REVISITING ELECTRIC AVENUE

Transcription

1 REVISITING ELECTRIC AVENUE BY SAM BELL A haphazard electrical diagnosis can take you down plenty of one-way and dead-end streets. Learning and remembering a few important concepts will greatly reduce the number of detours you ll encounter. 28 Electrical2.indd 1 11/18/16 10:01 AM

2 Photos: Thinkstock Last month I left you in the middle of describing the Statue of Liberty Play. If you didn t do the experiment, you may want to do it now. After washing your hands, take your DMM and walk out to the vehicle. Set your DMM to DC Volts and attach the black lead to the COM (negative) port and the red lead to the VΩ port. Clip the red probe on the positive battery terminal and the black probe on 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 positive terminal of the battery; with your other hand, touch the red probe and read your DMM. Most techs are surprised to find their DMM showing 12+V, as seen in the left photo on page 30. Wow! Is Lady Liberty s tablet really a battery in disguise? Could that explain the light in her other hand? The answer is no, not really. To see why, we need to conduct one more experiment. You ll need a 12V test light, or at least a 12V bulb and couple of pieces of wire to make things easier. We re going to set up exactly as we did for the Statue of Liberty Play. Take your DMM and walk out to the vehicle. Set your DMM to DC Volts and attach the black lead to the COM (or negative) port and the red lead to the VΩ port. As before, clip the red probe on the positive battery terminal and the black probe on 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 one hand, touch the positive terminal; with the other hand, touch the red probe and read your DMM. This time it should be no surprise that your meter is reading somewhere in the neighborhood of 12V. 29

3 Revisiting Electric Avenue Left: As Lady Liberty illustrates in the Statue of Liberty Play, full electrical voltage can pass right through your body! The voltage shown, 12.64V, is the result of a basic principle of electricity. Electrical potential, or voltage, passes through even major resistances undiminished until the circuit is subjected to a load. Put another way, there is no voltage drop without current flow. Right: Once asked to perform some actual work i.e., once subjected to a load by the introduction of a test light voltages which looked just fine may drop precipitously to 2.37V, as shown here. But what does that mean? Can you use that voltage to do some useful work? Clip your test light lead to the battery negative (where the DMM COM lead is) and pick up the probe end of your light with the same hand that holds the red lead from your meter. What has changed? Suddenly, our hand voltage has dropped to just a couple of volts, as seen in the right photo above. And yet our test light is not lit. How do we reconcile this with the readings we had before? Once again, there s no voltage drop when there s no current flow. But wait! There is a voltage drop; we just saw it drop. Does that mean there is Voltage tests of an open circuit may be misleading. Proper circuit analysis requires knowing where and under what conditions any voltage readings have been taken. current flow? And if there s current flow, why isn t the test light lit? Turn your DMM to the mvdc scale. Leave the COM (black lead) attached to battery negative, as before. Attach the red lead to the test light probe tip. As before, grab the red lead with one hand while your other hand squeezes the battery positive post. Read the meter. You should see something along the lines of a few millivolts. That reading is the amount of voltage being dropped through (by) the test light. All the rest of the battery s 12V or so are being dropped through your body. By the time the positive flavor of current reaches the red lead, there s simply not enough of it left to illuminate the test light. Bottom line: Voltage tests of an open circuit may be misleading. Proper circuit analysis requires knowing where and under what conditions any voltage readings have been taken. Tip: Voltage drop readings are always taken under load, and are always taken on the same side (positive or negative) of the circuit. Is Resistance Futile? Thus far, although we ve talked a little about resistance, we haven t actually tried to measure it directly. Primarily, this is because resistance tests via a DMM use very small voltages and currents. These voltages are supplied by the DMM s internal battery. Static resistance readings obtained in this way may fail to indicate the true state of affairs. For example, let s suppose that someone has put on one of those universal temporary emergency battery cable ends a few years ago. Inside 30

