Introduction: CBF 125 Headlight Circuit Modification

Given the popularity of the CBF 125 I was surprised to find little in the way of detailed information to help those who wish to prepare a bike for the installation of HID headlighting. Most of the information I did uncover was either uninformed or so unintelligible as to be worthless. With this in mind here’s what I hope is a simple step-by-step guide on how to convert the headlight to a ‘pure’ DC supply.

Step 1:

First, remove the seats, left fuel tank infill piece and left front and middle fairing panels. Detach the negative and then positive terminals from the battery and position a four-pin, 12v (normally off) relay next to the intake hole of the airbox on the left side of the bike just behind the tank. Run a length of wire from terminal 30 of the relay to the positive terminal of the battery. This wire should contain an inline fuse holder with its 10 amp fuse removed. I made the connection to the relay using a standard automotive female spade connector and at the battery end fitted a ring connector.

Step 2:

Now trace the wires from the dimmer switch (on the left handlebar) back to their block connector beneath the fuel tank. This connector contains six wires – white, yellow, blue, grey, green/red and black. Working on the loom side (in other words, to the rear of the bike), uncouple the connector and either remove the yellow pin or cut the yellow wire approximately an inch and a half from the block connector (see Figure 1). If the pin has been removed discard it, connect a new one to a length of new wire and insert the pin in the block connector. If the yellow wire has been cut, attach a bullet connector to the short length of wire protruding from the block and crimp it to a length of new wire (see Figure 2). In either case the redundant yellow wire on the loom side must be isolated. With this done, feed the newly fitted wire back to the relay, cut it to length and attach it to terminal 87 using a female spade connector.

Step 3:

Next, return to the block connector and locate the black wire on the switch side (to the front of the bike). This wire runs from subfuse A and carries DC power to the horn switch. As with the yellow wire on the loom side, either remove the relevant pin or cut the wire an inch and a half from the block. If the former, splice a new wire to the existing black wire, connect both to a new pin and locate the pin in the block. With the latter method, fit a bullet connector to the cut end of the black wire attached to the block. Bind a new wire to the other cut end, insert into the bullet connector and crimp (see Figure 3).

Step 4:

With this done, DC power has been reinstated to the horn switch and the new wire can be used to power a headlight switch. The mechanism I used was a small waterproof button switch that was fitted to a flat rectangular area at the front of the left tank infill panel. A second new wire was attached to the switch and fed back to the relay where it was connected to terminal 86. Another wire was attached to terminal 85 and connected at the other end to the common earth terminal located beneath the fuel tank (see Figure 4).

Step 5:

Here, a word of caution. To protect the electrical system from ‘blowback’ a diode should be fitted to the relay. Some relays contain an inbuilt protective diode. For those that don’t, a diode can be connected in parallel by binding its legs to the stripped ends of the wires attaching to terminals 86 and 85 prior to the fitting of the female spade connectors. It is important, however, that the striped end of the diode be connected to terminal 86.

At this juncture the 10 amp fuse may be inserted into its inline fuse holder and the circuits checked for continuity. If the system is working correctly all connections should be waterproofed with insulation tape or sheathing. The ring terminal may then be connected to the battery and the new circuits tested under ‘working’ conditions.

Step 6:

In summary, we have installed a relay connected at terminal 30 to the battery via an inline fuse. A small amount of power has been tapped from the horn switch circuit, fed through a newly installed switch and back to terminal 86 of the relay. With the switch in the ‘on’ position current passes through terminal 85 via 86 and grounds at the common earth terminal. As it does so it creates an electromagnetic field in the relay which actuates an internal switch, allowing battery power to pass through terminal 30 and on through terminal 87. The wire connected to terminal 87 is also connected to the yellow wire in the block connector situated beneath the fuel tank. This modified yellow wire now routes battery power to the dimmer switch and thus the headlight. As such, we now have a DC powered headlight which can be switched on or off irrespective of whether the engine is running. Using this configuration the bike is ready for the installation of an HID kit.

Here, it might be pointed out that this modifiction does not affect the sidelight. My initial objective was to reduce the drain on the battery as far as possible, to which end I concentrated solely on providing the headlight with DC power. With hindsight this was a mistake. Having installed an H4 HID system the sidelight looks weak and very yellow. At some point in the future, therefore, I’ll again tap into the black horn switch wire and make a connection with the sidelight’s supply wire. Once provided with DC power the sidelight can be changed for a whiter, brighter and much less consumptive T10 LED bulb, meaning that the sidelight alone can be used for normal daytime running. This will not only be kinder to the battery, but it should also extend the life of the HID headlight bulb.

Step 7: Here's How It's Done ...

Ensure that the seats, left fairing panel and left tank infill piece have been removed from the bike. Detach first the negative and then the positive battery terminals. The sidelight block connector is located behind the left fairing panel and is connected directly to the indicator relay suspended from the fairing panel bracket. Hence the easiest way to locate it is simply to trace back from the indicator relay. The block contains six wires (yellow, black, white, grey, blue and green), two of which (black and grey) are routed to the indicator relay. The remainder are connected to either the headlight or sidelight.

Step 8:

Working on the loom side (see Figure 5) uncouple the connector and cut the black and yellow wires approximately an inch and a half from the block. Strip the ends of the black and yellow wires still attached to the block, bind them together, insert the tail into a bullet connector and crimp. Take the remaining length of the cut black wire, strip the end and connect to the other end of the bullet fitting. With this done isolate the adjacent length of yellow wire, test for continuity and replace the battery terminals (positive then negative) once the testing has proved satisfactory. The bullet connector and isolated yellow wire should be waterproofed using either sheathing or insulation tape.

The sidelight fitting is coupled to the headlight shell by way of a grommet-like fixing which can be pulled from its housing with ease. Once detached its bulb pulls out just as easily. With our modified wiring configuration the stock bulb can be replaced with a T10 LED equivalent.