Headlight relay harness for Honda NT650 Hawk GT

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If you've ridden a Hawk at night you've probably noticed that the headlight leaves something to be desired. 

I've got two solutions to this problem. One is a suplementary headlight wiring harness that uses heavy gauge wire and automotive relays. It is described on this page.

The much more effective (and more expensive) is a complete headlight swap from a Honda CB600F/Hornet/599. Click here for details on this.

Answers to questions

What causes the light to suck?

It's is caused by two things.
  • older lens and reflector design
  • deficient wiring
The first one is hard to fix cheaply, you can swap it out with a headlight bucket from a Honda 919, or Suzuki SV650, or GSF400, or some other bike, but that requires special parts, and won't look stock, and... well, you'd have to think about it.

If you want to replace the whole reflector and lens, consider my Honda 599/hornet 600 kit, which I believe is the best stock-looking light you can have on the Hawk for (almost) any price.

The wiring is deficient for several reasons,

  • small gauge wire
  • too much of it
  • too many switches
The brightness of a bulb is determined by how much power you can pump through it.  The amount of power is determined by the voltage across it. The voltage across the bulb is determined by the source voltage and the resistance of all the wires and switches between the source and the bulb and back to the battery.

In the case of the Hawk the power has to go from the battery, through the main fuse, through the ignition switch, through the main fuse box, through the right hand side starter switch (the headlight turns off when you press the starter) across to the left side high/low beam switch, and then finally to the headlight.  All of the wiring is pretty small gauge, (18 I think) and each of the switches adds resistance.

When I did my measurements I measured 9.6 volts across the headlight with the headlight on, but not the engine, as compared to 12.8 volts at the battery.  When I hooked the bulb directly to the battery using big wire the voltage was 11.9 volts.

That 2.3 volts makes a significant difference in the amount of light put out by the bulb, as the relationship between power and brightness is not linear, a properly powered 100 watt bulb puts out noticably more light than two 50 watt bulbs.

Why shouldn't I just use a high-wattage bulb?

Some people will just use a high wattage bulb in the stock harness to get some more light.  This works, sort of, but it's not a good idea.  Here's why:

  • The resistance of the stock wiring will not get you the brightness you expect, as the voltage drop will be more apparent with a higher wattage bulb.
  • The added load on the stock wiring will generate heat in all the switches, potentially melting them.  This doesn't happen very often, but if it does you've got a lot of work on your hands replacing them, and it's likely to be expensive.
  • When you open a switch that has current going through it there is a spark.  The spark damages the contact points of the switch.  The size of the spark and the damage caused is proportional to the amount of power going through the switch.  The switches will fail sooner than they would otherwise.  The relays are designed for high load, and are easily replacible in the very rare circumstance that they fail.  They're made by Bosch, so failure is unlikely.
  • Read this testimonial

What do I get with the kit?

What this harness supplies is some marine grade large gauge wire from the battery directly to the headlight, switched by two Bosch automotive relays that are controlled by the stock wiring harness.  The relays draw very little current so heat and arcing in the switches isn't a problem.

The harness includes a male H4 plug so there's no cutting into the stock harness, if you want to remove it, the bike is still stock.

The kit includes everything you need to install the harness except for some common tools.

Tool list:

  • #2 philips screwdriver
  • #3 philips screwdriver (for the battery terminals)
  • wire cutters (to trim the cable ties)
  • 6mm allen wrench (rear tank bolt and headlight aim)
  • 10mm socket (front tank bolt and battery box bolts)

picture of the kit    Click for a very large picture.

What are the options?

There aren't a lot of options.

