AUTOMOTIVE ELECTRICITY BY KARL SEYFERT Electricity has been an integral part of the automobile since its earliest days. With gas-electric hybrids already here and fuel cell vehicles on the way, now s the time to brush up on your electrical diagnostic skills. Today s vehicles could be described as computers on four wheels. Although most vehicles still rely on internal combustion engines for propulsion, nearly every vehicle function is now controlled and monitored by computers. Impressive as that may seem, all of those computers are powerless to function as designed without one essential ingredient: electricity. While you might have been able to limp a 64 Chevy back to the garage with an out-of-commission alternator and a nearly dead battery, it s unlikely you d be able to achieve the same result with an 04 model. This emphasis on vehicle electronics and electricity has resulted in a shift in the skills needed to diagnose and repair today s vehicles. There may already be times when you feel more like an electrician than a technician, as you grapple with yet another computer- or electronics-related vehicle malfunction. These types of problems aren t going to go away. In fact, they re more likely to become the norm, as gas-electric hybrids and fuel cell vehicles increase in numbers on the road. In the years to come, you may be spending more time diagnosing electric motors and control sys- Photos: Karl Seyfert March 2004 57
Clamp your meter leads to a few too many dirty battery terminal clamps and it won t be long before the crud takes up residence on the tips. Crusty test leads add an unpredictable amount of resistance to your measurements and may send you on unwanted diagnostic detours. The number of digits a DMM can display is limited, so a shorthand notation method is required. Readings with the letters k and M attached allow the meter to display some really large numbers. The correct measurement range should always be selected, to assure maximum meter accuracy. tems instead of the idiosyncrasies of their gasoline predecessors. In this article, we ll cover some of the basics of automotive electrical diagnosis and repair. It s always dangerous to use the word basic when describing the subject matter of an article, as more experienced technicians may be likely to turn the page. Some of the material covered here may seem familiar to you, but that doesn t mean it s unworthy of review. It s much easier to get into trouble when we don t know what we don t know. A Trusted Companion The digital multimeter (DMM) is probably the single most important piece of electrical diagnostic equipment any technician can own. Fifteen years ago, DMMs were just beginning to find their way into auto repair; now there s at least one in every technician s toolbox. What more can be said about such a common tool? Plenty! How much attention do you pay to this trusted companion? Did you know that a DMM s accuracy decreases as its battery becomes discharged? If you re relying on a DMM with a Low Battery indicator that s been lit for the past couple of weeks, chances are the results you ve been reading off its screen have been something less than precise. DMM batteries last a long time, which is why they re easy to ignore. It s the same problem folks have with smoke detectors. If it makes it easier to remember, why not replace your DMM batteries twice a year, just like you re supposed to do with your smoke detectors? The seasonal time changes for Daylight Savings Time provide a convenient reminder. What condition are your test leads in? An auto shop is a pretty harsh environment, even for tools that were designed to withstand its rigors. If your test leads have been kicking around the shop for any length of time, chances are they ve sustained some sort of damage. Perhaps the insulation is frayed or the tips are corroded or bent. For a simple test, set the DMM on the Ohms scale, then touch the test leads together. The DMM should read 0 ohms, even at its highest resolution. If the reading is something more than 0, that extra resistance is being added to every reading you take. The accuracy required for today s testing makes that variable unacceptable. Some DMMs have the ability to compensate for any test lead resistance by rezeroing the display when the leads are connected. If your leads are in poor condition, a better option would be to replace them. If the tips have gotten dirty but the rest of the leads are in good shape, clean them before using them again. How well do you understand the readings on your DMM? Most meters offer several scales for readings of ohms, volts and amps. The meter reading will be the most accurate when the scale being used is as close as possible to the measurement being taken. For example, it s preferable to measure charging voltage on a 0-40 volt scale, rather than a 0-400 volt scale. The expected reading (about 14 volts) is safely within the selected scale, but there s not a huge amount of unused headroom above it. One way to make sure you re always using the best possible meter range is to scroll through the ranges until your meter indicates OL (out of limits or overload), then continue scrolling upward until you get a reading. Some DMMs have other ways of indicating that the reading has exceeded the limits of the selected range. Consult your DMM manual for specifics. Let s return to our charging voltage example. Suppose your DMM has four DC voltage ranges: 0-4000 volts, 0-400 volts, 0-40 volts and 0-4 volts. Most DMMs are limited to a display of four digits. So on the 0-4000 volt scale, the DMM reads 0014 volts. On the 0-400 58 March 2004
