Seat Heating, Front (Help Texts)

Current Measurement, Seat Heating

The current is measured in order to be able to provide a basic statement with regard to a defect in the seat heating system.

Measurement preparation
The current is measured at the lines under the relevant seat.
- Switch off seat heating
- Set seat to rear most and upmost position
- Measurement is carried out with a 50 A clip-on ammeter
- Partly unwind insulating tape from wiring harness under seat and attach clip-on ammeter to wire (white/yellow) for seat heating
- Start engine in order to ensure a system voltage of approx. 13.8 V

System voltage
If the system voltage drops below 11.4 V, the LEDs remain lit but the seat heating is switched off. The seat heating is re-activated as soon as a voltage of at least 12.2 V is permanently applied.

Measurement at DIS tester
- Select "measurement system"
- Measurement function: Current 50 A
- Measurement connection: Clip-on ammeter
- Type of measurement: Direct current =
- Measuring range: +/- 50 A
- Check system voltage with DIS tester
- Switch on seat heater on the side to be measured

Measurement evaluation
Generally applicable to all heating stages or heating settings: Each heating stage has a certain setpoint temperature. The control unit attempts to reach this temperature as fast as possible. The temperature is the kept constant by means of clocking (= pulsating direct current).
- Generally, the current is approx. 10 A during the heating-up phase (at a system voltage of 13.8 V)
- The heating output is reduced on approaching the setpoint temperature. The clocking function can now be distinctly recognized, i.e. the current pulsates between 0 and approx. 10 A.
- On finally reaching the setpoint temperature, the interval of the 0 A range is extended even longer.
- If the current temperature is higher than the setpoint temperature, no current is applied until the temperature has again dropped to the corresponding setpoint temperature.

Heating-up seat
The duration of the heating-up phase is dependent on the temperature measured at the temperature sensor! The setpoint temperature is reached more quickly at high ambient temperatures (e.g. in summer) or if the seat is preheated. In this case, the clocking cuts in correspondingly earlier.

Measured power
The measured power depends on many different factors. Both the level of the system voltage as well as the type of seat installed and operation of other loads (seat heating on other side, steering wheel heating) play a significant role. The specified value of 10 A is therefore only a reference value.

Setpoint temperatures
On E38 and E39 series vehicles, the seat temperature can be set by means of a potentiometer in a temperature range between 20 degree C and 42 degree C.

A two-stage seat heating system with the following heating stages is installed in E46 series vehicles:
- Stage 1 operation:
- The seat is heated to a setpoint temperature of 40 degree C

- Stage 2 operation:
- The seat is heated to a setpoint temperature of 37 degree C

Measurement evaluation
- A current of approx. 10 A is applied:
The seat heating is OK
- A current of approx. 5 A is applied:
Defect in heating mat of seat cushion or seat backrest
- No current is applied:
Defect in heating mat of seat cushion and seat backrest