This chapter explains which tools are needed in your tool arsenal to be ready to tackle the most common automotive electrical tasks. Furthermore, this chapter explains the differences between various types of measurement tools and how to properly use each of them. I don’t know about you, but any time I learn new ways to do something, that gives me an excuse to buy the right tools!
This Tech Tip is From the Full Book, AUTOMOTIVE WIRING AND ELECTRICAL SYSTEMS. For a comprehensive guide on this entire subject you can visit this link:
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Nowadays, specialized tools are more readily available than ever before. Obviously, the tool department at your local Sears is a great source. Your local hardware or home improvement stores can also be a great source. Professional tools, such as Snap-On and Matco, are sold from the tool trucks. These are best quality tools, but they are expensive. If you’re in a rural area, most of the tool companies have great websites at which you can purchase directly from them. Regardless of where you buy your tools, if I cover anything unique, I’ll tell you where I got it from to save you the time of looking for it.
OK, now you’re ready to dig in with both hands. What tools do you need to get started? At the minimum, I recommend the following:
- Diagonal Cutters
- Wire Strippers
- Razor Blade
- Wire Crimpers
- Electrical Tape
- Soldering Equipment
- Electrical Measurement Device
I use diagonal cutters often—from cutting the end of cable ties flush to cutting wire. What’s most important to me is that they fit my hand comfortably—not too big and not too small. Three common sizes are shown in the photo below, and all are available from Sears. I prefer the size in the middle. You may already own at least one pair of these.
This is probably the most personal tool I own. Ask any five mechanics what they prefer for stripping the insulation off of wires and you’ll likely see five different tools. Not that I recommend this, but I’ve even seen wires stripped with a cigarette lighter! Shown are the most common types; I prefer the ones on the lower right-hand side.
Now this is a trick that saves you time and effort trying to remove insulation from larger gauge wire. Since wire strippers for 8 AWG and larger wire really don’t exist, I always use a razor blade to:
- Score around the insulation of the wire, being careful not to cut too deep.
- Run from the score to the end of the wire lengthwise—no worries on going too deep this way, as the blade is parallel with the wire strands.
- Tear the insulation off by hand—if you scored it deep enough, it tears perfectly, right at the score.
A crimping tool is any tool that is designed to “crimp” a connector onto the end of a length of wire. There are numerous offerings in the marketplace. Buy the right pair of crimpers for the job—that one-size-fits-all pair won’t work well for crimping a 4-gauge ring terminal on the end of the charge lead from your high-output alternator. As a result, wire crimping tools are readily available to crimp connectors on wiring up to 4/0 AWG—really big stuff!
Ironically, the least effective kind of crimpers is also the most commonly found in tool boxes across the nation. Those are the “squeezy” kind that have crimpers, wire cutters, and bolt cutters all built into a single tool. And, just as you’d expect, none of them work very well.
I would say the tool’s operation is equally important as the tool itself. I could fill an entire chapter (or even a whole book) with photos of incorrectly crimped connectors. Chapter 3 covers how to correctly choose and use each of the pictured tools for the task at hand and how to make ideal crimp connections with them.
Hammers, pliers, and bench vises are all great tools, but crimpers, they are not. Never rely on them to get the job done of a properly designed pair of wire crimpers, because they can’t. Properly crimped connections have extremely low resistance and offer a solid reliable connection for years. Improperly crimped connections are high in resistance and pose numerous hazards—the least of which is the voltage drop through them.
A tool you say? You bet! In my opinion, this is one of the most important tools in your tool box. I’ve used Scotch Super 33+ Vinyl Electrical Tape for more than twenty years now and I’ve never had a problem with it—not one single problem. It’s very pliable, stretchy, easy to tear, and it sticks! I only recommend one kind of electrical tape.
Here’s an excellent tip when it comes to taping: Be sure that your hands are clean of dirt, oils, food, etc. As you invariably make contact with the sticky side of the tape in the process, clean hands help ensure that the tape sticks as it was intended to.
