EHC Control System
OVERVIEW OF EHC CONTROL SYSTEM
The control philosophy of EHC is to "Initiate a control sequence only when necessary". The system offers the following advantages:
- The control system operates independently from the vehicle's engine (no engine driven hydraulic pump system as per previous self leveling systems).
- Individual control of the rear wheels is possible
- An uneven load is identified and compensated for
- Uneven road surfaces are identified and not compensated for
- Automatic control is interrupted when cornering
- The system is diagnosable using the DIS or MoDiC
The air suspension system consists of the following components:
EHC SYSTEM COMPONENTS
CONTROL MODULE
The Control Module is mounted in the rear module carrier next to the battery on the right side. It contains the processing electronics and final stages for operation of the EHC system.
The control module receives the following inputs for its processing functions:
- KL 30 & 31 (Power/Ground)
- KL 15
- Left & Right Ride Height Sensors
- K Bus for:
- Vehicle speed
- Engine running
- Door/tailgate - open/closed
The control module incorporates two filters (slow/rapid) for processing the input signals from the ride height sensors. Depending on the operating mode, either the slow or rapid filter is used to check the need for a regulating sequence.
The slow filter is used during the normal operation mode to prevent normal suspension travel from causing the system to make adjustments.
The rapid filter is used during the pre-run and tailgate (LID) modes to ensure that the suspension is adjusted quickly while the vehicle is being loaded or checked prior to operation.
AIR SPRINGS
The air spring is made from a flexible rubber material. It forms an air tight cavity which provides the calculated spring rate required for the sport wagon.
As the spring compresses downward the bottom edge of the rubber material rolls along the vertical surface of the base mount cylinder.
Air is added or removed from the air spring through its top port. The top port of each spring is connected to a reservoir and the air supply pipes. The reservoirs are required to hold additional air due to the compact design of the springs.
REAR AXLE LEVEL SENSORS
Hall effect sensors are mounted on the left and right sides of the rear suspension for ride height detection. They are pivoted by a coupling rod through the rear axle swing arms.
The hall sensors produce a varying voltage input to the control module as the suspension height changes.
If the vehicle is equipped with Xenon headlights the right side sensor contains an additional sensor for the automatic headlight level adjustment system.
WARNING DISPLAYS
If the system is faulted and off-line or set in the transport mode, the following is displayed:
High Cluster: A message is posted in the high cluster matrix display.
AIR SUPPLY SYSTEM (LVA)
The air supply system is identified as the LVA in the diagnosis program and in the repair manual. It is mounted in the spare tire well compartment. The components are housed in a sound deadening carrier, through rubber bushings, to prevent operating noises from being transmitted through the vehicle's interior.
With the exception of the compressor relay, individual replacement parts for the air supply system are not available. If diagnosis determines a defect in any of the other air system components, complete replacement is necessary.
The air supply system consists of the following components:
- Compressor assembly with:
- Piston compressor
- Electric motor
- Air dryer (desiccator)
- Pressure relief solenoid valve
- Pressure maintenance valve
- Check valves
- Compressor Relay (Replaceable)
- Solenoid Valve Block (2 - two way valves)
- Lines - including distributor block
AIR SUPPLY SYSTEM (LVA) OPERATION
The single stage piston compressor produces a maximum pressure of 13.5 Bar. The compressor is maintenance free - provided it is used in a dust free environment. This includes the compressor's intake air filter.
The compressor is driven by a DC motor that is controlled by the compressor relay through the control module.
When the compressor is activated, the pressure builds up to a working pressure of 11.4 Bar (+0.8/-1.5 Bar). This is controlled through the pressure maintenance valve. The air under pressure is fed through the dryer and check valve to the solenoid valve block for the air springs.
There is one solenoid valve in the valve block for each air strut. This allows the system to compensate for uneven loads in the vehicle and maintain the vehicles ride height at all times.
Pressure is drained through the left or right solenoid valve (energized open) the pressure relief solenoid valve, restricter, check valve and dryer back to the inlet side of the compressor pump.
EHC SYSTEM OPERATION
A fully functional EHC system is controlled by one of three different modes of operation. The operation mode is selected by the control module based on current conditions provided by the monitored input signals. The main modes of operation are:
- Pre-Run/Post-Run Mode
- Normal Mode
- Tailgate Mode
Two special operating modes are also included in the control module programming.
- New/replacement mode (pre ZCS encoded). This mode provides basic operation.
- Transport Mode - Transport mode is set at the factory and raises the vehicle 30 mm to prevent vehicle damage during transportation. It must be deactivated with the DIS/ MoDiC prior to customer delivery.
PRE-RUN/POST-RUN MODE
The Pre-Run mode is activated when the vehicle is parked and the control module is in the sleep mode. Opening a door or the tailgate initiates a system wake up and the control module comes on-line.
The control module performs a self-check of the control electronics and sensors. It no fault is found, the system will check the ride height and institute a rapid regulation if the height varies by more than 40 mm.
NORMAL MODE OPERATION
Once the rear lid is closed, KL 15 switched ON and the engine started, the system switches into the normal operation mode. In the normal mode, the control module will constantly monitor the input signals from the ride height sensors and will activate a correction if the ride height deviates by at least 10 mm.
TAILGATE OPERATING MODE
The tailgate operating mode is activated if the gate is opened with KL - 15 On and the engine running. The difference between this mode and the normal operating mode is the response time is rapid instead of slow
SPECIAL OPERATING MODES
- ASSEMBLY LINE MODE (New control module)
The assembly line mode refers to control module manufacturing. New control modules are stored in a deactivated state. The control programming is not active and must first be ZCS encoded.
After installing a replacement control module, it must be coded using the DIS or MoDiC. The instrument cluster fault display will remain illuminated until the control module is coded.
CONTROL INTERRUPTS
CORNERING
To prevent unnecessary suspension adjustments while driving through corners, a "control interrupt" is built into the system. Above 30 MPH the control module monitors the left/right ride height sensors for a difference of 30 mm. Exceeding this difference will put the system into a control interrupt and no adjustment will take place. The control interrupt last for a duration of 5 minutes.
VEHICLE LIFTING
The ride height control is interrupted when the vehicle is raised on a lift or with a jack. The system monitors the ride height sensor inputs and when the height limit of 90 mm is exceeded, the control is switched OFF until the vehicle is lowered again.
EHC SERVICE INFORMATION
DIAGNOSIS/CODING
- The EHC control module is connected to the diagnostic link. The EHC control module activates the fault display in the instrument cluster to alert the operator of the off-line status of the system. The EHC control module stores up to three electrical/electronic faults.
- Diagnosis/troubleshooting of EHC is carried out using the fault symptom troubleshooting program of the MoDiC or DIS. The EHC system has an extensive diagnosis program.
- Replacement control modules are shipped in the factory mode. The control modules must be ZCS encoded using the DIS or MoDiC to activate the operating parameters.
DIS/MoDiC SERVICE FUNCTIONS PROGRAM
The Service Functions program of the DIS/MoDiC provides the Transport Mode activation/deactivation and Ride Height Offset functions.
Once the transport mode has been released, or if the system requires left to right side height adjustment, the ride height "OFFSET" must be carried out to ensure that the vehicles suspension has a base ride height level starting point.
The "HEIGHT OFFSET" is adjusted using the DIS or MoDiC. The procedure is as follows:
- Place the vehicle on a level surface unloaded.
- Access the Height Offset program in the service function menu.
- Measure the base ride height from the lower edge of the wheel housing to the center of the wheel hub.
- Check measured height against the specifications listed
- Use the DIS/MoDiC to correct the ride height if the value differs from the listed specification.