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Old 07-06-2013, 09:16 AM   #1
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Blowing fuses

When I try and replace a 15 amp fuse it blows. I took all the lights out on that circuit and fuse still blows. Do I have a short in the ground or power (+) side?
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12 volt circuit
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Old 07-06-2013, 09:19 AM   #2
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Power!
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Old 07-06-2013, 09:54 AM   #3
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Between them.
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Old 07-06-2013, 10:18 AM   #4
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Fuse Blowing

Short between POS+ and NEG- or, short between POS+ and grounded (NEG -) trailer frame, or a fault located in a connected load , or a short in a light fixture or a shorted light bulb or a short to another wire in the wiring harness ,ETC
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Old 07-06-2013, 10:31 AM   #5
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For give the dumb question. Short mean break in the line + or -?
or positive touching neg?
Could I use a continuity tester to check the line?
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Old 07-06-2013, 10:39 AM   #6
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Quote:
Originally Posted by Kevin K View Post
For give the dumb question. Short mean break in the line + or -?
or positive touching neg?
Could I use a continuity tester to check the line?
Electrical Short = Unwanted connection between two electrical points.


A continuity tester will show a connection, but not where the unwanted connection is located.
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Old 07-06-2013, 10:48 AM   #7
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One way to help find a short is to replace the fuse with a low wattage lamp. If a short exists, the lamp will light. Watch the lamp while checking sockets, connections, wiggling wiring, etc & when the lamp goes out you have found the short.
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Old 07-06-2013, 10:54 AM   #8
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Thanks Jon, it is always nice to know another simple diagnostic trick.
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Old 07-06-2013, 10:59 AM   #9
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Steve made a point you may not have picked up on, "short in a bulb" In all my years as a truck mechanic I don't recall having that happen,BUT, I did have it happen on one of my utility trailers a couple years ago, one filament broke and shorted inside the bulb and blew a fuse in the tow vehicle. I was all set to start tracing wiring to look for a short, but by dumb luck looked at the bulb first. Thanks for mentioning that Steve.
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Old 07-06-2013, 11:06 AM   #10
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No bulbs in any of the light bulb sockets on the circuit.
Jon Vermilye Would a LED light work as a lamp in the fuse holder?
Or should I disconnect all the sockets on the circuit and then try a fuse?
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Old 07-06-2013, 11:23 AM   #11
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A simple automotive test light is good to have, the kind with a pointy probe, and wire with an alligator clip. When you get really desperate trying to find a short you start cutting the wire to narrow it down. Something I've had to do.
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Old 07-06-2013, 11:37 AM   #12
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I just tried my automotive test light on the fuse holder and it lights up.
(means I have a short)
I also tried to wiggle the wires going into each bulb no change with light in fuse holder.
Next I will disconnect the wires the go to the right side of the trailer and see if the light goes out. If not I will try the left side.
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Old 07-06-2013, 11:59 AM   #13
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Actually my friends a short is often the most difficult fault to find. There are a few instruments made that help track down a short but they're expensive. So other techniques need to be used.
First try disconnecting one wire from a light fixture that connected to the offending circuit. Or if there's two wires of the same color disconnect both. If the short is still present you'll know that it's somewhere between that light fixture and fuse holder.
Disconnect all power sources and then use your continuity tester (ohm meter) between the fuse holder (it'll only be one side) and the negative battery line.
It can be difficult and frustrating. If you have too much trouble try to find somebody that knows a bit about trouble shooting electrical problems to help.

Wiggling wires can at time cause more problems. Wires break if wiggled too hard and too much. Wires can also be pulled from their connection the same way. That would make more problems.
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Old 07-06-2013, 08:24 PM   #14
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Originally Posted by Kevin K View Post
When I try and replace a 15 amp fuse it blows. I took all the lights out on that circuit and fuse still blows. Do I have a short in the ground or power (+) side?
Attachment 61512 1999 Casita SD
12 volt circuit
Here is my suggestion for you. First of all, my method I named it as..." Cold testings with Ohm meter with block-by-block to pinpoint the problem". I do not know if professionals have that name...That means one must un-hook battery, un-plug any power supply before tests. If you are new to Ohm meter, ask a friend to help, it just takes one minute to know how to use with a super cheap multi-meter. According to your photo, it appeared problem at circuit 3 from left to right with top-to-bottom 5 components, let's say 5 light bulbs..1/ Using Ohm meter, one probe is clamped at fuse of that branch(no matter which end of fuse as long as the fuse is still good), you can connect at the other end of circuit without fuse 2/ Put all the GOOD bulbs at its sockets 3/ The other probe of ohm-meter, start contacting FROM THE BOTTOM BULB, FIRST. 4/Reading Ohm meter: Zero means closed circuit or GOOD, indefinitely means O.P(open circuit)Bad. 5/Repeat steps 3 and 4 if the first bulb is good, and start from bottom UP until you find out an open circuit..When you pintpoint where is an O.P occured, inspect right there, the problem will be discovered soon. Be patient and remember your meter is only powered by an AA battery(1.5volts), usually. If any power supply goes into your multi-meter, IT WILL BE BLOWN, more exactly, the interior fuse of your meter will be blown...Just IMO. Good luck. One more thing: A short circuit happens when positive of power supply goes directly to negative or ground and BY-PASS your component/bulb...
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