Note that I am not listing soldering equipment as an option, as you really should have at the very least a soldering iron and a roll of rosin core solder in your tool box. The fact of the matter is that soldering is easy, and how to do it correctly is covered in the next chapter.
One of the oldest wives’ tales is that soldering has no place in an automobile because the connections are subjected to vibrations. This is simply nonsense. In fact, a properly soldered connection likely outlasts the vehicle. In addition, I’ve seen thousands of problems from mechanical connections of all types—crimp connectors, Scotchlok connectors, and T-Taps are the most common. And guess what? In 99 percent of the cases, these were improperly chosen or installed to begin with.
Today, there are many different types of soldering equipment available. Many companies offer butane powered soldering irons. As they do not require electricity, they are totally portable and can be used anywhere. To be properly equipped for any automotive soldering job, consider purchasing:
- Soldering Iron—It should have at least a 25-watt capability for soldering wiring up to 14-gauge or so.
- Soldering Gun—This should have at least a 150-watt capability for soldering wiring up to 10-gauge or so.
- Propane or MAPP Gas Torch—This solders connectors on the end of the big stuff
- 60/40 Rosin Core Solder (in various sizes)—I use .040 diameter more often than any other size.
- Third Hand Device—A tool used to hold a connection in place, leaving both of your hands free to do the soldering.
Fortunately, all of the above are inexpensive and readily available. The third hand device is found only at your local Radio Shack, which also has all of the above with the exception of the torch.
Electrical Measurement Device
This is a must-have for any do-it-yourselfer. Prices, even on DMMs, are affordable for almost everyone. At the very least, buy a computer-safe test light, but understand that its light typically illuminates between 10 volts and 16 volts—so at best it’s just a shot in the dark. Nod if you’ve actually got a meter of some type lurking in the bottom of your tool box but never use it…you are not alone. Soon, this will be as important a tool to you as it is to me.
The Test Light
As I mentioned in the Chapter 1, old-school incandescent test lights have no place in the modern auto-mobile because they have very low impedance. I won’t even check fuses with them—just too many horror stories that resulted in someone spending their hard-earned money to fix a problem that could have been easily avoided.
This is simple enough and is the single most common use for a test light—you need to locate a source of power to power something up or need to verify the presence of voltage to be sure something is working properly. Just follow these steps:
Connect the clip of the test light to chassis ground.
Touch the tip of the test light to the connection point in question; in this case I’ve connected it to the headlight connector.
Turn on the circuit to be measured; the test light lights, indicating the presence of voltage.
Another problem with a test light is that you really can’t tell much other than the presence of voltage or ground or detection of current flow; you certainly cannot equate the brightness of the bulb to an actual figure. You need to know how to use one. Following is how to correctly use one in a non-computer equipped vehicle, such as my 1972 Olds Cutlass, for example.
All measurements have been taken with a Snap-On CT4G test light.
Let’s say that you wanted to find a suitable place to ground an aftermarket device or electronic component. You would use this procedure to determine if the chosen spot has ground present or not:
Connect the clip to 12 volts.
Touch the tip to the proposed grounding spot of the vehicle’s chassis.
If the test light lights, the spot provides ground, if not continue probing, until you find a spot that causes the test light to light.
I know this one sounds kind of silly, but for years mechanics have used test lights to track down drains on the battery. Here’s how to do it. Since the Olds has dual batteries, I’ve disconnected the front one to simplify the explanation. In addition, it doesn’t have any low current items, such as under-dash lighting or a glove box light, so I’ve connected a small light bulb to the circuit breaker next to the batteries for this example.
Be sure all accessories in the vehicle are turned off, especially the ignition switch! The bulb within the test light can’t pass much current through it—try to pass too much and it will burn out.
Disconnect the negative battery terminal.
Disconnect the positive battery terminal.
