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The Motorcycle Headlight Electrical Diagnoisis Page...

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Electrical problems can exist in any motorcycle, new or used, and can be particularly daunting at first to try to diagnose. This page is designed to help you isolate your electrical problems and solve them.


Headlights are probably the highest-draw item on your motorcycle, consuming more power than any other single accessory. They are also among the easiest to cause problems, especially if you don't follow the manufacturer's rating for bulb usage. There are a number of areas of possible problems with headlights:

1. Wiring:

  • Symptom: headlight operates fine for a fixed period of time (minutes to weeks), but then spontaneously blows the fuse, most often when the motor is running.
  • Possible Cause: wiring from the power source to the headlight has a bare spot where the insulation is worn away. When the bare spot comes in contact with a bare metal part (acting as ground), the path to the headlight becomes short-circuited and the fuse blows.
  • Trouble Shooting: follow the wiring from the headlight plug backwards through the wiring loom, searching for exposed wire or questionable insulation (cracked, brittle, etc.). If found, use corrective action (next).
  • Corrective Action (A): if the wire is in poor condition, cut out the bad segment of wire and replace with a new length of multistranded automotive electrical wire rated at the same or lower gauge (i.e. - if the existing wire is 14 gauge, replace with 14, 12 or 10 gauge wire). Note that multistranded is recommended because of the vibrations on a motorcycle, and that automotive electrical wire is recommended because of the environmental exposures the wire will endure (including heat, humidity, possible water/oil/gas exposure, et cetera). Do not use single stranded or household electrical wire as a substitute.
  • Corrective Action (B): if the wire insulation is nicked or abraided in a single spot, isolate the spot and wrap well with automotive grade electrical tape or equivilent, taking care to wrap for at least one and half inches in each direction to the side of the exposure. Determine if it is feasible to reroute the wire so that it does not rub against the same spot again. Consider replacing the wire with a longer segment if necessary to acheive the rerouting.

2A. Headlight Bulb(s):

  • Possible Symptom #1: headlight operates fine for a fixed period of time, but then spontaneously blows the fuse or filament when switched to high-beam (within seconds or minutes, not hours or days). Wire to the headlight bulb may be warm or hot to the touch if checked.
  • Possible Symptom #2: headlight operates fine for a fixed period of time, then spontaneously blows the fuse. Wire to the headlight bulb is warm or hot to the touch if checked.
  • Probable Cause: bulb is drawing more power than the electrical system wiring, headlight selector switch, headlight retaining plug, and (if present) headlight relay is rated for.
  • Trouble Shooting: Remove headlight and check power rating (55/60 or 50/55 are typical ratings, where the first number is watts drawn on low beam, and the second is watts drawn on high beam); the information is imprinted on the metal ring of most headlights. Compare this to manufacturer's recommended wattages for your motorcycle (locate on the web if you don't have any owner's manual or an after-market guide). Do not exceed the power draw recommended by the manufacturer without rewiring the entire system to handle the extra power. Many of the supposed "super bulbs" and fake "HID-look" bulbs draw substancially more than OEM; we recommend using high-efficiency replacement instead, such as Sylvania SuperWhites (which still draw the stock 55/60 watts, but put out the light of a 80/110).
  • Corrective Action (A): Replace bulb with a bulb with the recommended rating.
  • Corrective Action (B): Replace wiring loom to headlight, plug, headlight-selector switch, and (if necessary) headlight relay with appropriate lower gauge wire and connectors to support the higher ratings desired. Consider using parts from a newer model bike designed to utilize the high rated bulb system.
2B. Headlight Bulb(s):

  • Possible Symptom: Headlight works only on the low or high beam setting, but not on the other setting.
  • Probable Cause: bulb has failed or defective filament.
  • Corrective Action (A): Replace bulb with a bulb with the recommended rating. Retest. If test still failed, go to section 3.

3. Headlight Selector Switch:

  • Possible Symptom #1: headlight operates fine for a fixed period of time, but then spontaneously blows the fuse when switched between high-beam and low-beam (switched either way, fuse blows within seconds or minutes, not hours or days), or headlight simply ceases to operate when switched into one position. Headlight selector switch may be warm or hot to the touch if checked while bulb is operating.
  • Possible Symptom #2: Sparking or electrical noises at headlight selector switch.
  • Probable Cause: Corrosion or water intrusion into the headlight selector switch, or wear and tear of extended usage may have caused a less-than-ideal contact in the headlight selector switch.
  • Trouble Shooting & Corrective Action (A): Disconnect headlight selector switch from electrical source and from headlight. Check ohmage readings using an ohm meter with the switch in each position (reading should be zero, very close to zero, or perfect continuity). If reading is 1 in either position, replace switch. If reading is under .5 but above .008, go toTrouble Shooting & Corrective Action (B), below. If switch is replaced, pack new switch with dielectric grease before assembly to help prevent future problems.
  • Trouble Shooting & Corrective Action (B): Disconnect headlight selector switch from electrical source and from headlight and remove from motorcycle. Disassemble in well lighted area and physically inspect for presence of burnt plastic insulation, corrosion, carbon build-up and/or water. If on physical inspection, burnt plastic is found, replace switch. If water is found, clean with WD-40 or equivilent. If carbon build-up or light corrosion is found on the contacts, remove by sanding lightly with emory cloth. If heavy corrosion is found, replace switch. If reusing existing switch, pack with dielectric grease before reassembling to help prevent further problems.

