Part I: topic628344.html

So, once you think you have a wiring diagram for the fuse that's blowing, what are the pitfalls, and what's the best way to utilize the information? The biggest initial pitfall is having an incomplete diagram without realizing it. Let's say you have a parking lamp circuit that's popping. You go the Nissan ESM and pull up the diagram for EXL -> Parking and you think you're good to go. Except maybe you aren't. The interior backlighting on every Nissan is driven off one of the parking lamp circuits, front or rear. The diagram in EXL won't show you that extra connection. For that, you need to consult the PG section for the overall "Battery Power Supply" diagram. That will tell you whether you also need the backlighting diagram in the INL section, or anything else that might be connected to the same fuse.

Now that you're sure you have all the needed diagrams, how should you best use the info? I mentioned earlier that "short sniffers" can be dangerous toys, but that isn't to say that a tool can't help you. Probably 50% of short circuits are from failed components and not wiring, so a tool that gives you a quick "short-no-short" indication while you're unplugging everything will potentially help a lot. It's another version of, "Let yourself get lucky," because maybe the indicator goes "no short" when you're halfway through and you just saved a lot of time.

https://drive.google.com/file/d/1FrGazX ... sp=sharing

To my knowledge you can't buy one of these, but I have no idea why. Ever since polyfuses were invented, I've wondered why every multimeter in creation doesn't have a similar circuit built-in. But you can DIY one for under $10 and it may save you a lot of time. The principle it works on is simple. Polyfuses are self-resetting silicon devices filled with granular carbon. The carbon heats up and separates when the fuse rating is exceeded, turning the fuse into a high-value resistor and breaking the circuit. Once it resists it becomes self-heating, which keeps the circuit broken until you remove the short. The light bulb across it won't light unless the polyfuse blows, because the fuse normally has much lower resistance than the bulb and takes all the current. Once the fuse blows, it has higher resistance than the bulb, so the bulb gets all the current and lights up. SInce the polyfuse will automatically "reset" once the load is removed and it cools off, it gives you a fairly quick "on-off" indication when you've found your culprit. Plus it's passive and harmless, unlike a short-finder that artificially injects current into the circuit.

The polyfuse rating is the amps at which it won't blow, and the "instant trip" value is generally 50% higher than the "sustain" value. So the 9 amp version shown can test circuits up to about 13 amps. In my job I also keep a 5 amp version around for testing more delicate circuits with 3, 5 or 7.5 amp fuses. The polyfuse shown is a Littelfuse 16R900GMR, but Bourns, Schurter and others all make similar stuff. You can get one from any electronics distributor like Digi-Key, Mouser, Allied or Newark, but I suggest Digi-Key because they'll ship First Class on onsie-twosie orders, saving you about $4 shipping. Incidentally, you might also consider soldering one in place of the standard fuse for the "10 Amps" measuring circuit in your multimeter. Then, next time you forget to move the probe wire from amps back to volts and the fuse blows, you won't need to open the meter and replace it. The polyfuse will reset itself by the time you're done shaking your head and saying, "Duh!"

Using the device is simple, just plug it in place of the offending fuse before you start disconnecting things. The polyfuse will blow just like the factory fuse, lighting the bulb. Then if you happen to hit on a component part that's causing your short, the bulb will go out and you got lucky.

The order of disconnecting things matters too. Many engine controls these days are both powered and grounded via the ECM, so disconnecting the ECM first can seem to make the short disappear, leading you to conclude that the ECM is bad instead of the component. Needless to say, that sort of mistake can result in a lot of needless expense. To avoid it, the simple rule is, always disconnect the driven devices first and the driver devices last. That pretty much goes for every computer on the car, whether it's a BCM, TCM, HVAC, etc. Virtually all of them have power cross-connections of some sort, and the only way to isolate a problem inside is to first disconnect every device they drive. The dome light circuit on a modern Nissan is a typical example. The BCM powers it on with a simple internal on-off switch, but creates the "fade out" effect and "power saver" functions using a PWM driver on the ground side of the circuit. If you disconnect the BCM first, you may find a wiring problem, but it won't tell you that the problem also caused the PWM transistor to melt internally. Essentially you'll have two shorts and not know it. When you hook back up, now the dome light works but won't go out, and you scratch your head wondering why.

Once everything is disconnected and your ohmmeter still shows a short, now a sniffer like the ECT2000 can be employed. It can't harm anything once it's only connected to wires and not components, and it can save you a lot of time and money. The problem with shorted wiring is that you can't "wire around it" like you might with an open circuit. It has to be located precisely so you can repair the injury without replacing the whole harness or tearing it to pieces. Sniffers will accomplish that, leading you to the precise spot in the loom that needs to be opened up. While you're in there, inspect all the adjacent wires very carefully for stress or hardened insulation, and cut-out and splice any suspect sections. Harness failures are almost always due to fatigue or chafing caused by a twist or bend, so chances are there is more than one compromised wire, even if only one or two are causing the immediate problem. For a lasting repair that won't bite your butt with a different failure down the road, make sure you fix everything that might need fixing.

In part three I'll talk about that most fearsome monster of all, the intermittent short. Until then, happy motoring!