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20 November 2016 BY SAM BELL If you don’t know where you’re going, any path will take you there, eventually. But with electrical troubleshooting, your destination must be identified before you begin the journey. STREET SMARTS FOR ELECTRIC AVENUE

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Page 1: STREET SMARTS FOR ELECTRIC AVENUE · Street Smarts for Electric Avenue November 2016 23 Circle #12 In many ways, electrical troubleshooting is the exact opposite of mechanical repair

20 November 2016

BY SAM BELL

If you don’t know where you’re going, any path will take you there, eventually. But with electrical troubleshooting, your destination must be identifi ed beforeyou begin the journey.

STREET SMARTS FOR ELECTRIC AVENUE

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21November 2016

STREET SMARTS FOR ELECTRIC AVENUE

The basic ideas present-ed at the beginning of this article will devel-op in ways designed to help your under-standing of advanced

circuits to grow. The goal is to help you learn to troubleshoot electrical circuits quickly, accurately, confi dent-ly and, of course, profi tably.

All procedures described are spe-cifi cally designed for use on 12V auto-motive systems only. Work on higher voltage systems, such as those found in hybrid-electric or full electric-drive vehicles, requires different proce-dures and equipment. High-voltage systems present a lethal danger.

It’s all about feeding the beast. Whether it’s something as simple as turning on the brake lights or mea-suring coolant temperature, or as complex as determining whether to deploy an air bag, every electrical cir-cuit exists for only one reason: to do some kind of work. Whatever device or “load” will be doing the work in a particular circuit is a beast that must be fed with an adequate diet of elec-tricity. The job of the circuit is to feed the beast; the job of the beast is to do some work.

Sound troubleshooting procedures can always get you to the correct an-swer eventually, although sometimes the journey will be long and “circu-itous” (pardon the pun!). Following sound procedures can identify all three types of possible circuit faults: open faults (think of a broken wire or inoperative switch); shorts (unintend-ed circuit paths, whether caused by abrasion, a stuck switch or some oth-er cause); and voltage faults (usually reduced voltage most often due to corrosion, but occasionally overvolt-age due to charging system faults).

Troubleshooting can identify failed electrical components as well. Fi-nally, troubleshooting can also help identify the root causes of some fail-ures, and may often even help iden-tify conditions which are likely to cause future failures.Ph

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Page 3: STREET SMARTS FOR ELECTRIC AVENUE · Street Smarts for Electric Avenue November 2016 23 Circle #12 In many ways, electrical troubleshooting is the exact opposite of mechanical repair

diagnosis, we spend most of our time fi nding out where the problem isn’t!

Circuit Measuring BasicsCircuits are circular in that they al-low energy to flow from a source, perform some work at the load and, eventually, to return to that same source. But if the energy has done its work, why isn’t it “all used up”? The short and simplified answer is that, for our purposes, there are two kinds of energy in a circuit (positive and

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Street Smarts for Electric Avenue

In many ways, electrical trouble-shooting is the exact opposite of me-chanical repair. Mechanical faults are generally visually apparent and easily diagnosed, though they may require lengthy work for access and repair. Electrical faults are often hidden, requiring a long time to di-agnose, while the actual repair (run-ning in a new wire, replacing a cor-roded terminal, etc.) is generally rel-atively quick once the fault has been located and identifi ed. In electrical

negative), and we need both of them to make the work fl ow. And to make that happen, we need a return path back to the energy source. And yes, the energy does get used up along the way. Some people fi nd it helpful to think of the two kinds of electricity as examples of the equal and opposite forces of action and reaction.

Let’s look at an example circuit. Fig. 1 above shows a very simple flashlight circuit. We start with a flashlight so simple that it doesn’t

Simple Flashlight Circuit(all readings VDC )

Flashlight Off

Flashlight Off

Flashlight Off

Flashlight On

Flashlight On

Figure 2

Figure 3 Figure 4

Figure 5 Figure 6

Figure 1

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Page 4: STREET SMARTS FOR ELECTRIC AVENUE · Street Smarts for Electric Avenue November 2016 23 Circle #12 In many ways, electrical troubleshooting is the exact opposite of mechanical repair

even have a switch. If good batteries and a bulb are installed, the light simply shines until the battery becomes too weak or until the bulb’s filament breaks. One end (or pole) of the bat-tery is positive and the other end is negative. Each end of the battery is connected to the bulb at opposite ends of its fi la-ment. When the two types of energy, positive and negative, meet inside the fi lament, it lights up from the heat of their combi-nation. The heat, in turn, causes the bulb to glow. I know you’re more inter-ested in working on cars than on fl ashlights, but bear with me.