Re-connect the negative battery terminal.
Connect the clip of the test light to the positive battery post.
Connect the tip of the test light to the positive battery clamp—notice that the bulb in the test light is illuminated slightly—this indicates current flowing through the test light.
Disconnecting the small light bulb causes the light to go out entirely which eliminated the source of the current draw—if only it were that easy!
The Computer-Safe Test Light
A “computer-safe” test light is just that—a version of the test light that is safe to use in modern vehicles. Typically, they look and function similar to a traditional test light, but the similarities end there.
Computer-safe test lights have high internal impedance and two LEDs (light-emitting diodes) to indicate the presence of voltage or ground. Additionally, they require power, so they typically have alligators for power connections. They can also be designed to be plugged into a cigarette lighter. You can also purchase a complete kit, which typically includes:
- The computer-safe test light with a cigarette lighter plug.
- An adapter harness to allow the light to be powered by means of a positive and negative clip lead that can be clipped directly to the battery posts or power points.
- Different adapters or probe tips that can be affixed to the probe point for ease of probing just about any kind of circuit.
Computer-safe test lights are not designed to measure or detect the flow of current, so we cannot use them for such.
Let’s do some of the same measurements as above with my MAC Tools Model ET125C computer-safe test light.
The Digital Multi-Meter
I like a DMM because it has high internal impedance, typically around 10MΩ, which makes it computer safe. In addition, the numeric display takes the guesswork out of your measurements. You know exactly what you’re dealing with, not an approximate.
Detecting Voltage and Ground
Connect the red clip to +12 VDC and the black clip to ground.
Touch the tip of the computer-safe test light to the headlight connector—notice the green LED is illuminated, indicating the presence of ground as the headlights are off (the light is detecting ground through the filament of the passenger side headlight to the chassis, as both headlights are wired in parallel).
Turn on the headlight switch, the red light is illuminated, which indicates the presence of voltage.
In addition, the cost of a DMM has come down to a point where you can get one at your local Sears or Radio Shack pretty cheap because they’re available for as little as $25 or so.
DMM Basic Functionality
A DMM is quite a marvel, especially the nicer ones. Quite simply, it can measure voltage, resistance, and current. Personally, I’ve had my Fluke 87 since 1992, and it’s a tool that I rely on often. While it wasn’t an inexpensive investment, it’s certainly paid for itself on many an occasion. Technology in DMMs has come a long way since then, and the current model Fluke 115 has many of the features that were once only available in more expensive meters, at a much lower price. A really nice DMM, like the Fluke 115, can be had nowadays for less than $200 and can be obtained locally at most Sears tool departments.
Be sure the DMM you select can measure current, as some manufacturers offer some really nice meters that do not. This is easy to tell because the selector switch lacks an A or mA setting. Furthermore, you should select a DMM that can measure at least 10 amps of current safely, or it really won’t do you much good for troubleshooting current draw problems. No need to get carried away here as you’ll not be using your DMM to measure how much current the starter motor on your big-block Chevy draws on a cold winter day.(Although, if you did need to know that, a clamp-on style Ammeter is the ideal tool for that job.)
Before using your new DMM, I have a few cautions:
- Always be sure that the probes are in the correct position given what you’re trying to measure. Measuring current is the only time the Red Probe is connected to the mA or A terminal!
- Familiarize yourself with the type of fusing your meter has. Most DMMs have an internal fuse (or two) that you can’t find on a Sunday at your corner grocery store. It will pay off in spades to have spares on hand—trust me, you’ll need them when you least expect it.
- Never loan your DMM to a buddy. Why? Simple—they don’t know how to use it anyhow. Anyone that knows how to use a DMM owns one. They’ll invariably put the probes in the wrong spots and blow the internal fuse(s)—unbeknownst to them—and return it to you that way. You’ll find out when you use it next…on Sunday.