4 the mass of corrosion that has built up over time there are only five strands of wire making resistance-free contact between the battery post and the auxiliary lead to the underhood power distribution center (PDC). If we do a resistance test, we ll see essentially zero ohms on our meter, indicating that those few strands are intact. But if we turn on the headlights, the rear window defogger and the high blower fan (all fed via the PDC) and measure the voltage drop from the battery post itself to the PDC positive feed stud, we may see a drop of several volts. This is because the dynamic resistance of the circuit is altogether different from its static resistance. Dynamic resistance refers to the cause of voltage drops due to planned or unplanned resistance in actual use, as opposed to static resistance, which is measured in an isolated circuit under no-load conditions. For some components usually sensors static resistance measurements may be sufficient, but for most components such measurements merely confirm continuity. To judge actual dynamic resistance, both voltage and current measurements are required. In effect, this makes voltage drop testing the gold standard for most troubleshooting purposes. Voltage Drops We ve all heard of voltage drop tests, time and again, in class after class, yet somehow most of us still find them confusing. What are voltage drops, and how do we do these darned tests? Remember the Statue of Liberty Play? There s no voltage drop without current flow. That means you do the test with the circuit turned on. There are two sides to every circuit power and ground. Proper circuit testing requires that you examine each side. To test the power side, connect the red lead of your DMM (set up for Vdc) to the battery positive post. Connect the black lead to the positive side of the load connector, as close to the load as you can get without piercing any wires. Make sure the load is switched on. Record your results. To test the ground side, connect the black lead of your DMM (set up for Vdc) to the battery negative post. Connect the red lead to the negative side of the load connector, as close to the load as you can get without piercing any wires. Make sure the load is switched on. As you See how you can finish the job right Circle #16 31

5 Revisiting Electric Avenue did before, record your results. Now evaluate your results. In a good circuit, both power and ground voltage drops will be relatively small. For large power consumers, like a starter motor or rear defroster grid, a good rule of thumb is that voltage should drop no more than.1v per connector whether on the power or ground side. For low-current electronic circuits, voltage drops should be closer to.01v per connector. If the load does not operate but your voltage drops are normal, substitute an appropriate dummy load. (By substitute, I mean unplug the original device and plug in your dummy load at the original connector, using appropriate connector pins and jumper wires as needed.) What makes a substitute appropriate or inappropriate is the magnitude of its current draw. For example, you could use a high-beam headlamp (around 4A) to substitute for a small electric motor like a fuel pump, while you d want something heftier, like a wiper motor, to judge a starter circuit. For most computer-controlled circuits, dummy loads should be limited to something under.1a, like a #18 or #2174 lamp. (Computer-controlled ignition coils or injectors may draw a full amp or more. Most other computer-controlled circuits consume far less current.) When in doubt, look at the fuse rating for the circuit you re testing. You don t want or need to pop the fuse; if you substitute a load that draws about a third to half the fuse rating, you should be okay for further testing. Just because your test load now works the light shines, or the motor turns don t jump to the conclusion that the wiring is fine. Repeat your voltage drop tests with the dummy load on. (It s hard to judge whether a light is fully bright or only 90% as bright as it should Fig. 1 Lady Liberty obligingly reveals her secrets. The voltage drops shown are proportional to the resistances of the conductive paths connecting them. The total is equal to the available battery voltage, while the individual drops on both the power and ground sides of the circuit add up accordingly. Bottom line: Voltage drop measurements must be taken under load. This photo was taken KOEO on a Toyota, but it could have been almost any vehicle. Each of the two voltmeters has its black (COM) lead connected to the battery s ground terminal. Each positive lead is connected to a back-pin lead inserted into a different terminal of the injector harness. Note that both sides of the harness show the same 12.05V. All we know so far is that the injector has continuity and that, somewhere, the power is on. 32

