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

On vehicles equipped with a switching centre in the centre console, the current is measured directly at the switching centre.

• Switch off seat heating
  
• Remove centre console switching centre (A169)
  
• Measurement is carried out with a 50 A clip-on ammeter
  
• Unwind insulating tape from wiring harness under seat and attach clip-on ammeter to the wire to be measures.
  
• Left seat heating: Measurement on HZFA wire (white/yellow, pin 1)
  Right seat heating: Measurement on HZBF wire (white/green, pin 13)
  
• 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 for all heating stages: 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, e.g. when, after a longer period of operation, the heating is switched back from stage 1 (3 LEDs) to stage 3 (1 LED), 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

The following setpoint temperatures are reached in the three-stage seat heating system:

• Stage 1 operation (3 LEDs lit):
  The seat is heated to a setpoint temperature of 35 °C
  
• Stage 2 operation (2 LEDs lit):
  The seat is heated to a setpoint temperature of 39 °C
  
• Stage 3 operation (1 LED lit):
  The seat is heated to a setpoint temperature of 44 °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