Automatic headlight control

Ever forget to switch-on the headlight ? No more a problem with this mod. It works when the bike moves, as soon as a gear is engaged.

This mod is primarily intended for TDM900 2002, TDM850 - TRX850 - XTZ750, and actually all machines with a manual headlight switch. It will fit easily on TDM900 2003 + (with automatic headlight control), but is it less useful then...

How does it work ?

The circuit uses a relay to control the headlight. It is activated when a gear is engaged, and reset itself only when the main switch is switched off.

The circuit controls the headlight only, not the pilot lights. The original light switch may still be used as usual - actually it will be used only to switch on the pilot lights.
Anyway check your local daylight riding regulation concerning the headlight / pilot light.

Circuit description

(image)Composants :
• T1 : SCR (thyristor) C106A or equivalent.
• OPT : opto-coupler LTV816 / 4N25 or equivalent.
• D1 : diode, 1N4001.
• LED : any standard type.
• C1 : capacitor, 100nF.
• C3 : capacitor, 100µF.
• R1-R4 : resistors 5% 1/4W.
• K : 12V/12A power relay.

All these components are very common and available from all electronic components retailers.
Use a hot glue gun to secure the components & the wiring.


My casing comes from a salvaged car indicator relay, the relay inside was ok for the job. I had to remove the other components, modify the PCB and then add new components. The original connectors are kept and a small PC connector is added to connect the LED.

The casing is located right next to the ECU, hooked to an existing support where the 2003+ headlight relay is located.


• HeadLight : headlight live wire, to the main light switch connector (located under the airbox).
• VCC HL : +12V power from the main light switch connector.
• VCC : +12V from the main light switch connector.
• RUN : chassis.
• Neutral : neutral switch from the blue connector located behind the black left-hand side panel.

Use wires with adapted gauge (at least 10A current) for HeadLight and VCC HL. All other wires are ok with smaller gauge, just avoid too thin or rigid wires - vibrations will destroy them !


Salvaged part : an old indicator relay.
Inside : the relay is kept, other components are added.
The casing is located next to the ECU (2002).

TDM900 2003+

Models from 2003 on (2002 on OCE) have an automatic headlight relay. The relay is switched on by the ECU as soon as the engine is running.

Adapting the circuit above is relatively simple since the relay is already present (near the ECU).
The headlight will then switch only when you start riding, not when the engine is running.

Original headlight relay wiring
Modified headlight relay wiring : modified wiring shown in red color.
Modified headlight circuit diagram.

• The circuit is inserted between the oem headlight relay and the ECU command.
• The relay is connected to the circuit at RELAY.
• The ECU command is connected to the circuit at RUN.
• +12V is connected to the circuit at VCC.

TDM900 - 2002

The TDM900 2002 has no ECU controlled headlight relay, although this feature is available on the ECU but left unused (2002 EUR models only, 2002 OCE models have the relay).

It is easy to add this feature on any 2002 EUR model : just mod the ECU connector, then add a relay + diode and some wiring like on any 2003+ models.

Since the ECU connector pins are quite special, the best is to reuse the pin from the airbox solenoid (that means two mods at the same time).
• remove the ECU connector and unlock pins by pushing the white inserts.
• remove the Brown / Black wire / pin (red, see diagram below).
• relocate the pin on the blank green pin location (swap the cap seal).
• replace the airbox solenoid (under the air filter box) with a relay + diode and make all connections required for the headlight (see wiring diagram 2003+).
• or simply replace the airbox solenoid with the above circuit.



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