volt scale, it reads 014.2 volts. Knowing that the alternator is putting out a little more than 14 volts may be enough to make a diagnosis, but perhaps you d like to have a more accurate reading. Switching to the 0-40 volt scale, the DMM display moves over one more decimal point and now reads 14.25 volts. Without switching to this more accurate scale, you never would have known that the alternator was putting out that extra.05 volt. Switching the DMM to the 0-4 volt scale causes it to display OL, because the meter s internal protection circuit prevents it from being overloaded by the charging system s too-high voltage. Some DMMs have an autoranging feature, to make the job of range selection a little easier. Each time the meter is powered up, it defaults to the range it thinks you re most likely to use. On DMMs destined for automotive service, the default voltage range is usually 0-40 volts, since most vehicles (excluding gas-electric hybrids) aren t expected to produce voltages outside that range. But don t let the autoranging feature lull you into complacency. On the voltage scale, an autoranging DMM may autorange only upward, not downward. If the measured reading is above the default range, the DMM will autorange upward to compensate and protect the meter. But if the reading is safely within the default range, the DMM will leave the range as-is. It s still up to you to manually adjust the selected range downward when greater accuracy on weaker meter readings is required. Numbers & Letters The DMM can display only a limited number of digits. This requires electrical shorthand, especially when larger meter readings are encountered. For example, suppose we re measuring the resistance of a heater blower motor resistor. The DMM has five ranges for measuring resistance: 400 ohms, 4k On many vehicles, the battery ground cable may not be connected to the starter motor. A complete voltage drop test of the entire ground side of the starter circuit should include every step in the ground path between the battery and the starter. ohms, 40k ohms, 4M ohms and 40M ohms. When the 4k- and 40k-ohm ranges are selected, the k informs us that any reading on the DMM display must be multiplied by a factor of one thousand (1000). When the 4M- and 40M-ohm ranges are selected, any reading displayed on the DMM must be multiplied by a factor of one million (1,000,000). The letters k and M allow the DMM to display some really large numbers, all within the limitations of a four-digit display. As you ll see, the placement of that decimal point on the display becomes very important. Back to our blower motor resistor. The segment we re testing is supposed to have a resistance of 5 to 8 ohms, so we ll start out on the 400-ohm scale because its range is the closest possible to the expected reading. The DMM displays 6.6 ohms. Moving to the 4k-ohm scale, the meter displays 0.007 ohm. We know the readings on this scale must be multiplied by 1000, so the actual reading is 7 ohms. As you can see, the reading has been rounded off by.4 ohm. Not good. Moving to the 40k-ohm scale, the DMM now displays 00.00 ohms. The meter is too far away from the preferred range, so it can no longer display a meaningful or accurate reading. Moving to the other remaining resistance scales only moves the decimal point further to the right, still displaying all zeroes. Some readings you ll be taking will be much larger, requiring the use of the DMM s larger resistance scales. Once again, the letters k and M come into play. This time we ll measure the resistance of a spark plug wire (remember those?). The manufacturer says the wire is supposed to have a resistance of about 1000 ohms. On the 400-ohm scale, the DMM reads OL. Moving to the 4k-ohm scale, the DMM reads 1.205k ohms. We know the meter readings on this scale must be multiplied by 1000 (k), so the actual wire resistance is 1205 ohms. We ve found the DMM s most accurate range, and determined that the ignition wire is probably okay. On the 40k-ohm scale, the DMM reads 1.20k ohms, for an actual reading of 1200 ohms. We ve lost only 5 ohms of accuracy, which probably wouldn t make much difference in this case. Things don t get any better on the 400kohm scale, where the DMM reads 1.2k ohms (1200 ohms actual). On the 4Mohm scale, the DMM reads 0.001M ohms (1000 ohms actual). The DMM has dropped another 200 ohms from its reading, just because it s on a less accurate range. By the time we get to the 40M-ohm scale, the DMM is unable to display a meaningful reading, so it shows 00.00M ohms instead. Your DMM is a very versatile instrument. Use that versatility to its best ad- 60 March 2004
By attaching one test lead to the battery terminal and the other to the terminal clamp, we ve isolated the battery connection itself. The terminal and clamp look relatively clean, and a voltage drop test revealed no significant loss with all normal electrical loads applied. Longer or more complicated circuits require a disciplined approach to locate voltage drops. Refer to a wiring diagram to find the location of connectors that can be used to easily break the circuit into smaller, more manageable parts. vantage. If your meter has autoranging capabilities, use the convenience of that feature with both eyes open. Make sure either you or your meter has selected a range of measurement that will produce the most accurate results possible. And don t hamstring a quality instrument with poor quality or defective test leads. Remember: garbage in, garbage out. Field Tests Electrical tests that are conducted while a circuit is powered up are called dynamic tests. Perhaps the single most powerful dynamic electrical test is the voltage drop or volt drop test. The biggest enemy of any electrical circuit is unintended or unwanted resistance. It doesn t matter whether it s a circuit with a huge electrical load like the starter motor circuit, or a relatively low-load circuit like a parking lamp circuit. Unwanted resistance in either will wreak havoc. While it is possible to conduct simple static resistance tests on a circuit with a DMM, the results can be misleading at best and totally wrong at worst. This is because many circuit defects will reveal themselves only when the circuit is powered up and under a load. Testing a live electrical circuit for resistance with an ohmmeter will damage the tester, so voltage readings must be used to deduce the resistance in the circuit. Noted crime sleuths of the past have used what they knew to help them uncover the truth about what they didn t know. This is called the power of deduction. Electrical sleuths rely on something called Ohm s Law to help them use the known to reveal the unknown when diagnosing electrical circuits. Without boring you to tears with an overlong explanation, Ohm s Law defines the completely predictable relationship among volts, amps and resistance. For reasons that are best left for another time, George Ohm assigned letters to these three electrical measurements. The letter E equals voltage in volts, I equals current in amps and R equals resistance in ohms. The first