6 Revisiting Electric Avenue Sometimes, installing a bypass around an inaccessible voltage drop (this one was deeply buried beneath this Prius corroded main fuse panel) may be the most practical and economical solution. Be sure to document your bypass by attaching an indelible label and by providing your customer with a modified schematic to keep in the glovebox. Fig. 2 This schematic indicates the proper procedure for voltage drop testing. The load is turned on and a voltmeter measures the voltage drop on each side of the load, both power and ground. High current consumers may tolerate drops on the order of.1v per connection, while electronic circuitry may be adversely affected by drops barely in excess of.01v per connection. Field tolerances may vary. Checking known-good vehicles helps you gain familiarity with what constitutes normal. be on the basis of sight alone. Your voltage drop test will reveal the difference, if there is one.) If your voltage drop readings are okay, odds are the original device or component has failed; replace it with confidence. If the readings are not normal, repair the circuit wiring as needed. In a pinch, for testing purposes, you can usually bypass suspect wiring by unplugging the affected component and independently supplying it with both power and ground. Polarity (which wire gets power and which gets ground) is important in many cases. Reversing power and ground may damage or destroy a perfectly good component, so make it a habit to maintain correct polarity whenever you bypass the original wiring. For bypass testing only, when in doubt as to which wire is which, always consult the wiring diagram. If no diagram is available, unplug the load, then turn the circuit on and measure voltage at each pin of the harness (vehicle side) plug. Since the circuit is now open, you should expect to find voltage on one pin and ground on the other. It s usually not advisable to use this technique on components with more than two wires, especially if you re relatively inexperienced in electrical troubleshooting. Some sensor circuits may be engineered to operate at a reduced voltage. This is usually a 5V reference, but may be a different value, such as 7.0 or 9.0V. In these cases, first unplug the sensor and verify the correct open-circuit voltage supply level. You may also need to check the circuit diagram and the principles of operation. In some cases, such as with temperature sensors, it s normal to see a drop of several volts at operating temp. My readings are too high! Now what? Leave whichever test lead 34

7 1.67A 1.67A 1.67A (-) 1.5V (+) (-) 1.5V (+) 1.5V 3.0V (-) 1.25V (+) (-) 1.25V (+) 1.25V 2.5V (-) 1.5V (+) 0.5V (-) 1.5V (+) 1.5V 3.0V Fig. 3 Top: Our famous flashlight has gone Napoleonic! Yep, it s a Blownapart. Here we ve put an in series at various points in our flashlight. The reading of 1.67A would be the same no matter where we took it, and is correct for a 3V, 5W lamp. Center: Here we ve put an in series at various points in the flashlight. The reading would be the same no matter where we took it, and reflects the same lamp but a reduced voltage supply due to weak batteries. Above: Here we ve again put an in series at various points in the flashlight. The reading of would be the same no matter where we took it, and reflects the same lamp but a reduced voltage supply due to a corroded ground connection. was connected to the battery where it was when you found the excessive voltage drop. Move the probe closest to the load to the other side of the load connector (to the harness end of the plug) in the same circuit. As always, use appropriate backprobe pins rather than risk future damage from piercing the wire s insulation. Is your reading now okay? If that's the case, try disconnecting and reconnecting the connector to clean off any corrosion. Repair as needed, then retest, making sure that the circuit is turned on. If the reading did not improve when you changed sides at the last connector, consult your wiring diagram to find the next one closer to the battery. Take the same readings on each side of the connector cavity. In some circumstances, it may be more practical for you to test at more accessible connectors, remembering any you skipped in case it becomes truly necessary to test them. In any event, remember that the load must be switched on throughout. Still no joy? Keep working your way back towards the battery, whether you re on the power side or the ground side. When working on the ground side of the circuit, remember that while many devices ground directly via their mounting hardware, others may plug into harnesses featuring multiple connections in series, frequently including in-line splices which may become problematic after years of exposure to water, salt or high humidity. In some cases, even the chassis itself may have become unusable as a ground path. Test, don t assume! Sometimes it may become necessary to install a bypass around a severe but inaccessible voltage drop. Best practice here is to tag your repair. You can (and should) even print out the original circuit Electrical2.indd /18/16 10:03 AM

