A friend asked me to help her repair her bathroom light switch, which
had stopped working. By the time I got there, she had already
installed a new switch, but it still wouldn't work. .It's a one-way
switch, controlling two wall fixtures.
The house (which is in the USA) is about sixty years old and has two-
prong plugs, everywhere. The light switch has two screws, with a
black wire to each screw. There are two white wires in the switch
box, spliced together (and they had obviously been that way for many
years). One lamp fixture has two black and two white wires, with the
two blacks connected to the fixture's black wire and the two whites
connected to the fixture's white wire. The other fixture has one
white and one black going to it. So far, so good. Apparently, the
fixtures' wiring is somewhat newer than the rest of the house's
wiring, because they also each have a ground, which is connected to
each box, mounting plate, and fixture housing.
I couldn't see anything that was obviously wrong. And the light
fixtures and the old switch had been working for years. So I used my
multimeter (Tektronix DMM916) and did some very basic measurements,
but will probably need to go back and do some more. Anyway, here is
what I have measured, so far:
With the switch on, and no bulbs installed in the fixtures, the
voltage (VAC RMS) across the switch terminals is very low, i.e.
approx .04 VAC. At each fixture, white-to-ground measured 54 VAC,
black-to-ground measured 120 VAC, and black-to-white measured 34 VAC.
WITH bulbs installed, and the switch ON, both white-to-ground and
black-to-ground measured 120 VAC, and black-to-white measured anywhere
from 14.6 VAC to 18VAC on the first fixture and about .04 VAC on the
But, after emptying some dead insects out of the first fixture, it,
too, measured .04 VAC from black-to-white. (I neglected to measure it
again, without bulbs, to see if the 34VAC from black-to-white had then
changed.) The black-to-white measurements without bulbs also matched
the measurements between the socket bases and threaded bulb holders.
With the switch OFF (both before and after emptying-out the insects in
the first fixture), the voltage across the switch terminals (two
blacks) was about 70 to 76 VAC (varied between measurements).
Also, with the switch off, the resistances, at the fixtures, between
any two of black, white, and ground all appeared to be infinite, as
did the resistances between the socket bases and threaded portions..
Can anyone figure out what's going on, from that? Or, what else
should I measure, or try?
Could it be a problem in the breaker box, itself? I did happen to
notice, about six months ago, that her breaker box was open, slightly
(not the door, the whole front panel!). But it appears to be closed-
up OK, now. The breakers look very old, though. Also, everything
else in the house appears to be working as it always has. And that
circuit does not have its own breaker.
Any DIY suggestions will be appreciated.
- Tom Gootee