P h o t o s : K a r l S e y f e r t
AUTOMOTIVE ELECTRICITY
B Y K ARL S EYFERT
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.
T
oday’s vehicles could be described as computers on four wheels. Although most vehicles still rely on internal combustion en-gines for propulsion,
nearly every vehicle function is now controlled and monitored by comput-ers. Impressive as that may em, all of tho computers are powerless to func-tion as designed without one esntial ingredient: electricity. While you might have been able to limp a ’64 Chevy back to the garage with an out-of-commis-sion alternator and a nearly dead bat-tery, it’s unlikely you’d be able to achieve the same result with an ’04model.
This emphasis on vehicle electronics and electricity has resulted in a shift in the skills needed to diagno and repair today’s vehicles. There may already be times when you feel more like an elec-trician than a technician , as you grapple with yet another computer- or electron-ics-related vehicle malfunction. The types of problems aren ’t going to go away. In fact, they ’re more likely to be-come the norm, as gas-electric hybrids and fuel cell vehicles increa in num-bers on the road. In the years to come,you may be spending more time diag-nosing electric motors and control sys-
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 u the word “basic ” when describing the subject matter of an article, as more ex-perienced technicians may be likely to turn the page. Some of the material covered here may em familiar to you,but that doesn ’t mean it ’s unworthy of review. It ’s much easier to get into trou-ble when we don ’t know what we don ’t know.
A Trusted Companion
The digital multimeter (DMM) is prob-ably the single most important piece of electrical diagnostic equipment any technician can own. Fifteen years ago,DMMs were just beginning to find their way int
o 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 decreas as its battery becomes discharged? If you ’re relying on a DMM with a Low Battery indicator that ’s been lit for the past cou-ple of weeks, chances are the results you ’ve been reading off its screen have been something less than preci.
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 suppod to do with your smoke detec-tors? The asonal time changes for Daylight Savings Time provide a conve-nient reminder.
What condition are your test leads in? An auto shop is a pretty harsh envi-ronment, even for tools that were de-signed 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, t 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 resis-tance is being added to every reading you take. The accuracy required for to-day ’s testing makes that variable unac-ceptable.
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 veral scales for readings of ohms, volts and amps. The meter read-ing will be the most accurate when the scale being ud is as clo 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 ex-pected reading (about 14 volts) is safely within the lected scale, but there ’s not a huge amount of unud “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 me-ter indicates OL (out of limits or over-load), 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 lected range. Consult your DMM manual for specifics.
Let ’s return to our charging voltage example. Suppo your DMM has four DC voltage ranges: 0-4000 volts, 0-400volts, 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
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 measure-
ments and may nd you on unwanted diagnostic detours.The number of digits a DMM can display is limited, so a short-hand 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 al-ways be lected, to assure maximum meter accuracy.
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 read
s 14.25 volts. Without switching to this more ac-curate 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 caus it to display OL, becau 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 lec-tion a little easier. Each time the meter is powered up, it defaults to the range it thinks you’re most likely to u. On DMMs destined for automotive rvice, 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 volt-age 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 lected 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 electri-
cal shorthand, especially when larger
meter readings are encountered. For程序员面试宝典
example, suppo we’re measuring the
resistance of a heater blower motor re-
sistor. The DMM has five ranges for
measuring resistance: 400 ohms, 4k
ohms, 40k ohms, 4M ohms and 40M
ohms. When the 4k- and 40k-ohm
ranges are lected, 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 lected, any
reading displayed on the DMM must
be multiplied by a factor of one million
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(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 e, the
placement of that decimal point on the
display becomes very important.
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Back to our blower motor resistor.舆情管控
The gment we’re testing is suppod
to have a resistance of 5 to 8 ohms, so
we’ll start out on the 400-ohm scale be-
cau its range is the clost possible to
the expected reading. The DMM dis-
雅思和托福哪个难plays 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 read-
ing is 7 ohms. As you can e, the read-
ing 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 read-
ing. Moving to the other remaining re-
sistance 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 u of the
DMM’s larger resistance scales. Once
again, the letters k and M come into
play. This time we’ll measure the resis-
tance of a spark plug wire (remember
tho?). The manufacturer says the wire
is suppod 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 read-
ings 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 ca.
