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NC30 Low Beam Relay Installation¹


One of the most common mods bike owners make is to upgrade their bikes' OEM lighting. Usually, this involves higher wattage H4 halogen bulbs, or sometimes inefficient-but-trendy "blue" bulbs. But 400cc Honda sportbikes, which come with twin 35/60w headlight bulbs, can particularly benefit from certain bulb upgrades. As shown on the first wiring diagram excerpt below, NC30s¹ have a relay only on the hi-beam circuit. Accordingly, if you want to upgrade your NC30's headlights² and not risk damaging the headlight switch, you should install a low-beam relay by following the procedure outlined below. A standard 20A automotive relay will work fine (available from Maplins, Tandy, Radio Shack or any auto alarm shop). The new relay on the NC30 is stuffed into the rubber boot in the center of the photo to the right:

NC-30 Low Beam Relay

This modification can be done without running any new wires (long ones, anyway). The left-hand side middle and lower fairings will need to be removed, because the access point is a set of connectors on the lower left-hand side of the bike. You'll need basic electrical tools to do the mod. Secured in a metal bracket are two six-pin connectors (each with only five wires) covered by a PVC hood. Once the wiring is completed, it is possible to push the relay up inside the hood "up-stream" of the connectors, so everything remains more or less protected from the elements.


NC-30 Low Beam Relay Focus

UK-spec NC30s came with low beam relays and 55/60w H4 headlight bulbs as standard. Essentially, this modification attempts to mimic the UK-spec headlight circuit without having to modify the bike's wiring harness. On the J-spec NC30 headlight wiring diagram to the right, the white wire goes from the low beam element on the 35/60w H4 bulbs directly to the handlebar switch. If you simply fit higher wattage bulbs without adding a relay, the switch (which is available only as part of the switch pod) will see more than 55% additional current on a repetitive basis—every time you switch the headlights on, or from "Parking" to "On."

J-spec NC-30 Headlight Wiring Diagram

Pull the headlamp connectors off both of the bulbs, then, using a continuity tester, find the white (ish) wire (lo beam) on one of the "up-stream" connectors. You will also need a ground—there's a black wire with a green band around it on one of the connectors that works fine (even if it may be a floating ground). Then, you will need to find the "Bat5" power lead—black with red stripe—it's the same wire that powers the hi beam relay on the other side of the bike. Wiring the relay is a matter of tapping into (without necessarily cutting) the ground and power leads (temporarily releasing the contacts from the connectors with a tiny screwdriver makes this easier) and cutting and intercepting the white (ish) lo beam lead. Note that the "hot" side of the white wire connector is the one that you are working on, i.e., the power comes from the handlebar switch and runs down and over to the other side of the bike before going back up to the headlight bulbs.

Modified NC-30 Headight Wiring Diagram

The aforementioned "standard automotive relay" has four contacts, conventionally labeled 85, 86, 87 and 30 (see the new relay in the diagram above). The contact labeled "30" is the input power (black with red stripe) lead; 85 is the ground; 86 goes to the handlebar switch; and 87 goes to the headlight bulbs. Solder and wrap all of the connections, plug in the headlight bulbs, and see if it works! If you ever want to remove the relay, make sure you use "female spade" connectors on the relay contacts; then you can quickly connect the two white (ish) wires back together, and you're back to Honda spec.

Will you be my friend?


(1) Although this modification procedure relates specifically to NC30s, a similar modification would work for other J-spec 400s, such as NC35s and NC29s, and for certain mid-90s European-spec, dual-headlight Hondas (e.g., German-market CBR1000Fs) that illuminate only one of their two H4 bulbs on dipped beam.

(2) For a good description of how to use regular H4 (i.e., 55/60w or even brighter) bulbs in a Honda, see this mirror of Jeff Knights' now defunct NoyzToyz H4 Bulb Modification Page; for a description of how to modify a (VFR750F) headlight unit to permanently accept normal H4 bulbs, see Bob Peloquin's H4 modification page. Home Copyright © 1999-2006 [Source: Jeff Knights' now-defunct H4 Modification page]