Car 12v 5w halogen incandescent lamp erratic

내 문제

One of the two registration plate lamps on my 2002 Toyota Corolla was randomly off or on.

A slight tap to the back of the car could turn the lamp off or on. The lamp was a standard 12v 5w push-in halogen.

나의 수리

As the lamp did illuminated, I initially thought the lamp must be ok and it must be a problem with the wiring or the contacts. Furthermore the lamp had a satisfactory resistance (about 40 Ohms)

I tried spraying 'contact cleaner' into the lamp socket and pushed the lamp in and out a dozen times. No Joy, I then cleaned the contacts in the socket and on the bulb with paper socked in 'contact cleaner'. No joy. No amount of bending the supply wires forced the lamp to turn on or off.

내 조언

Then I viewed the lamp under a x20 hand lens and saw there was a discontinuity in the coiled tungsten filament (See red arrow in image). So I assume the filament had broken but the two ends of the filament were touching, so my (low voltage) resistance meter did not detect the breakage. However, when in use in the vehicle with 12 v applied, the contact points could either spot weld themselves together and glow or some localised heat could blow the two coil ends apart and leave some tungsten oxide so current couldn't flow.

So, after all that messing about, I went to get a new lamp (£4) and it worked fine.