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FYI: Electrical basics 1 -- testing a lamp with a multimeter

Charging System, Wiring, Lighting
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e3steve
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FYI: Electrical basics 1 -- testing a lamp with a multimeter

Post by e3steve » Wed Sep 08, 2010 10:43 am

This may give some of your arses an 'eadache, as it's pretty basic; but it may help some to test their lamps without the aid of a charged battery or a charger.

You can check the continuity of a lamp (bulb) with a multimeter on its Ohms (Ω or 'R') range; always short the meter probes together before continuity testing just to get an idea of how the meter is calibrated at full continuity (short circuit):
  • Disconnect the lampholder, if possible, from its supply wire, leaving the lamp in place
    Hold or clip one of the probes to the bike's ground; I prefer to use the batt -ve or a cylinder fin
    Place the other probe in contact with each wire into the lampholder(s) and note the reading*
If the reading shows infinity (∞), or a very high resistance -- >1KΩ (1000Ω) -- then remove the lamp from the lampholder and check it independently. Try to make sure that you don't hold both probes' conductors at the same time, as this will produce a false reading if the lamp is open circuit (blown), albeit a resistance up in the Megohms range.

With the lampholder still connected to earth**, resistance readings between those noted below and, say, up to 5KΩ (5000Ω) would be a reasonable indication of a poor connection within the lampholder or a grounding point/path. Where the lampholder is still grounded on the bike and the reading is high, as mentioned, leave the meter connected via clips or clip-leads and physically 'work' lamps, connections & brackets to see if the reading changes drastically, thus indicating the approximate location of a poor (resistive) connection. Connecting the +ve meter probe gradually further upstream towards its 12V source -- i.e. moving from the taillight connection behind the licence plate, then the fender connection, then terminal 'TL' at the ignition switch (follow the wiring diagram logically for each circuit) will gradually narrow down the source of a resistive connection.

*The lamps on our bikes should vary between 2.4R (&#937;) for a 12V/60W(!) headlight, 4.1R for a 35W headlight, 6.9R for a 21W brake light, 28.8R for a 5W taillight and 72R for a 2W instrument lamp. These figures are accurate for a 12V lamp, and a bit anal, but for most lamps on our bikes a reading of <100R, bare lamp taken from its holder, is close enough to indicate a serviceable lamp.

** Not so the neutral light; this gets its ground from the neutral switch (except when the trans is in gear) so disconnect both wires and test the lamp & holder independently of the bike.

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