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Old 04-08-2013
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Short in lighting circuit affects radio?

Just fixed a problem with my interior lighting circuit. Lights downstream from a bad ground wire stopped working. When I would switch on one of those lights, the AM/FM radio would go off!. Turn the light switch off...the radio would come on again. The one light that was upstream from the bad wire worked, and didn't affect the radio.
Why would a bad ground wire affect a radio on a completely different circuit?

Last edited by L124C; 04-08-2013 at 12:17 PM.
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Old 04-08-2013
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Re: Short in lighting circuit affects radio?

If I recall correctly, a radio needs a very steady ground; it needs to measure signal level deviation in the antenna. If the ground is bouncing around (e.g., because of noise induced by a bad light), that would mean that the measurements would be faulty, and you wouldn't get good reception, at a minimum. All of the grounds on the boat are probably (and should be) tied together, so even though the noise was on a different circuit, it still would impact the ground.
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Old 04-08-2013
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Re: Short in lighting circuit affects radio?

You were probably getting a ground loop (voltage loop) when the light was turned on, it "became" an imperfect ground path, so the voltage flowing through the light stole voltage to the radio, or presented an intermediate (not zero volt but something slightly higher) ground, which caused the radio to see less than 12 volts and appear to turn off.

If you placed a voltmeter across the radio power leads, you'd probably have seen them go down to something like 9 volts or 10 volts when the light was turned on, effectively turning off the radio.
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Old 04-10-2013
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Re: Short in lighting circuit affects radio?

Quote:
Originally Posted by L124C View Post
Why would a bad ground wire affect a radio on a completely different circuit?
Answer: Because something is really messed up.

I might guess that your bad light is actually drawing excessive current and bringing down the voltage at the radio (are other lights dimming?). Like hello says, the first step is to get a voltmeter on the radio and see what's happening there.
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Old 04-11-2013
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Re: Short in lighting circuit affects radio?

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Originally Posted by asdf38 View Post
Answer: Because something is really messed up.
Cute, but inaccurate. As I indicated in the OP, I know what the problem was - a bad ground wire in the interior lighting circuit. Rather minor and not that "messed up" actually. I simply by-passed it and everything is working perfectly (though...it inspired me to re-wire the entire circuit).
The question was why that would have an effect on the radio. Poignantly addressed by others. As always, my thanks to them.
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Old 04-11-2013
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Re: Short in lighting circuit affects radio?

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Originally Posted by L124C View Post
Cute, but inaccurate. As I indicated in the OP, I know what the problem was - a bad ground wire in the interior lighting circuit. Rather minor and not that "messed up" actually. I simply by-passed it and everything is working perfectly (though...it inspired me to re-wire the entire circuit).
The question was why that would have an effect on the radio. Poignantly addressed by others. As always, my thanks to them.
...My point is that a frayed wire in one circuit *shouldn't* have any effect on another circuit. Not if they're completely parallel. Just because the problem went away doesn't mean the question is answered or guarantee there is no more problem.

Hellosailor could clarify but I read his response to imply the same thing - some unexpected connection between the lighting and radio circuit existed causing a ground loop. And I don't think jimgo's answer is likely because the radio ground connection isn't in question. Though it's possible, but given that you describe the radio as going "off" (did you mean lost reception?) it doesn't seem likely. If you meant lost reception instead of off then it becomes a pretty reasonable explanation.

Last edited by asdf38; 04-11-2013 at 01:09 PM.
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Old 04-24-2013
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Check this out!

As I indicated I fixed the problem in the OP, but decided to rewire the entire interior lighting circuit. Due to the modifications, it also made sense to re wire the neg circuit from the bow nav lights.
The new interior circuit worked perfectly, then I cut the neg wire for the Starboard bow nav (running) light. Suddenly, the interior, spreader, anchor and steaming lights no longer worked, and switching any of them on turned off the am/FM radio. If I turned the circuit off, the radio would come on in about 5 seconds.
Except for grounding at the main panel, those circuits are completely separate from the bow nav lights. Interestingly, the only light that wasn't affected was the Port bow nav light, which shares a switch with the Starboard light (of course) but is wired independently (separate hot and ground wires).
I expected that cutting the ground on the Starboard light would disable that light (of course). Why would it affect the other circuits? To be clear, this isn't a bad ground as I suspected with the problem in the OP. I kind of understand how the system would be affected when a circuit is trying to function with a bad ground. However, this was a severed ground. I would think the current on the cut circuit would simply not flow, and the other circuits could go about their business.
When I reconnected the ground on the Starboard nav light, everything functioned perfectly!
Someone please enlighten me!

Last edited by L124C; 04-24-2013 at 04:13 AM.
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Old 04-24-2013
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Re: Short in lighting circuit affects radio?

To what else was that ground connected?
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Old 04-24-2013
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Re: Short in lighting circuit affects radio?

L-
It would take some brain-wracking to try to figure out the exact problem, but the bottom line is that those two circuits, which you think are separate, are in fact not separate. Somehow they've been wired together. Possibly at some time in the past someone added something or repaired something and said "I'll just connect it over here" and got the wiring crossed. The only way to be 100% certain that you've got it fixed?
Right, rewire all the affected circuits, 100% to the panel. Either that or you'll spend more time following wires, tagging wires, chasing wires, tryinh to find the spot where someone cheated.

Since the bow running lights are always turned on as a pair (or wired with the stern light, all three on one switch) there is no reason at all for them to be wired up separately. If the wiring is adequate, I'd suggest moving the both over to the side you know is isolated (Port side?) or putting them both on one new wiring run as well.

If someone ran dedicated grounds all the way down the length of the boat to the bow lights, that could give enough voltage drop to cause voltage loops (aka ground loops, whatever) in other wiring, which again explains odd behavior in other circuits.
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Old 04-24-2013
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Re: Short in lighting circuit affects radio?

Quote:
Originally Posted by jimgo View Post
To what else was that ground connected?
All the circuits ground at a bus bar at the main panel, with the exception of the mast lights which ground at a bar near the batteries.
Maybe the fact that the running light circuit supports the other bow and Stearn light, makes the circuit more complicated. Therefore, the cutting of the Starboard ground was not as absolute as I first thought, as indicated in post #7?
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