Suppressing "Bulb failure low beam" when converting from halogen to LED

[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 (and probably more P2 cars), headlamp bulb (dipped beam / low beam) 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 and signals „BULB FAILURE LOW BEAM“.
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. The wiring in the engine compartment needs to be altered; risk of moisture damage.
Solution B: Increase the resistance of the shunt and thus the voltage drop.

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 instead.
In my case (Osram Nightbreaker Speed H7 450, 16w, 64210DWNBSP-2HB), I connected two 0.1-ohm 1W resistors in parallel (resulting in 0.05 ohms).
The shunt board or resistors do not get noticeably warm during operation.

What did not work:
Drilling a hole in the circuit track (which was a tip for a xenon conversion, 2x33W http://www.xenonvalot.com/xenonfixvolvo.html).
Osrams load resistor "H7 LEDriving Smart LEDSC01" also did not work (if I remember correctly, they have 21w).


The hole in photo 3 was drilled to test the tip from the xenon conversion page.
Yes, I know: it’s not neatly soldered, resistor marking not in the same direction.

[Cees Klumper]

Thanks for posting this information. It's convinced me never to attempt this conversion!

(By the way I've always been happy with the output of the conventional halogen lights on my 2002 S80).