Seat Functions
Comfort Seats (Standard On E38 750iL, Other Models If Equipped)
This option provides additional back support in the shoulder area. An additional adjustment in the upper backrest allows forward "contour" that is controlled by a rocker switch in the side panel.
When the ignition is on or off, the two way rocker switch can be pushed until the desired setting is achieved (forward/backward tilt). The motor drive operates the same as the seat motors.
The shoulder support adjustment is also stored in memory for recall.
Active Seat (E38 - if equipped)
Purpose of the System
Actively changing the surface of the seat helps to prevent muscle tension and fatigue particularly on long trips or sitting in one position for a long period of time. The seat causes weight transfers which are slight and imperceptible enhancing driving comfort. The basic seating position is not changed by this.
In the seat's upholstery beneath the surface of the seat is a cushion of fluid on the left and right. The fluid is slowly circulated back and forth between the cushions by a pump.
Principle of Operation
To activate this feature, the ignition must be in the KL R or KL 15 and press the switch located in the Center Console Switch Center (driver and/ or passenger front seat).
The switch provides a ground to the control module and an LED in the switch will illuminate. If there is a fault in the system the LED will not illuminate.
The control module, fluid bladders (2 and 3) and pump (1) is contained in each seat. The pump contains a Hall sensor to monitor the number motor revolutions and two pressure sensors to monitor each bladder for vacuum (indicating empty).
The fluid bladders (2 per seat) contain water-glysantin mixture and are connected to the fluid pump. The fluid bladders are equal in volume at rest (15 mm. high) and will raise from 0 up 30 mm. when filled alternately from left to right.
NOTE: The pump unit and fluid bladders are replaced as a unit
The control modules (one per seat) are connected to the P-Bus. The driver's side control module contains an acceleration sensor that monitors transverse acceleration of the vehicle. Active Seat operation is temporarily interrupted during high acceleration.
Rear Power Seat (E38 750iL)
The rear seats in the 750iL provide electric adjustments for:
1. Backrest Angle - Control switch powers the motor.
2. Headrest Height - Control module behind each upper seat.
3. Lumbar Support
The headrest automatically extends when a passenger fastens the safety belt and will automatically retract when the safety belt is released. The power rear seat is a purely electric control system. No electronic diagnostic communication is possible.
Seat Heating Operation (from 99 MY)
Principle of Operation
The front seat heaters are adjustable through three ranges of heating output temperature.
Pressing the respective seat heater button once provides stage 1:
- All three LEDs illuminate and the heating elements are provided regulated output current producing a seat temperature of 111 °F.
Pressing the button a second time provides stage 2:
- The top LED switches off and the heating elements are regulated to an output temperature of 102 °F.
Pressing the button a third time provides stage 3.
- The top and middle LEDs are off and the heating elements are regulated to an output temperature of 95 °F.
The SZM monitors the seat heating element temperature via an NTC feedback signal to regulate the output current which maintains the seat temperature.
Seat heating is switched of by pressing the button a fourth time, pressing and holding the button for more than 1 second or when the ignition is switched off.
System Components
Carbon Fiber Heating Pads: In each seat is a two section heating pad wired in parallel. The heating pads are resistors which when powered produce radiant heat.
The seat base heating element also contains a temperature sensor for feedback to provide the temperature regulation output control.
Center Console Switch Center (SZM): From 1999 MY E38, E39 and E53 vehicles are equipped with a SZM to control the front seat heating and provide a diagnostic interface with the DISplus/MoDiC via the K BUS.
The SZM directly controls:
- Front Seat Heating
- Rear Window Roller Blind
SZM also provides a unitized switching center for:
- Park Distance Control (PDC)
- Dynamic Stability Control (DSC III)
- Electronic Damper Control (EDC).
The switch signal output for these systems is a direct output signal. All diagnosis functions are carried out through their respective control systems.
SZM Monitoring of Seat Heating
Battery Voltage: The SZM switches current supply to the heating elements off when battery voltage drops below 11.4 volts. However, the heating stage LEDs remain on. Regulated output current resumes when battery voltage raises above 11.2 volts for more than 5 seconds.
SZM Internal Temperature: The power output stages for the seat heating elements generate a considerable amount of heat when in stage 1 operation. The SZM monitors it's own internal temperatures and reduces the heating output when internal temperatures rise to a temperature of 185 °F or switches it off completely above 203 °F. As with battery voltage monitoring, the heating stage LEDs remain on when these temperatures are exceeded.
Fault Monitoring: The SZM monitors the temperature sensors and heating mats for faults. Detected faults are stored in the SZM. Fault Symptom Troubleshooting in conjunction with stored faults will initiate the diagnostic paths using the DISplus/MoDiC. The following faults can be recognized:
- Shorts or opens in the wiring circuits.
- Shorts or opens in the temperature sensors.
- Open in heating element.
If a short is detected in the temperature sensor, the seat heating is switched off to prevent overheating. The Stage LEDs are also switched off with this fault present.
Rear Window Roller Sunblind Operation
The roller sunblind is activated by momentarily pressing and releasing (one touch) the sunblind rocker switch in the SZM. The SZM activates the motor in the appropriate direction. The SZM switches the motor off when the motor amperage increases indicating the blind has reached the end of its intended travel.
Fault Monitoring: The SZM continually monitors the motor drive amperage loads to determine jamming or blockages. If current values exceed preset values, the motor is switched off immediately.
Faults are stored in the SZM for opens or shorts in the motor or motor's wiring circuit. The SZM also provides diagnostic requests to monitor the switch input status via the DISplus/ MoDiC.
System Components: Inputs - Processing - Outputs