The standard ones are:

  • Korean made, no-name 100/80 watt bulb.  This is not a "tinted" bulb.  read this note
  • Extra parts for dual light setup.  This doesn't include the lenses or reflectors or mounting hardware or anything.  This is if you already have a dual light setup and need the harness to plug into it.
  • Removable Diode to keep the low beam on when the high beam is on.  This is quite useful if you've got a dual light setup that has one low and one high beam, or if you just want more light.  The stock reflector has a plastic lens, and I've heard that over time the plastic can discolor from the heat, although I don't think I've heard any first hand reports of this, just that it's possible.  If you've got a glass lens (like from an SV650) it shuoldn't be a problem.
    I wouldn't recomend using both the diode and a high wattage bulb, that is a lot of power for a single bulb and a lot of heat.  Also, 180 watts is s significant portion of the total available output from the Hawk's alternator.

I've got an aftermarket fairing with dual headlights, what about that?

For $10 extra (and some measurements) I'll make it work with your fairing.  I'll need some information about the fairing before I can build the harness first:
  • How far apart are the bulbs? (center to center)
  • What kind of bulbs does it use? H4? H7? Something else? How would you like them wired? Some setups use one for high beam, one for low beam, others use one for both, one for high only, others use both for high, both for low.
  • probably some other question that I can't think of right now.

What about my CBRGSXRYZF 600 or my '72 Bultaco?

What's listed here is for the 88-91 Hawk GT only.  If you can give me lots of measurements, I'll entertain setups for other bikes, but you'll be expected to do a lot of the measurements and the research.  If we do something, you will take full responsibility for the fit, and any potential benifit.  I'll gladly help you, but I can't guarantee how well it will fit, or if it will help.

If you want to know if it could help, measure the voltage across the headlight when it's on, if there's a significant drop compared to the voltage at the battery, then the wiring is wimpy and could be helped.

If you can find a place for the relays to mount gracefully, and a way to get a bunch of wires from there to the battery and headlight, then there's potential.

How much does it Cost?

Stock harness$58
no-name Korean made 80/100
watt bulb (80 watt low)
8   (ea) Out of stock, sorry.
no-name Korean made 55/100
watt bulb (55 watt low)
8   (ea)
dual light setup10
removable diode for low beam
use with high beam
harness shipping 8-12   depending on location
paypal charge for harness3   (ea)
Honda 599 kit$330   Click here to see details on this.
599 kit shipping 10-15   depending on location
paypal charge for 599 kit11   (ea)

Unfortunately Paypal charges me money to accept payments.  My margin on these is small so I've got to make it up.  Sorry about that.  However, you can pay with a credit card through paypal, or send me a check or money order.

Shipping is $8 - $15 for the lower 48 states, depending on where you are, if it's to a business or home, etc.  For AK, HI, Canada, or other countries, send me mail and I'll find out what it costs.  I'll entertain faster/other shipping options, and even rush "Build me a custom one now!" orders, but since I don't necessarily have a finished harness waiting around to be shipped, anything faster than ground might be a waste of money. 

For weird setups (project bikes, funny fairings, stuff like that) Send me mail and we'll talk about it.

How do I order one?

Send me mail and tell me what you want.  I accept paypal or money orders or even personal check if I have some idea of who you are or if you're willing to let it clear before I ship.

Is installation hard?

Nope, it's pretty easy.

No knowledge of electricity or mechanics or physics is required. 

If you can read and you aren't color blind, you can do this.

Paper instructions are included, and an HTML version is available here: www.mahonkin.com/headlight-harness/instructions.

Typically I hear that it takes about 1/2 hour.

Do I have to worry about power consumption?

Not really.

Technically, if the bulb is running brighter, more power is going through it, which means the alternator has to generate more power to keep up with the draw. 

The power draw is increased if you use a dual light setup, or high-wattage bulbs.

However, it's not really a concern unless you've got two 100/80 watt bulbs running both high and low beams, and you've got an electric vest, and electric socks, and gloves, and pants, and ......

I'm exaggerating, but really you don't have to worry about it.  The alternator is rated at 240 watts at 5,000 RPM, and the bike (without headlight, but with gauge and running lights) draws about 60 Watts.  If you include 110 watts of dual 60/55 watt low beam, it's still only 170, leaving a resonable margin.