On Sat, 05 Dec 2015 19:00:02 +0000, dennis@home wrote:

On 05/12/2015 09:59, JD wrote:
wrote in
news:4a56e299-a5da-41b3-8d2a-df8077ea4ae3 @googlegroups.com:

I checked the single-filament bulb. It's fine


What check did you make? Visual? Electrical?


Ohm meter and visual. The Ohms reading indicates it has resistance, so
the filament must be intact, and it looks intact too.

JD


How much resistance?
A working 20A fuse should read pretty close to zero with an ohm meter.

He was referring to the lamp filament resistance, not the fuse link.

I have a couple of 12v 55W quartz halogen capsule lamps which are
probably identical to the ones JD's testing. The hot, working resistance
calculates out to 2.62 ohms.

However, since the hot resistance of a tungsten filament lamp is
typically an order of magnitude greater than its cold temperature, a
resistance check with a multimeter on the ohms scale will be trying to
measure a mere quarter of an ohm resistance value which, with most
multimeters, can be damned difficult to read or distinguish from the test
lead's own resistance.

Whilst many modern DMMs are calibrated to tenths of an ohm on their 200
ohms scale, thermocouple effects and test probe contact resistance make
getting 'tenth of an ohm' readings rather problematical. With a lot of
care and determination, I can usually get the short circuit test to
stabilise at 0.6 ohms for the test lead resistance and with similar care
when testing the lamp plus test lead resistance, see a 0.8 to 0.9 ohm
reading on my 55W test lamp.

In practice, of course, I'm happy to see a reading that approximates a
short circuit when testing such low voltage lamp filaments when all I'm
doing is simply to prove they haven't gone open circuit.

In the early days of automotive electrics, it was standard practice to
eliminate any fusing protection on headlight circuits to avoid the
consequences of a total blackout due to a tired fuse blowing whilst
driving along an unlit road.

Any resulting electrical wiring fire was deemed less of a risk than a
possibly fatal collision due to a sudden blackout - the theory being that
at least one of the headlamps would remain lit long enough for the driver
to bring the vehicle safely to a halt, pop the bonnet release (in later
vehicles) and jump out, fire extinguisher in hand and possibly a suitable
wrench to disconnect the battery (or, at the *very* least, *safely*
abandon the vehicle along with any passengers before the vehicle became a
raging inferno).

Later on (sometime in the fifties or sixties?) the manufacturers started
fitting safety fuses initially one per headlamp circuit (protecting both
dip and drive filaments on each side) then 4 fuses to protect each
filament or lamp. When relays started to used to control the vehicle's
driving lamps, it was most likely a single changeover relay to begin with
before this single point of failure was duplicated then quadruplicated to
eliminate the risk of a total blackout by any one component failure.

I'd imagine all modern vehicles (circa ten years old or less) will have
not just a seperate fuse link per lamp filament but also a seperate relay
for each lamp filament (not just duplicate contacts on a single coil
relay).

I don't envy JD's task of tracking down this particular fault. Vehicle
electrics wiring diagrams are generally a total and utter shambles,
seemingly designed more as a technically challenging puzzle designed to
exercise the minds of individuals possessed of the intellect of the
eponymous "Sherlock Holmes" character than as an aid to diagnosis by us
mere mortals.

--
Johnny B Good