I have Bajaj Pulsar 180 UG4, it has a 35 watt HS1 Helonix headlamp bulb. I want to upgrade the stock headlight to make it brighter, so I found H4 Xenon bulb 60/65 watt. I asked this question question and got this answer from mac. He says I should go for a relay as I want to use a more powerful bulb, but I am not a mechanic so I don't know which relay to use and how to wire it.

Can you please help me in finding correct relay for my bike and how to wire it.


3 Answers 3


Your question raises more issues than you probably thought.

With respect to the issue of a relay, then assuming your bike has a 12 volt, negative ground, electrical system, all you need is a generic automotive relay, which looks like this (sometimes called a Bosch relay, regardless of whether it is actually a Bosch unit). Here is an article, which looks accurate at a skim, that describes how relays work, starting with general information and narrowing down on automotive relays specifically (including the standard pinout of 30, 85, 86, and 87).

Before you get that far, you need to consider the output of your generator (I'm guessing your bike has a generator...but it could have an alternator). Cheaper bikes often have undersized generators, and if your energy requirement exceeds your charging capacity, your battery will run flat even while you ride. On a 12 volt system, the stock bulb pulls approximately 3 Amps (35 Watt / 12 Volts). The replacement will pull around 5-5.5 Amps depending on whether you end up using the low or high beam of the H4 bulb (that's an important difference between the bulbs you mention, and I'm sure you saw this already...the H4 has 3 prongs, a common ground in the middle flanked by the positive leads of the high and low beam filaments). If the only powered accessories on the bike are the lights, this could actually be significant (bikes with provisions for other accessories usually have some headroom).

Next is the wiring going to the headlamps. Obviously, go directly from the battery. Make sure you use a thick-enough wire to prevent excessive voltage drop, which will make the light appear dimmer than it should be and overheat the wire. At the amperage of a single 60 Watt bulb, and the distance involved on a motorbike, 18 gauge (American Wire Gauge, or AWG) is the minimum recommended, but I would suggest 16 or even 14 (no such thing as too much wire, so long as the weight penalty is minimal). Make sure you make your connectors weather proof -- that means sealing any solder or crimp joints with marine-grade shrink tubing (which includes adhesive), unless you actually manage to get your hands on a proper, multi-hundred-dollar crimping tool, in which case just crimp and ride. Otherwise your connectors will eventually corrode and fail, and bikes get wet, so this will happen much sooner than you imagine.

Finally, don't expect miracles. Motorcycle lights usually don't work well, but not because they are underpowered -- because the optics don't work well (reflector and lens). In your case, the 35 Watt bulb is also underpowered...but I'm sure the optics are pretty bad as well. I've been riding bikes for about 25 years, and so far I've owned only one with headlights that were good enough to ride with at night with the same level of visibility as a car. Ironically, that was a relatively inexpensive Kawasaki KLR650 (relatively inexpensive by global standards...very cheap by U.S./Canadian standards). I've had to put auxiliary lights on much more expensive and exotic bikes. All those bikes ran 55 or 60 Watt bulbs, but the inefficient reflectors and/or lenses diffused the light excessively, rendering the headlights effectively dim.

Good luck, have fun with the project, and ride safe.

  • 1
    +1 very good comment on potentially outstripping the generator/alternator's ability to charge the battery. Add-on LED lighting may be more efficient.
    – mac
    Nov 16, 2012 at 16:04

If you want to build the relay setup yourself instead of using a commercially produced motorcycle headlight relay kit, you'll need two automotive relays, commonly called "Bosch relays". They should have a rating of at least 10A, which they nearly all do (generally you find them at least 20A). You need one relay for the low-beam, and one relay for the high-beam.

You'll run power wires of 14 or 16 GA with a 15A fuse from the battery to the front of the bike, maybe in the headlamp housing--wherever you can hide the relays.

  • The new +12V wire (with a fuse!) goes to pin 30 on each of the relays
  • The new ground wire goes to the ground pin on each of the headlight bulbs
  • Take the old +12V wire that went to your existing low-beam bulb, and wire it to pin 85 of one of your relays, this is now your low-beam relay.
  • Take the old ground wire that went to your existing low-beam bulb, and wire it to pin 86 on your low-beam relay.
  • Take the old +12V wire that went to your existing hi-beam bulb, and wire it to pin 85 of the other relay, this is now your hi-beam relay.
  • Take the old ground wire that went to your existing hi-beam bulb, and wire it to pin 86 on your hi-beam relay.
  • Connect pin 87 on your low-beam relay to the +12V low-beam terminal of your new H4 bulb
  • Connect pin 87 on your hi-beam relay to the +12V hi-beam terminal of your new H4 bulb

This is not a super-complicated job, but can be made simpler by purchasing a pre-made kit, which will have the H4 bulb socket, relays, wiring, fuse holder, and instructions.

Basically what you're doing is connecting pins 85 and 86 of the relay to where your old bulbs used to be. Now your handlebar switch turns on the relays instead of the old bulbs. The relays, in turn, turn on the new, higher powered bulb, which is connected straight to the battery with new, larger wiring.

  • for what it's worth, this source says that an H4 bulb will fit an HS1 socket, which means that this H4 Single Headlight Relay Kits should be a direct fit for your motorcycle. If you choose to make one yourself, you're basically replicating this kit.
    – mac
    Nov 16, 2012 at 16:20
  • Thanks for this nice explanation. But I have a question that why do we need two relays can't we use single SPDT relay?
    – Sandy
    Nov 17, 2012 at 15:36
  • you said two relays would be needed then what type of two relays I would need SPST or SPDT?
    – Sandy
    Nov 17, 2012 at 15:37
  • my concern is that I am setting up new single bulb so don't you think single SPDT relay would be enough.
    – Sandy
    Nov 17, 2012 at 15:49
  • your single bulb actually has two filaments in it--one for the low beam and one for the hi beam. each is on its own circuit, so you need two relays.
    – mac
    Nov 19, 2012 at 16:42

If you're not comfortable with this sort of thing you need to find someone who is. You could burn up your whole electrical system if you wire in a relay and headlight poorly. If I were in the same room as you I could walk you through it, but you have to understand that this is just a question answer site.

  • 1
    I understand your concern and I totally agree with it that I should goto a mechanic or someone who have understanding of these things. I asked this question so that I can understand these things and I should be able to do it myself but anyways I appreciate your response.
    – Sandy
    Nov 16, 2012 at 7:39

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