Headlight upgrade: Relay circuits

Headlight upgrade: Relay circuits

As Sebastians Article about restoring the W124 headlights was a good point to catch up i thought about showing how further improvement can beachieved. Theres a quite simple way to upgrade your headlights and gain more light ouput without either destroying the OEM look nor being highly skilled in electronics. This can be done with building in relay circuits. But what is the point of that?

As you surley can imagine the electricity has to go long distances until reaching the headlights. From distributor X4 to the light switch, over to the high beam switch. Then moving through the fuses, the headlight controller with final destination being the headlight itself. When metering the no-load voltage the result where 14,3 volts at the battery at 800rpm idle. The right headlight only received 13,13 volts, the left one 13,35 volts. The voltage loss is due to resistors and doesnt sound too bad. The results after the upgrade will teach us better!

Lets get started!

A specialty of my circuit will be the that it wont leave any marks within the system. No wires will be cut that would need soldering once removed. Nobody will ever remove this gadged again but anyway! Some minor circuit plan reading skills are needed as i wont describe the building process.


This is the circuit plan


The relay circuit ready to built in










Main goal was to have a solution that wont affect the stock look without cutting any wires. There are solutions with OEM relays but these parts lack 2 connections and cutting wires would be the result. Besides a voltage and resistance measurement i also measured the headlights brightness in LUX, before and after the upgrade.




…and after the upgrade. Distance was 170cm, more space was unfortunately not available









The installtion

Installing the circuit is quite easy. First of all disconnect the battery. Then open up the fuse compartment by unbolting the 6 screws on top of it. Another screw is placed within the compartment right in the front and then you are ready to proceed. No you can lift the relays and have them mounted to the hood. No you need to move underneath the dashboard:



The trim below the dashboard needs to be removed to access the main distributor X4. You will find it upl left above the brake/clutch pedal


Having a wire attached to cables helps to make the way down to X4.










Connecting it


Once attached to the X4 distributor we ´ll proceed within the fuse compartment.  No we open the wires underneath the fuses, 2,5mm in white for low beams; AB) and 2,5mm in yellow for high beam des Fernlichtes marked (FL) and replace them with the short wires coming in from the relays.


Having done this you need to attach the wires coming from the car to the relay, yellow to high beam (FL) and white to low beam (AB).












To have the relays working we need to ground them on clamp 86. You can search for suiteable connections within the fuse compartment like we did or use any connection you might find!

img_3395 img_3396







Last point on the agenda, shrink hoses around the wires and dump the complete setup down into the fuse compartment.


To reduce another voltage loss i replaced the 8 ampere fuses with 16 ampere on place 13 – 16. But this is up to you!



I hope i dont need to tell you that working on the electronic system is totally on your own risk.







At the end i collected all metering results which i had before and after the upgrade. Hope you enjoyed this little article and maybe it helps  to lighten up some of you! 🙂




Battery no-load voltage 12,57 volt
Battery no load voltage at 800 rpm 14,34 volt
Before After
Left headlight
Fuse compartment before fuses 13,82 volt 14,05 volt
Fuse compartment after fuses 13,59 volt 13,93 volt
At the headlight (low beam) 13.35 volt 13,67 volt
(ohne Last 14,0 Volt)
Right headlight
Fuse compartment before fuses 13,79 volt 14,00 volt
Fuse compartment after fuses 13,59 volt 13,90 volt
At the headlight (low beam) 13,13 volt 13,46 volt
(no load voltage13,98 volt)
Brightness in LUX 9.390 11.700
-All metering results against ground at the battery
-8 ampere fuses replaced by 16 ampere




Regards Oliver (ollib)


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