Light Switch & Fixture Problem

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 second fixture.
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
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It seems that the white wire is not connected somewhere. It should go all the way back to the braker box and hook to the neutral side of the wiring. It may have came apart in the switch box when the switch was replaced.
At no time should the white wire have any voltage on it to the ground. With the meter you may see some of the induced voltage , but it should not be 120 volts. Sometimes the white wire will be used in a switched circuit, but should be marked with tape to indicate that..
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Check to see if a GFI outlet nearby tripped. It could be feeding that switch.
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Mike,
If a GFI had tripped would there still be 120 v at the light switch?
Dave M.
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He never actually says theres 120V at the switch, he says he gets 0V across the switch when on, and 76V when the switch is off. What should be done is to check the voltage across the black and white feed at the switch ( switch removed, just measuring the incoming feed). What would help is to get a test light, like a pigtail with a bulb in it, and test from point to point. You really don't need to measure resistances. Either you have an open or not.
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On Jan 25, 1:42 am, snipped-for-privacy@fullnet.com wrote:

buffalo ny: broken wire at new switch, forgot to turn power back on, circuit breaker must be turned fully off then on, you might find and reset an upstream GFI, you might change the light bulbs with tested working bulbs. but first get your clipboard out and take notes for your future use, perhaps a floor plan with circuit numbers for each floor of the building. unscrew the bulbs and identify the circuits. find the breakers and label the outlet and switch covers to match the breaker numbers stamped into the breaker box metal. do this throughout the house as you test the outlets with radios of the type that go on when plugged into a live outlet. and portable lamps with regular bulbs not just a voltmeter. with one hand in your pocket and the other hand writing a check to an electrician, ask him to show you how he checks the power under the panel cover safely as he measures the proper voltage at the hot to common 110vac arriving in the panel, and hot-to-hot for 220vac if available. the common white is usually grounded to earth ground at the panel but that location may vary. black screw outputs at a fuse or breaker output should have 110vac measured to common white. the half- voltages you are measuring might be due to a fluorescent or transformer or motor or compressor or pump with a ground fault problem. unplug these devices if on your bathroom circuit to see. also, the power to the bath could arrive from anywhere even a different meter or a mystery feed from the garage that the neighbor wired to his meter fifty years ago when he rented it from the previous homeowner. -b
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You need to check voltage from the hot wire on the switch to the two neutrals that are spliced together. If you get 120 v, you have an open neutral between that box and the first lighting outlet. If you don't get 120v, you have an open neutral wire somewhere upstream of the switch box. I would check nearby outlets and switches for the bad connection. It could be an upstream GFCI, if one exists, but not likely as that would disconnect the hot leg to your circuit.

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In addition to what RBM said, I would suggest that you stop using that multimeter and just use a pigtail socket and standard light bulb to test the wires. The results will be more definitive. All of those odd voltages that a multimeter displays just confuses a do-it-yourselfer.
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Those digital meters are about the worse test device a home owner can use. They do seem to give all kind of funny voltages, even when there is no real connection involved. That is probably where the 54 and 34 volts are comming from . Also that .04 volts is not worth mentioning, just say zero volts.
I work as an electrician and instrument technician at a large plant. I have a $ 300 digital meter, but mostly use the old Simpson 260 analog meter for simple tests. I have also used the light bulb and pigtail. Usually get laughed at by the co-workers when I pull that out, but it usually finds the problem where the others fail with their digital meters.
You have to know what to expect when using any test device.
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Every time I read posts where people are testing circuits with a variety of multi meters, I've been tempted to tell them to chuck the meter and get a pigtail socket. Like Ralph, and you, I've got all manner of test equipment, but the pigtail or wiggy tell me everything I need to know, almost all the time. It's simple and it works

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On Thu, 24 Jan 2008 22:42:28 -0800 (PST), snipped-for-privacy@fullnet.com wrote:

I would break the white wire at the switch and then test between the hot and the white.
120V there will tell you that the problem is in the circuit going to the light. If you don't have 120V there then the problem is in the circuit going home.
You have a bad connection in the white somewhere.
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