Best way to find a short in a wiring harness.
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Leatherneck
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Best way to find a short in a wiring harness.
What is the best way to find a short in your harness, where do you start and how do you pin point it.
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Badbugtwo
- Posts: 496
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Re: Best way to find a short in a wiring harness.
Well you could put in a bigger fuse and then look for the smoke…Leatherneck wrote:What is the best way to find a short in your harness, where do you start and how do you pin point it.
No don’t really do that!!!!
There are several types of shorts, but here is one example…
If you know that you have a short to ground and are blowing fuses, you could connect a test light across the terminals that hold the fuse (with out a fuse in the box). If there is any current flow the test light will come on, but slow the current enough to not damage the circuit. Then you could isolate the circuit(s) until the light goes off. This will show you at least which branch has the problem. You may have to go as far as a visual inspection to truly solve the problem which can be tedious.
If this was a rhetorical question, I hope this makes sense? If your looking for an answer to a specific problem, give me more info and I will try to answer…. if I can!
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500LbGorilla
You start by measuring current flow through the various parts of the harness. THink of current the same way as water flowing through a forked river.. as the water flows before the fork, it has a certain flow rate. When it splits, a percentage goes down one fork and the other percentage down the other.
The short has to be found while it is shorting. So start at the source.. the main wires from the battery. Interrupt it, and put an ammeter between. Measure current there. That measurement is the amount that is being supplied to everything. At some point, your wires go do a distribution point, namely the fuse panel.
Systematically remove fuses and use the ammeter as if it was the fuse. Measure the current through each fused circuit. Knowing what each fuse is for, does the measurement make sense? For instance, if one is for the horn, and you are not using your horn, is there supposed to be current? No, there shouldn't be any current flowing. If there is, that wire probably has a short.
That's the beginning of tests. If you find one circuit that makes no sense, i.e. substantial current flow with the accessory not in use, then trace the wire until you find something that shouldn't be. Most shorts can be found this way.
There are other ways to find a short, for special cases and using different methods, like undervoltages and such, but our wiring systems in general are not complicated enough to have to do that most of the time.
The first indicator should be an accessory that is not working as it should. If it's not a matter of that, then the current test can help a lot by process of elimination.
Not long ago, my wife's Taurus was killing the battery. The battery was tested, and it was bad, had a cell that would not respond correctly, probably either a dead cell or a mineral spur or something in it. I replaced the battery, and the problem was decreased but did not go away. I started to measure the current, and found erratic results, sourcing current from the battery out to the system. So I went to the fuse panel, and started pulling fuses and found that the breaker (some modern cars use breakers on some circuits) was hot enough to singe my fingerprints clean off. The plastic fuse block was discolored under it too. Measuring across the breaker terminals with the breaker out I was measuring anywhere from about half an amp to 8 amps each time. Erratic measurements.
So this breaker was for the horn and cigarette lighter. I started with the lighter, and all the wire from the panel to the lighter was ok, but when I got to the lighter to check for shorts to the body, I looked down the lighter socket and found a penny in there. I got the penny fished out of the socket (the change tray was next to the lighter socket) and the penny had arc spots all along the edge.
What was happening was that the penny was microwelding to the the lighter terminals and causing different quality connections each time the car rattled it, and each time the circuit was interrupted, the penny would lose its connection. When reconnected, the contacts (which were loose since it was just sitting in there) would be different. Mystery solved.
I am sure others have some good suggestions as well, but this is something I have done a few times to track things down. Took me about 20 minutes.
The short has to be found while it is shorting. So start at the source.. the main wires from the battery. Interrupt it, and put an ammeter between. Measure current there. That measurement is the amount that is being supplied to everything. At some point, your wires go do a distribution point, namely the fuse panel.
Systematically remove fuses and use the ammeter as if it was the fuse. Measure the current through each fused circuit. Knowing what each fuse is for, does the measurement make sense? For instance, if one is for the horn, and you are not using your horn, is there supposed to be current? No, there shouldn't be any current flowing. If there is, that wire probably has a short.
That's the beginning of tests. If you find one circuit that makes no sense, i.e. substantial current flow with the accessory not in use, then trace the wire until you find something that shouldn't be. Most shorts can be found this way.
There are other ways to find a short, for special cases and using different methods, like undervoltages and such, but our wiring systems in general are not complicated enough to have to do that most of the time.
The first indicator should be an accessory that is not working as it should. If it's not a matter of that, then the current test can help a lot by process of elimination.
Not long ago, my wife's Taurus was killing the battery. The battery was tested, and it was bad, had a cell that would not respond correctly, probably either a dead cell or a mineral spur or something in it. I replaced the battery, and the problem was decreased but did not go away. I started to measure the current, and found erratic results, sourcing current from the battery out to the system. So I went to the fuse panel, and started pulling fuses and found that the breaker (some modern cars use breakers on some circuits) was hot enough to singe my fingerprints clean off. The plastic fuse block was discolored under it too. Measuring across the breaker terminals with the breaker out I was measuring anywhere from about half an amp to 8 amps each time. Erratic measurements.
So this breaker was for the horn and cigarette lighter. I started with the lighter, and all the wire from the panel to the lighter was ok, but when I got to the lighter to check for shorts to the body, I looked down the lighter socket and found a penny in there. I got the penny fished out of the socket (the change tray was next to the lighter socket) and the penny had arc spots all along the edge.
What was happening was that the penny was microwelding to the the lighter terminals and causing different quality connections each time the car rattled it, and each time the circuit was interrupted, the penny would lose its connection. When reconnected, the contacts (which were loose since it was just sitting in there) would be different. Mystery solved.
I am sure others have some good suggestions as well, but this is something I have done a few times to track things down. Took me about 20 minutes.
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2088 bob
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Leather have a look at this
http://www.mytoolstore.com/kd/kdbatt07.html
this may make finding your short fairly simple a friend of mine has one and swears by it
I googled " short circuit detector "and came up with a fair few sites offering the same or similar device
http://www.mytoolstore.com/kd/kdbatt07.html
this may make finding your short fairly simple a friend of mine has one and swears by it
I googled " short circuit detector "and came up with a fair few sites offering the same or similar device