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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 identifi ed before you begin the journey.

    STREET SMARTS FOR ELECTRIC AVENUE

    FuelTrim2.indd 1 10/24/16 10:28 AM

  • 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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    Electrical.indd 2 10/21/16 12:47 PM

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

    Circuit Measuring Basics Circuits 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

    22 November 2016

    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

    Electrical.indd 3 10/21/16 12:47 PM

  • 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

    23November 2016

    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!

    Electrical.indd 4 10/21/16 12:47 PM

  • 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 suite