What if we decided to make our

fl ashlight a bit more practical by add-ing a switch. Fig, 2 shows our light

with the added switch in the OFF po-sition. What’s happening here? Since

the switch is off, there’s no fl ow of current, no work happens, no light.

This is an open circuit.Let’s add in a voltmeter

now, to see if we can learn a little more about the circuit’s properties. Fig. 3 shows the readings we get at each of the indicated points when we hook up the voltmeter’s negative lead to the left end of the left battery and move the voltmeter’s positive lead to each of the points shown.

The black lead goes in the COM port, while the red lead goes into the VΩ port. The black probe tip is connected to the left end of the

left battery, while the red probe tip is connected to each of the points

Street Smarts for Electric Avenue

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Circle #12

In many ways, electrical troubleshooting is the exact

opposite of mechanical repair. In electrical diagnosis, we spend

most of our time fi nding out where the problem isn’t!

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right battery all the way to the tip of the switch, and a conductive path-way all the way from the negative pole at the left end of the left bat-tery all the way along the negative bus, into the negative terminal of the bulb, and all the way out of the other end of the bulb right up to where

26 November 2016

Street Smarts for Electric Avenue

shown. The selector switch for our meter is in the VDC position.

What have we measured? So far, we’ve looked only at source voltage, the amount of electrical pressure found in the batteries. But we’ve al-so learned that there’s a conductive path from the positive pole of the

the switch contact would close if we turned our fl ashlight on.

Okay, now let’s turn our fl ashlight back on and do the same measure-ments (Fig. 4). We hook up our volt-meter the same way and check the voltage at each of the points again. You can see the change at the left

Current probes, or “amp-clamps,” make use of one of the fundamental elec-tromechanical properties

called induction. A current trav-eling through a magnetic field induces a voltage in that field. This induced volt-age is proportional to the strength of the original current, and varies accord-ing to the design of the probe.

And that’s their big ad-vantage: You can measure voltage more safely and easily than you can mea-sure current. High-current probes generally output a voltage of 1mV/A of cur-rent. They’re very useful for looking at high-current power circuits like starter motors, alternators, wip-ers and rear window de-foggers. Low-amp probes generally output either 10mV/A or 100mV/A. They’re very helpful when evaluating low- to mid-range currents such as those powering injectors, ignition coils, fuel pumps, etc. Microamp probes gen-erally output 10mv/mA, and are extremely useful in evaluating certain types of wheel speed sensors, air/fuel sensors, vacuum switching solenoids, blend door motors, etc.

Most current probes have a zero knob or dial. To zero a current probe, place the probe around the wire whose current is to be measured with that circuit turned off. Make sure to turn the

probe on and connect it to your scope or meter. (Most probes re-quire a 9V battery.) Turn the knob until the meter reads 0. Low-amp and microamp probes are easi-ly overwhelmed by high current

fl ow, which can cause their jaws’ magnets to become temporarily too strong. Open the jaws and let them snap closed on a piece of paper to dissipate the excess and

restore normal function.A current probe’s output is gen-

erally in millivolts (mV) and corre-sponds with the ratio printed on the probe.

High-amp probes are frequent-ly used to do relative compression testing and to assess actual current consumption or output. They usually feature a large jaw opening capa-ble of surrounding even very large battery ca-bles. Low-amp probes are best used for circuits in the 100mA to 20A range. Jaw diameter is usually insuffi cient to allow their use on a main battery ca-ble. Microamp probes are well suited to very small power consumers that use 400mA or less. Their much smaller jaw open-ing is intended for small-gauge single wires.

High-amp probes can-not generally be used to home in on small current draws of the sort that might leave a battery dead in a vehicle un-driven for several days. Low-amp probes may lack sufficient precision to pinpoint very small c i rcu i t draws below about 100mA. They’re al-so generally unreliable when currents exceed 40A. Microamp probes

are well suited to small-current circuits and to identification of small-magnitude parasitic draws, but their small jaw size limits their use on larger cables.

CURRENT PROBES

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end of the bulb. The closed switch now allows positive current to fl ow from the batteries to the filament, then back to the other end of the batteries. Meanwhile, negative current flows from the other end of the batteries through the spring, through the fl ashlight, into the bulb, through the fil-ament, through the switch, and back to the positive end of the bat-teries. This is an exam-ple of a closed or com-pleted circuit. Since the load (in this case the bulb) is on, the cir-cuit is said to be “load-ed.” The black probe tip is connected to the left end of the left battery, while the red probe

tip is connected to each of the points shown in turn.

We’ve already seen what changed, so now let’s look at what stayed the

same. The readings at each end of each battery are the same in Fig. 4,

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and so is the one at the bottom of the bulb. The reading at the battery end of the switch has also remained the same. In fact, only the voltage at the

left end of the bulb, nearest to the switch, has changed.

