlled Air Suspension) System Layout

COMPRESSOR8–12.5 barAIR DRYERMoisture removalAIR RESERVOIRSuspension tank8.0–12.5 barECAS ECUElectronic controlCAN bus connectedSOLENOID BLOCKInflate / Deflate valvesCONTROL SIGNALFRONT BELLOWSLeft + Right2 air springsREAR BELLOWSLeft + Right2–4 air springsHEIGHT SENSORFront axleHEIGHT SENSORRear axleFEEDBACK SIGNALPPressure sensor

Ride height is measured from the bottom of the chassis frame rail to the ground at each axle. Measurements must be taken on level ground with the vehicle unloaded (or at specified load). The side-to-side difference must not exceed 15 mm — greater difference indicates air spring leak, sensor fault, or frame damage.

Ride Height Measurement Points — Coach / City Bus

BUS BODYFrame / ChassisFARAGROUND LEVELH1Front ride320–380 mmH2Rear ride340–400 mmKNEELING MODEEntry side drops 60–80 mmSide-to-side difference: MAX 15 mm (left vs right)
1

Air System Pressure Check

Measurement: Pressure gauge on suspension air reservoir
Expected values:
8.0–12.5 bar = Normal system pressure
Below 6.0 bar = Low pressure — compressor or leak fault

The suspension reservoir should maintain pressure even with the engine off. If pressure drops overnight, there is a leak in the system.

2

Air Spring Visual Inspection

Measurement: Walk around vehicle, inspect each air bellow
Expected values:
All bellows fully inflated, no visible cracks = Normal
Collapsed or cracked bellow = Air spring leak — replace
Do not work under a vehicle supported only by air suspension. Always use jack stands.
3

Height Sensor Test via Diagnostic Tool

Measurement: Read ECAS sensor values, manually raise/lower vehicle
Expected values:
Smooth voltage change 0.5–4.5 V as height changes = Normal
Fixed voltage regardless of height = Sensor or linkage fault
Erratic/jumping values = Loose connection or damaged sensor

Height sensors are typically rotary potentiometers connected to the axle via a linkage rod. The rod can bend, disconnect, or corrode — always check the mechanical linkage before condemning the sensor.

4

Ride Height Measurement

Measurement: Tape measure from frame rail to ground at each axle, level surface
Expected values:
Front: 320–380 mm (check OEM spec for model)
Rear: 340–400 mm (check OEM spec for model)
Side-to-side difference MAX 15 mm
5

Solenoid Valve Actuation Test

Measurement: Command each solenoid via diagnostic tool, observe bellow response
Expected values:
Bellow inflates when fill valve activated = Normal
Bellow deflates when exhaust valve activated = Normal
No response = Check solenoid coil (20–40 Ohm), wiring, and valve mechanism
6

Leak Test — Soap Spray

Measurement: Spray soapy water on all air connections, bellows, and lines
Expected values:
No bubbles = System sealed
Bubbles at connection = Tighten or replace fitting
Bubbles on bellow surface = Replace air spring
7

Shock Absorber (Damper) Inspection

Measurement: Visual inspection and bounce test
Expected values:
No oil on damper body = Normal
Oil leak on damper = Replace (in pairs per axle)
Vehicle settles within 1–2 bounces = Damper OK

Interactive Suspension Troubleshooting

step 1

Is the vehicle sitting unevenly (one side lower, front/rear tilted, or completely dropped on air springs)?