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Old 30-06-2015, 13:34   #1
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strange 12v accessory short circuit

A few days ago I blew a fuse when plugging my tablet into my 12v accessory plug. The system had been flawless up until that point and I had charged the tablet numerous times. The 12v accessory circuit powers the cigarette lighter plug as well as the stereo system. The stereo is working fine but as soon as I plug anything into the 12v plug it blows the fuse. I removed the 12v plug assembly and tested it for continuity. Everything looked good (no continuity). I replaced and hooked up the wires and tested again, theres a small amount of continuity present. All other electrical circuits are working fine. With the 12v accessory switch at the control panel turned off, I still read continuity between the positive and negative sides.

With everything wired up and a new fuse in, I measured 12.9v between the 12v plug assembly and my ground buss with the switch in the on position. When I turn the switch off the voltage slowly drops over maybe 2 minutes until it settles at 0.05 volts, but fluctuates. When I measure continuity between the same circuit it immediately registers no resistance, then jumps around sporadically. There are no obvious crossed wires or obvious signs of a short circuit.

I've just measured voltages across all the switches on the panel in the off position and all of them read 0 except for the 12v accessory and the switch for my instruments (depth sounder, knot meter etc) which read 0.05 and 0.4v!!!!

Are my switches faulty? How could the 12v plug blow the fuse but the stereo does not?

Does this sound like a familiar scenario and how can I prevent it? I like my wiring to be solid.

I should mention the electrical system incorporates a 120w solar panel connected to a trace30 charge controller, a 10a house charger, 80a alternator and the bank consists of 3 batteries, 1 starter and 2 house.
Will Vivian is offline   Reply With Quote
Old 30-06-2015, 13:42   #2
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Location: Sitka, AK
Boat: 1984 Slocum 43
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Re: strange 12v accessory short circuit

Have you checked the other end of the wire going to the receptacle?
Mycroft is offline   Reply With Quote
Old 01-07-2015, 08:42   #3
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Posts: 1,274
Re: strange 12v accessory short circuit

It's probably a faulty USB plug.

Trying putting it in without the tablet plunged into it

The other stuff is normal. You can't use an ohm meter on live wires. the readings are meaningless

The voltages you see are from the stereo and other electronics that hold power. And why it slowly drops when switch is turned off
smac999 is offline   Reply With Quote

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