8 Revisiting Electric Avenue schematic, draw in your modification and include it with the bill. Ask your customer to keep it in the glovebox for future reference. This makes everyone s life easier. KOER voltage levels in a vehicle with a good charging system are lowest at the alternator or generator case, highest at its positive terminal. The closer meter s positive lead is connected to the B+ terminal of the alternator, while its negative lead is connected to the battery s positive terminal. It reads 00.11V. The far meter has its positive lead connected to the battery s negative terminal and its negative lead connected to the alternator s case. It reads V. Note that both DMMs show positive values. 14.4V 12.6V 5.0V pulsating direct AC riding on DC square variable AC Fig. 4 This drawing illustrates several types of voltages and currents. The magnitude of the voltage is indicated by its distance from the 0V line as shown on the vertical axis on the left side. Time flows from left to right on the horizontal axis. The red trace shows an example of an alternating current with both the direction of flow and voltage changing over time. The black trace, labeled variable, is an example of a typical sensor output, such as a throttle position sensor or MAP sensor. The yellow trace, labeled direct, is typical of constant battery output. The blue trace, labeled pulsating, depicts typical alternator charging voltages, and is sometimes characterized as rectified AC riding on DC. The dotted line intersecting the bottom of each blue hump indicates the DC voltage level. The purple line depicts a superimposed three-channel scope trace of rectified AC voltage riding on DC voltage approximately 1V AC riding on 9V DC in this example. The green line shows a typical DC square wave. Such square waves are what you would expect to see used in sensor, control or communications circuits. Measuring Current So far, we ve stuck mostly to measuring voltages. Let s look briefly at amps, or current. Standard DMMs are set up to measure current directly, by being placed in-line with the circuit being analyzed. Let s look at our familiar flashlight once again, but this time we ll have to disassemble it, at least partially, in order to hook up our DMM. Here, the red lead is plugged into the Amps DC socket. The black lead remains plugged into the COM port. The meter dial selects the Amps function. Your meter is equipped with a special internal fuse, usually 10 or 20A. Many technicians install a lower amperage fuse in-line to protect the meter and its remarkably expensive fuse. You can splice in an in-line fuse holder from your local supplier, or you can buy a commercial version such as Electronics Specialties Part No The vast majority of blown meter fuses occur when technicians hook up their test leads in parallel, like across the load, without first moving the red lead back to its voltage measuring port and without returning the meter selection dial to voltage. This oops! moment essentially uses the meter to short circuit power to ground. If only the internal fuse is damaged, count yourself lucky for having bought a well-engineered DMM. With the flashlight on, we measure the current flow through the meter from the left end (negative post) of the left battery to the contact spring (Fig. 3 top on page 35). We use a jumper wire to complete the circuit from the base of the spring back to the negative conduc- 36

9 SERIES AND PARALLEL CIRCUITS Current limiting helps protect circuits from overload. This is because the current flow in any series circuit is the same throughout the entire circuit. You could think of it this way: If all the cars on a two-lane highway are traveling bumper to bumper as fast as they can go, then two more lanes are added before the next interchange, the earlier two-lane restriction continues to limit maximum possible traffic flow. At most, only the same number of cars per minute tor. Our reading, 1.67A, is correct for a 5W bulb in a 3V circuit. (Of course, if we turn the flashlight off, there will be no current flow.) This gives us a calculated resistance of 1.8 ohms for the bulb. (We have to use a calculated value because a direct resistance measurement won t apply enough current to cause the bulb s filament to incandesce. Filament temperature is a major factor affecting dynamic resistance of an incandescent bulb.) What amperage would we expect to find if we measured the current to the same bulb after the battery voltage had dropped to 2.5V? If the bulb s dynamic resistance remains constant at 1.8 ohms, our current reading will drop proportionately to 1.39A, and the bulb would appear noticeably dimmer. In the real world, however, that dimness also signals a cooler bulb filament. This, in turn, reduces the actual dynamic resistance of the filament, just enough that amperage may remain ever so slightly higher than expected, like the shown in Fig. 3 center. What amperage would we expect if we measured the current flow in our corroded flashlight? We start with the same battery voltage of 3.0V. Our same 5W bulb has a calculated resistance value of 1.8 ohms. But the 3V we start out with can pass a given signpost by the highway. This is analogous to a current-limiting resistor. If our highway ends with an Exit Only multilane ramp, it was part of a series circuit. But if only some of the lanes must exit (and there are no new entrance ramps), then our roadway was part of a parallel circuit. By definition, a parallel circuit offers multiple travel paths to get from power to ground via different loads, while a series circuit offers only one path. soon drops to 2.5V as the current passes through the extra resistance of the corrosion. Seems like we re losing one-sixth of the voltage before we get to the intended load. That means current should be reduced proportionately from 1.67 to (Fig. 3 bottom). Although in this instance the corrosion is an unintended resistance, it illustrates an important point namely that current limitation can be achieved via resistance. This is the reasoning behind current-limiting resistors such as those found in many early electronic fuel injection systems and in so-called pull-up or pull-down sensor circuits, including many Vref (reference voltage) sensor circuits. Additionally, current limiting may help protect power switching transistors from overload (see Series and Parallel Circuits above left). This article can be found online at 37