4. Headlight Bulb Connector:

  • Possible Symptoms: Headlight operating inconsistantly, smell of burnt plastic, headlight fuse or headlight filament blowing.
  • Possible Cause: Time & temperature, corrosion, water intrusion, or over-rated bulb in the headlight bulb plug caused the headlight bulb spades to overheat, burning plastic bulb retainer and possibly damaging headlight bulb spades (corrosion, carbon build-up).
  • Trouble Shooting & Corrective Action: Disconnect headlight bulb and examine both the spades on the bulb and in the retaining plug for the headlight. Examine the plastic surrounding the bulb retaining spades. If contacts are dirty, plastic is burnt or part is questionable, replace. Pack replacement with dielectric grease. If no problems found, pack existing headlight retaining connector with dielectric grease before reassembly.

5. Headlight Relay:

  • Possible Symptoms: Headlight doesn't always turn on with the bike, headlight not operating (filament and fuse fine), headlight operation sporatic, or headlight works only in one setting (high beam or low-beam, but not both).
  • Possible Cause: In order to reduce the electrical load going to the headlight selector switch and the ignition system, many motorcycles (and cars) use a relay to connect the power source to the headlight bulb. Relays are magnetically driven switches, so a small amount of power can throw an electrical switch to permit another (higher powered) circuit to operate.
  • Trouble Shooting & Corrective Action (A): Remove relay and replace with a known good relay; recheck.
  • Trouble Shooting & Corrective Action (B): Remove relay and place on work bench. Using an electrical diagram, feed 12 volt power to the activation spades and check for continuity across the switched pathway. Remove power and recheck.
  • Trouble Shooting & Corrective Action (C, or the 'college student method'): Remove relay and place on work bench. Disassemble cover carefully (viable only with relays using a metal cover) by bending tabs straight. Visually examine relay contents, paying particular attention to the contacts and the swing-arm of the contacts. If contacts are scorched or carbon build-up is present, clean with emory cloth. Feed 12 volt power to the activation spades and visually check for engagement of swing-arm contact. Remove power and recheck for disactivation of swing-arm contact. If no movement is present when energized/de-energized, replace relay. If movement is present, but contact is not making good contact, bend arm slightly to reduce gap distance, and retest. If movement is present, but contact is refusing to separate on power-off, bend arm slightly to increase gap-distance when not powered & retest. Do NOT use any grease or oils in this assembly! If repaired, reassemble carefully and reinstall.

6. Charging System Over-Voltage/Under-Voltage:
Also SEE: Full Charging System Diagnoistics

  • Possible Symptoms: Headlight dims or brightens excessively with RPM, headlight filament or headlight fuse blows intermittently, blinker and other bulbs give out unexpectedly.
  • Probable Cause: Voltage regulator bad and/or battery defective, permitting charging system to vary power outside of expected range. Power variations damaging to various electrical components, including bulb filaments.
  • Trouble Shooting & Corrective Actions:
    - Using a voltage meter, check the voltage at the battery with the engine & ignition off (reading should be above 11.8 volts and below 13.2 volts). If readings are below 11.8 volts, remove battery and charge on a trickle-charger. After fully charging, check with a hydrometer and recheck voltage readings (should be 12.2 or higher after charging). If battery fails hydrometer check or voltage recheck after charging, replace with fresh battery before proceeding.
    - Start engine on center stand, and recheck voltage at battery while varying RPM's (valid range should be 12.4 volts through 14.2 volts). If readings are under 12.2 or above 14.8 while at any RPM with engine running, replace voltage regulator and/or alternator/generator as needed. Note that most shops will replace both at the same time rather than replacing just the voltage regulator/rectifier pack (although the voltage regulator/rectifier pack is normally the part that goes bad).
    - Further Troubleshooting (altenator/generator): remove primary leads to alternator/generator and check for continuity as per electrical diagram. If continuity can not be established, replace. If continuity is valid as required, replace only regulator/rectifier pack if voltage is out of range while engine is running and battery is in range with engine off.


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