Finding Electrical Shorts On Multiple Circuits – UnderhoodService

Finding Electrical Shorts On Multiple Circuits

When I was a kid, I saw a movie called "Juggernaut" with Richard Harris as the main character. It was about a cruise liner that was about to blow up because a terrorist had planted 55-gallon drum bombs all over the ship. The premise of the story line is that Harris was going to save all the people on the ship by defusing the bombs. All through the movie, there were references to the different relays and electrical circuits.

When I was a kid, I saw a movie called “Juggernaut” with Richard Harris as the main character. It was about a cruise liner that was about to blow up because a terrorist had planted 55-gallon drum bombs all over the ship. The premise of the story line is that Harris was going to save all the people on the ship by defusing the bombs. All through the movie, there were references to the different relays and electrical circuits.

I would have to say, without a doubt, this is where my long-time involvement with electricity started. It’s where I learned the difference between an N.O. and an N.C. relay, and a few other electrical terms and techniques. (Watch the movie and see if you can spot the N.C. relay.)

What does this have to do with diagnosing today’s cars? Plenty. I call it “playing bomb,” just like in the movie when Harris asks, “Cut the red wire or the blue wire?” I use the same method when I’m diagnosing hard-to-find electrical shorts.

Here’s an example: 2002 Hyundai Sonata 2.7L with a no- start condition and the ECU/EGR fuse blows as soon as you turn on the key. There’s no spark, no fuel pump, no injector signal and no service light.

Coding the car wasn’t an option. For one thing, the fuse that kept blowing also powered up the ECU. So there weren’t any codes that could be read because the ECU wasn’t responding. The big issue was how to find the short that was causing the problem.

Looking at the wiring diagrams just made it worse. There were seven different components tied to the same fuse — from the top of the motor to the bottom, and from the trunk to the front of the engine. I guess you could have disconnected each and every one of the components. That would have been time-consuming for sure, but helpful only if the short was from the connector to the component, and not the ­entire wiring harness.

At this point, you don’t have a clue as to what component or at which part of the car the problem is originating, so how do you narrow down the problem into smaller sections, that you then can narrow down even further? My method was to find a spot from which all of these components get their positive voltage. The obvious spot was the fuse box.

I turned the fuse box over and removed the protective cover to expose the wires. With the battery still hooked up and the fuse removed, I used a standard test light with the end hooked to the positive post of the battery, and probed each of the wires in the circuit with the light. If I came across the correct “grounded” lead, the test light would light up, showing that it was the circuit that was grounded.

The leads on the back of the fuse box were in pairs, two to each terminal on the back of the fuse box. After a few tries, I found the pair that was grounded. Now I had to separate the two remaining wires, and get it down to one lead. This is where playing bomb comes in…which one do I cut? In this case, they are both the same color, so which one? Leaving the test light connected, I cut one of the wires…BOOM…and the light went off. I guess I know which lead to check now. 

With this one lead out of the way, I could put a fuse back into the fuse box, and see if all the other systems came back on. They sure enough did. But, I still don’t know which of the seven different components this particular lead went to.

The next thing to do was to try to start it with this one lead cut. Of course, I wasn’t expecting it to start, but what I was hoping for was a code and, if this particular lead wasn’t the direct voltage to the ECU, I should get something — P0335 or crank sensor circuit. So far, so good; time to locate and go directly to the crank sensor and disconnect it.

With the sensor disconnected, I could then reattach the lead that I just cut and see if it was still shorted. It wasn’t, so the problem had to be between the connector and the crank sensor itself. Sure enough, the three leads of the sensor had all of their insulation rotted off and the three wires were all tangled together.

I replaced the crank sensor, repaired the wire I cut and reinstalled the fuse box, finishing the job up in record time.

Oh, and by the way, this car was at another shop for four days and no one could find this problem. All said and done, I spent about 30 minutes diagnosing it. Playing bomb is a fast and effective way of finding these types of problems.

This method works well when you have multiple wires on the same circuit, and can also be helpful when there are only two wires in the ­circuit.

The big question is, do you cut the blue one or the red one? Tick-tick, tick-tick, BOOM!

Sidebar: Hyundai Voltage Drop Diagnostics

Fuse Fiasco

More than one OEM has sent out a TSB or internal memos warning dealerships that some discount, no-name blade-style fuses can lead to damage to a vehicle’s electrical system. Even the National Highway and Traffic Safety Administration in 2007 investigated the problem due to some vehicle fires.

300,000 sets containing up to 120 fuses were sold to the public. This means as many as 35 million fuses are waiting for you to find them in your customers’ vehicles. The fuses are supposed to be 5-30 amps, but may have been made with the wrong amperages. The housings are prone to melting because the fuse is made of aluminum instead of zinc, and the housing material is not high-temperature resistant.

If you get a vehicle in your shop with electrical problems like blown fuses or damaged wiring, ask the customer if he/she replaced any fuses on their own. The fuses can come from a variety of sources. But, most of the fuses were sold at retail tool stores and some auto parts stores. It’s difficult to tell the defective fuses from high-quality fuses. Knowing if the customer replaced a fuse can help you rule out a diagnostic problem.

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