The Problem with Shorts, Part I

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VStar650CL
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2004 Nissan Altima 2.5 S

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Most people have never chased an actual "short" circuit in an automobile. They don't happen frequently, even in older vehicles. The vast majority of electrical issues are "open" or "high resistance" circuits, where something is making little or no electrical contact and current can't flow. Short circuits, by contrast, are where the power side of a circuit somehow contacts the ground side, causing way too much current to flow. Anyone who has actually tracked down a short knows it's a tedious, complicated, and often frustrating process. Electrons are invisible, and there are no magic glasses you can put on to show where the extra current is going. The closest to that is a "short finder" like an ECT2000, which works well if you know where to look and what to disconnect. The problem is, in circuits like interior lighting that have many "normal" paths to ground through bulbs, tracers will simply lead you down rabbit holes unless you know what to disconnect. They can also easily destroy fragile computer inputs if you connect them wrongly. Even your ohmmeter can do that. So there are good reasons why most OEM's don't supply a device like an ECT2000 in their "required" tool kits. The potential for damaging expensive hardware with inept use is very great.

If you didn't gather it already, that means the second step in locating any short is to get a copy of the wiring diagram, so you can understand the loads attached to the fuse that's blowing. The first step is a simple-but-thorough visual inspection, because you might get lucky. Many shorts are caused by mechanical failures like shredding drive belts, road debris damaging harnesses under the car, or even invading rodents chewing on wires in the engine box. Give yourself a chance to get lucky by taking a hard look. Like they say on the golf course, "Never up, never in."

If you don't get lucky, then you're going to need a complete list of everything supplied by the blowing fuse. There are two simple reasons. First, your short can be in a component or a wire, and you may never know which one unless you disconnect all the components. Second, any ohmmeter reading you take with a load attached will be meaningless at best and will fool you at worst. One example of this is the noise-suppression condenser most Nissans have on the supply for the spark coils. It's just a capacitor and ideally it won't pass current when fully charged, but your ohmmeter will actually charge it and give you a transient false reading if you leave it connected. Worse, if it's electrically-leaky it will give you a false reading all the time. It won't be your culprit, but it very well could look like the culprit to your ohmmeter.

https://drive.google.com/file/d/103Cup0 ... sp=sharing

To understand the need, that's a schematic for the ECM fuse circuit on an '08 Versa. It's relatively primitive compared to newer stuff, but you can see that there are 11 different connections. Hmm, only count 10? The fuse counts too. If you don't remove that, then you may be ohming through the entire 12V bus, depending how the circuit is arranged. On many cars the fuse is south of the relay, so if you aren't certain, always pull it. You can see that 3 of the 10 loads on the Versa are grounded through the ECM, so if they're shorted but the ECM hasn't been damaged, the fuse will only blow when the ECM tries to operate them. But the ECM could be damaged. In certain kinds of failures, like IVT wires being "whipped" by a bad belt, it's actually unusual for the ECM not to fail. See this post: topic628199.html

In any case, you can see the point. Ohming through the circuit without disconnecting anything will give you a reading through the MAF internals and coil condenser, plus anything that's blown inside the ECM. In other words, it's a useless measurement. For your ohmmeter to tell you anything useful about the wires, it has to be ohming just the wires. If it shows infinity to ground with everything loose, then you can be confident the problem isn't in the wires. With that confidence, you can then reconnect things one at a time and see what pops the fuse, knowing that something will pop it. Yes, it's arduous, but if you don't want to chase your own tail or waste money throwing wrong solutions at the problem, it's the only method that works.

Case in point: A few years back a Sentra came into our dealership on a hook with a popping ECM fuse. The original tech pulled nearly everything loose in the engine box with zero results. With only a cursory look at the WD, he didn't notice that one component, the Evap Vent Valve, was all the way at the back of the car on 3 different harnesses, just like the Versa in the diagram. When I got it, I found no damage under the car. Disconnecting the valve made no difference, but breaking the SMJ connector between the engine room harness and body harness did the trick. With that knowledge, tracing the body harness back to the valve revealed damage inside the car, behind the ledge trim for the trunk. A rat had got inside, and his gnawing shorted the 12V supply for the valve to a taillight ground. It took me under two hours to locate it, after almost a full day of other people thrashing over it. Knowledge of the circuit, along with a systematic approach, will get you there every time.

In installment two I'll talk about that system, and show you a DIY tool that really does work without harming anything.


Buzzman
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When I saw your headline, and before I clicked on your post, the first thing that went through my mind was, what's wrong with wearing shorts?
I wear them all the time. Very comfortable in warm weather.
Then I started reading your post. Oops.

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AZhitman
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Yet another brilliant article by The Guru.

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VStar650CL
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2004 Nissan Altima 2.5 S

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Knowledge is the only priceless thing that even a pauper can bestow on a prince. I'm neither, but what I pass on will long outlive me. Thanks enough, methinks. :)

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AZhitman
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Posts: 71061
Joined: Mon Apr 29, 2002 2:04 am
Car: 58 L210, 63 Bluebird RHD, 64 NL320, 65 SPL310, 66 411 RHD, 67 WRL411, 68 510 SR20, 75 280Z RB25, 77 620 SR20, 79 B310, 90 S13, 92 SE-R, 92 Silvia Qs, 98 S14.
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I read everything you write with rapt attention.


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