Things don’t get any better on the 400k-
ohm scale, where the DMM reads 1.2k
ohms (1200 ohms actual). On the 4M-
ohm scale, the DMM reads 0.001M
ohms (1000 ohms actual). The DMM
has dropped another 200 ohms from its
reading, just becau it’s on a less accu-
rate 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 instru-
ment. U that versatility to its best ad-
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.
vantage. If your meter has autoranging capabilities, u the convenience of that feature with both eyes open. Make sure either you or your meter has lected 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 T ests
Electrical tests that are conducted while a circuit is powered up are called dy-namic 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. Un-wanted resistance in either will wreak havoc.
每天的拼音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 becau many circuit defects will reveal the
mlves 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 ud to de-
duce the resistance in the circuit.
Noted crime sleuths of the past have ud what they knew to help them un-cover the truth about what they didn ’t know. This is called the power of deduc-tion. Electrical sleuths rely on some-thing called Ohm ’s Law to help them u the known to reveal the unknown when diagnosing electrical circuits.Without boring you to tears with an overlong explanation, Ohm ’s Law de-fines the completely predictable rela-tionship among volts, amps and resis-tance. For reasons that are best left for another time, George Ohm assigned letters to the three electrical measure-ments. 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 deter-mine its voltage. Multiplying amperage times resistance gives us the circuit volt-age. Applying what is known allows us to determine what was (previously) un-known.歌乐山森林公园
The formula can be rearranged, de-pending 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 4R (voltage di-vided b
y resistance equals amperage).When we need to know the resistance
of a circuit, Ohm ’s Law states R = E 4I (voltage divided by amperage equals re-sistance).
The mere mention of Ohm ’s Law is enough to make some techs ’ heads ex-plode, 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 cau a change in the other two, as you ’ll e in the following volt-age drop tests.属鸡哪年出生
Drop Dead
Winter is when many starter circuit problems reveal themlves, 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, com-plaining that the starter motor doesn ’t crank fast enough to start the engine when the temperature is below freez-ing. By the time the vehicle gets to you,it ’s all warmed up and ems 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, un-wanted resistance is robbing the starter of needed voltage. The voltage is being
By attaching one test lead to the battery terminal and the other to the terminal clamp, we’ve isolated the battery connection itlf. 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 ud to easily break the circuit into smaller , more manageable parts.
“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 ther-mome
ter 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 re-sistance? Do you measure the resis-tance of the negative and positive bat-tery cables? More than likely, measur-ing the resistance from one end of the cable (the battery) to the other end (the starter) will not reveal a problem. That ’s becau your DMM places an almost
unmeasurable load on the cable when it measures its resistance. Becau 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 bat-tery 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 re-veal 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 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 volt-age reading shown reprents the volt-age that has been dropped between the battery and the start
er 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 the modest amounts.
Voltage drops may occur at any point in a circuit. It may not be practical to re-place all of the suspect wiring, so it will be necessary to pinpoint exactly where the voltage drop is occurring. It ’s rela-tively easy with something like a battery cable becau there are only a few joints or connections in the circuit. If there ’s a voltage drop, the likely suspects are the cable itlf or the terminals at each end.Cleaning the connections and replacing the cable should take care of the prob-lem. But what if the wiring is buried somewhere deep in the car and the cir-cuit is veral feet long?
Quite a few years ago, I was strug-gling with a problem on a digital dash-board. 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 rvice manager to e 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? H
is 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 con-vinced 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 focud by using the “split half ” method. Divide the circuit in half, then perform a volt-age 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 gments 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 the circuits will have a direct effect on their ability to perfo
rm as designed. Voltage drops measured in tenths or even hundredths of a volt can be significant and will cau problems. Be prepared to spend
even more time with your good friend the DMM in the years to come.
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.