This illustrates a couple of important properties of electrical circuits: In a cir-cuit where the voltage on each side of the load (the bulb) is the same, no cur-rent will fl ow. In a loaded circuit, the two sides of the load will have differ-ent voltages, and current will fl ow. In our example, one side of the bulb has 3V and the other side has 0V. The voltage “drops” from 3 to 0 “across the load.” In

fact, in any complete circuit, all of the voltage will be dropped, or consumed,

A voltmeter does not become a part of the circuit to which it’s

attached. There is no appreciable current fl ow from the circuit

through the voltmeter.

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or broken? If you figured out that the voltage readings would be exact-ly the same, give yourself a gold star. If you also figured out that there would be no current flow, give your-self another one!

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Street Smarts for Electric Avenue

along the way. In good circuits, essen-tially all of this drop will occur across the load(s).

What would the voltage readings be if the flashlight were switched on, but the bulb’s filament was burnt out

Let’s try this thought experiment: How would our readings change if we now switched the flashlight off, still with a blown bulb? The big change would come at the positive end of the bulb, where our voltage readings

METER LOVE

Think of “meter love” as sort of a three-way hookup! Meter A (let’s call it Alex) at the left

in photo 1 is set to the volts scale. As you can see, it’s con-nected to meter B (Bobbi) to the right, which is set to ohms. What does it tell us? Bobbi out-puts a small voltage (.62V, as reported by Alex) to measure Alex’s impedance (internal re-sistance). Bobbi shows us that Alex’s impedance is 10.09 meg-ohms (10,090,000 ohms). The current Bobbi used to make this measurement is about .62mA (.00062A), as shown in photo 2.

Why does this matter? Since both the voltage and the cur-rent used to measure ohms (re-sistance) are very small, they may not reflect the true state of affairs when it comes to large, power-hungry circuits, like those for a starter, or even for less needy consumers like an on-board computer. Additionally, a voltmeter with a lower imped-ance of less than 8 to 10 meg-ohms (8,000,000 to 10,000,000 ohms) may “load down” sen-sitive circuits like those of, say, an oxygen sensor, causing them to change their behavior and resulting in unreliable volt-age readings, possibly leading to an expensive misdiagnosis. I have even seen cheap meters change the spark characteristics of an ignition coil so greatly as to cause a no-start! Note that different meters may use differ-ent currents during resistance measurements, as shown in photo 3, where Alex and Bob-bi switch roles, and in photo 4, where Bobbi hooks up with Red, a stranger whose current, at 1.01mA, is roughly equal to Alex and Bobbi’s combined output.

1 2

3 4

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would now be fl uctuating. Why? Be-cause there is no longer a continuous path between it and ground.

Back to our fl ashlight once again, but this time with a twist. We’re go-ing to introduce a little corrosion where the spring at the negative end of the left battery meets the ground path going to the bulb (Fig. 5). The bulb still glows, but because the volt-age drop across the bulb is now on-ly 2.5V instead of the 3V it was de-signed for, the bulb is dim. The total voltage drop (across the batteries) remains 3V.

So far so good, right? Here’s where it starts to get a bit more interesting. What are the voltages we’d measure in our dim fl ashlight if we turned it off (Fig. 6)?

They all look the same as they did without the corrosion! How is that possible? If the corrosion is so bad that it drops half a volt across it when the light is turned on, why doesn’t it affect our readings when the light is off? The answer: There’s no voltage drop when there’s no cur-rent fl ow, and current fl ow requires a complete path.

A voltmeter does not become a part of the circuit to which it’s at-tached. It measures conditions at var-ious points relative to one another. There will be no appreciable current flow from the circuit through the voltmeter. This is a function of the meter’s internal resistance (imped-ance), as mentioned in “Meter Love” on the previous page, and is one of the reasons you want a good meter.

When I teach classes in electrical troubleshooting, techs tell me after-ward that the following is the most eye-opening experiment of the class. I call it “The Statue of Liberty Play.”

Take your DMM out to the car. Set it to DC volts and attach the black lead to the COM (or nega-tive) port and the red lead to the VΩ port. Clip the red probe to the positive battery terminal and the

black probe to the negative; your reading should be about 12.6V. Now remove the red probe from the positive battery terminal and lick your fingers on both hands. With your left hand, touch the pos-itive terminal of the battery; with

your other hand, touch the red probe and read your DMM. We’ll pick it up right here next month.

Street Smarts for Electric Avenue

29November 2016

This article can be found online at www.motormagazine.com.

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Circle #14

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