expression of Ohm s Law is E = I x R. So if we know the amperage and resistance of a circuit, we can determine its voltage. Multiplying amperage times resistance gives us the circuit voltage. Applying what is known allows us to determine what was (previously) unknown. The formula can be rearranged, depending on what is known and what needs to be known. If we need to know the current flow in a circuit, Ohm s Law can be restated as I = E 4 R (voltage divided by resistance equals amperage). When we need to know the resistance of a circuit, Ohm s Law states R = E 4 I (voltage divided by amperage equals resistance). The mere mention of Ohm s Law is enough to make some techs heads explode, so we won t spend any more time on it. The important thing to remember about Ohm s Law is the simple way it explains the unbreakable relationship among voltage, current and resistance in a circuit. Even the smallest change in one will cause a change in the other two, as you ll see in the following voltage drop tests. Drop Dead Winter is when many starter circuit problems reveal themselves, due to the extra strain the cold weather places on all of its components. No doubt you ve had a customer bring a car to you, complaining that the starter motor doesn t crank fast enough to start the engine when the temperature is below freezing. By the time the vehicle gets to you, it s all warmed up and seems to start just fine. The starter spins freely and the battery looks like it s fully charged. What could the problem be? Most likely the problem is a voltage drop on either the positive or negative side of the starter circuit. Added, unwanted resistance is robbing the starter of needed voltage. The voltage is being 62 March 2004
dropped at the point of the unwanted resistance. This means that instead of getting the battery s full voltage during cranking, the starter motor has to get by on what s left over. When the thermometer drops below a certain point, what s left over just isn t enough, and the starter slows to a crawl. So how do you find the unwanted resistance? Do you measure the resistance of the negative and positive battery cables? More than likely, measuring the resistance from one end of the cable (the battery) to the other end (the starter) will not reveal a problem. That s because your DMM places an almost unmeasurable load on the cable when it measures its resistance. Because the load is so small, the DMM will show a very low resistance reading, as long as even just a few of the strands in the battery cable are still good. The DMM can t tell the difference between a good cable and a bad one with this test. What s needed is a test that will reveal the cable s performance when it s in operation and under a load. To test the negative battery cable, attach the DMM s negative lead to the negative battery terminal. Don t attach it to the terminal clamp; we want to test the whole circuit from end to end. Attach Split half testing gradually narrowed the source of this voltage drop down to high resistance inside a single connector. By placing the DMM leads on either side of the connector, its voltage drop reading is revealed. the positive DMM lead to the starter motor body or the engine block. Set the DMM to the 0-40 volt DC scale, then have an assistant crank the engine while you watch the DMM display. Any voltage reading shown represents the voltage that has been dropped between the battery and the starter motor. Typically, a ground cable that s in good condition will drop.1 volt or less. Don t accept a voltage drop that s greater than.3 volt. A cable that s causing starting problems may be dropping far more than even these modest amounts. Voltage drops may occur at any point in a circuit. It may not be practical to replace all of the suspect wiring, so it will be necessary to pinpoint exactly where the voltage drop is occurring. It s relatively easy with something like a battery cable because there are only a few joints or connections in the circuit. If there s a voltage drop, the likely suspects are the cable itself or the terminals at each end. Cleaning the connections and replacing the cable should take care of the problem. But what if the wiring is buried somewhere deep in the car and the circuit is several feet long? Quite a few years ago, I was struggling with a problem on a digital dashboard. It was a brand-new car and the owner of the dealership was concerned about getting it fixed so it could be sold. That afternoon, he wandered out into the shop with the service manager to see how I was doing. It s probably just a bad connection, I still remember him remarking in an offhanded manner. At the time, his words infuriated me. How could he even pretend to think he knew what the problem was? His automotive technical knowledge could have fit on the tip of my little finger. But in the end, he was right. It was a bad connection. More than a few electrical problems are due to poor connections, which is what made the owner look pretty smart back then. What he didn t know was where the bad connection was located. I had to struggle for a few more hours to find that out. If I had known more about voltage drop testing then, I m convinced I could have shaved quite a few hours off that diagnosis. When a circuit is longer and more complicated than a battery cable, save time and keep your diagnosis focused by using the split half method. Divide the circuit in half, then perform a voltage drop test on one half at a time. Find a convenient connector somewhere in the middle of the circuit to mark your halfway point. Conduct a voltage drop test on the front half of the circuit while it s under load. If no significant voltage drop is found, move to the rear half of the circuit, then retest. Keep dividing the remaining segments of the circuit in half until you ve narrowed it down and have conclusively located the voltage drop. Many circuits on today s vehicles are designed to carry very low voltage and amperage. Ohm s Law reminds us that any added resistance in these circuits will have a direct effect on their ability to perform as designed. Voltage drops measured in tenths or even hundredths of a volt can be significant and will cause problems. Be prepared to spend even more time with your good friend the DMM in the years to come. Visit www.motor.com to download a free copy of this article. 64 March 2004