11-19-2013, 01:55 PM
Officially Welcomed to the 'Fest
Join Date: Mar 2013
Mein Auto: 08 e83 LCI
Ah OK well that's easy to fix.
Check Fuse F16 25A
he seat heating system heats the seat cushion area and backrest. The electronics for controlling the selected heating temperature (3 stages) and fault monitoring are integrated in the switch centre of the centre console.
Seat heating can be switched on in three heating stages as from terminal 15 by pressing the button. Three LEDs built into the switch centre light to indicate operation. "Terminal 15 active" is signalled to the centre console switch centre via the K-bus.
Initially, heating stage 1 is activated when the seat heating is switched on (button pressed once). The seat surface is heated up to approx. 44 °C. All three LEDs light to indicate operation.
By pressing the button once more, the topmost LED goes out and heating stage 2 is activated. The seat is now only heated up to 39 °C.
The seat heating system is switched to heating stage 3 by pressing the button once again. The bottom-most LED lights. The seat is heated up to 35 °C.
The seat heating is switched off by pressing the button once again or switching off the ignition. The seat heating can also be switched off in any heating stage by pressing the push button for longer than 1 second.
The seats are heated by means of carbon fibre heating mats in the main section and steel wires (steel filaments) in the bolster. A carbon fibre heating mat is installed in both the seat cushion section as well as in the backrest. The centre sections of the seat and backrest as well as the bolsters of the seat and backrest are always heated. Heating filaments or carbon fibre mats are additionally integrated in the thigh support on sports seats. The seat and backrest bolsters are not heated on the series E39 sports seat with fabric cover.
The temperature is measured by a temperature sensor in the seat upholstery and passed on to switch centre.
The electronic seat heating control switches off the power supply to the heating mats if the battery voltage in the vehicle is less than 11.4 V ±0.3 V. The function LEDs however remain on. Power supply to the heating mats is restored when the battery voltage is greater than 12.2 V ±0.3 V for at least 5 seconds.
Due to the fact that the power input of the seat heating system is very high (120 W), the temperature of the pc-board in the switch centre is monitored by temperature sensors. Initially, the heating power is reduced as the temperature increases to over 85 °C. The heating is switched off if the pc-board temperature exceeds 95 °C. The function LEDs however remain on. The seat heating cuts in automatically again when the pc-board temperature drops below 85 °C.
The electronic seat heating control system monitors the temperature sensor and heating mats for faults. Detected faults are stored in the fault code memory of the centre console switch centre.
The following faults can be recognized:
Short-circuit or line break
Short-circuit or line break of temperature sensors
Break in heating mats
If a break or short occurs at the temperature sensor, the seat heating is switched off in order to prevent overheating. The function LEDs are switched off.
Here's how they used to flash a fault, bu tI guess they only support it via INPA these days. I'd be under the seat with my multimeter, expect about 2-5R per mat if memory serves.
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.
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
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
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.
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.
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.
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
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
The seat heating electronics monitors the temperature sensor and the heating elements for faults.
If a break or short occurs at the temperature sensor, the seat heating is switched off in order to prevent overheating.
If a fault is recognized, the function LED installed in the switch flashes for 12 seconds when the fault occurs, or, if the fault is already present, when the seat heating is switched on.
A flash code shows what faults the electronic circuitry has currently detected.
Flash code output is started by following procedure:
Switch off ignition
Press and hold the button for seat heating
Switch on ignition (terminal 15) with button depressed
Release the button and press it again briefly
The following faults can be indicated:
Switch faulty (potentiometer or electronics) or switch overheating due to a short-circuit
Temperature sensor has break or short
All 4 heating elements broken or have a short (probably break or short in wiring harness)