Converting to LED bulbs Topic is solved

[azev]

The S80 I (and a number of other models) are fitted with a bulb fault monitoring system. Any failure is indicated on the DIM / dashboard.
In pre-facelift S80 I models, headlamp bulb monitoring is implemented via a shunt resistor (part number 9442190A). It is fitted in the CEM in slot SH4.

The function (for halogen bulbs) works roughly as follows:
CEM SH4 pin 1 was not assigned/connected.
Pins 2 and 3 are connected in series with both bulbs. The two headlamp bulbs appear to be connected in parallel.
When the lights are switched on, 2 × 55 W flows through pins 2 and 3. The connection has a (low) resistance, which I was unable to measure with the multimeter, but which causes a voltage drop.
This is presumably measured between pins 4 and 5.
If a bulb fails or is replaced with LEDs (2 × 16 W = 32 W), the power and voltage drop decrease, and the CEM detects a failure.
If the shunt resistor is removed or the connection between pins 2 and 3 is broken, the headlamp bulbs will not light up.

Solution A: Connect power resistors in parallel with the bulbs. Drawback: They get very hot. Disadvantage: The wiring in the engine compartment needs to be cut; risk of moisture damage
Solution B: Increase the resistance of the shunt.

Solution B is relatively simple: Expose the circuit traces (e.g. with an abrasive disc), sever the connection between pins 2 and 3, and solder in resistors.

In my case, I connected two 0.1-ohm 1W resistors in parallel (resulting in 0.05 ohms).
Drilling a hole in the circuit track (which was a tip for a xenon conversion) didn’t work.
The shunt resistor or resistors do not get noticeably warm during operation.


(Yes, I know: it’s not neatly soldered)