Using a DMM
Selector Switch: Right smack dab in the middle of any DMM is a switch or dial that allows you to set it according to the measurement you are taking. Set it incorrectly, and you could damage the circuit you’re measuring or even the meter itself, although this is quite uncommon. In most cases, the settings are not written out, but rather abbreviated. The side-bar on pages 26 to 32 illustrates the meaning of the abbreviations on my Fluke 87 DMM. This meter is used throughout the book, as it is still quite current in looks and functionality when compared to meters in the marketplace today.
Range Switch: Some inexpensive DMMs combine the selector switch and range switch into a single switch with numerous setting locations. They are typically labeled clearly and are self explanatory.
Most higher-end DMMs have a separate range button that allows you to manually adjust the range. As pictured, the range button in the upper row of buttons on the Fluke 87 allows for that. Most premium meters also have auto-ranging capability, as does the Fluke 87. Whether the meter does it automatically or you do it manually, setting the range is designed to maximize the display reading corresponding to the measurement being taken. Equally important is the accuracy of the measurement you’re taking—do you need to know what you’re measuring to the hundredth of a volt? No problem.
The Fluke 87, like most good DMMs, has a four-digit readout. The range can be set manually as follows for taking voltage measurements. This is how the display looks as you change the range (by pressing the range button):
- 0.000—Max range of 9 volts, with maximum accuracy of 999 thousandths of a volt.
- 00.00—Max range of 99 volts, with maximum accuracy of 99 hundredths of a volt.
- 000.0—Max range of 999 volts, with maximum accuracy of 9 tenths of a volt.
- 0000—Max range of 9999 volts with no further accuracy —Note that the meter is clearly labeled “1000V MAX.”
In most cases, I set the meter for auto-range. This is the default mode when turning on most DMMs. If you’re taking a measurement that exceeds the auto-range setting with this meter, the display typically reads OL—this means overload and is an indication that you need to manually range UP the DMM. (I’ve seen some that read OUCH—same difference.) This does not damage the meter—no worries if you see this!
Note that you should not attempt to make resistance measurements with a DMM on live circuits. If you do so by accident, most DMMs are internally protected from this, but this is a good rule to live by.
Probe Location: As I mentioned earlier, it is extremely important to have your probes in the correct location given what you’re measuring. Even though you have the legend to refer to, I’m going to go over this just so that you’re sure. Following are the four locations on the Fluke 87 and what they mean.
- mA μA—milliamps, microamps
- VΩ—volt ohm
OK, now the black probe is always in the black COM location, but what about the red one? It can go in any of the other three locations and they are all red. Do not let this confuse you! This is how to deter-mine this:
- A—Red probe goes here when measuring current of 10 amps or less. (Note that the meter says 10A MAX FUSED between this location and the COM. This means that you can safely measure 10 amps of current, and the meter is internally fused to protect it from more than that.)
- mA μA—Red probe goes here when measuring current of 400 milliamps or less. (Note that the meter says 400mA MAX FUSED between this location and the COM. This means that you can safely measure 400 milliamps of current, and the meter is internally fused to protect it from more than that.)
- VΩ—Red probe goes here when measuring voltage or resistance (Note that the meter says 1000V MAX between this location and the COM. This means that you can safely measure 1,000 Volts without risk of damage to the meter. You shouldn’t have to worry about exceeding that in your ’32 Ford.)
Now that you know the basics of how a DMM ticks, let’s put it to use by tackling the same three examples from the test light section with our DMM.
(Please note that the labeling of your meter may be slightly different, but the functionality is the same.)
Be sure the probes are inserted as follows:
a) Red probe in VΩ
b) Black probe in COM
Turn the selector on your DMM to measure DC Voltage.
Connect the black probe to chassis ground.
Connect the red probe to the connection point in question; in this case I’ve connected it again to the headlight connector.
Turn on the circuit to be measured. The display indicates how many volts are present.
By pressing the range button, I can now see the actual voltage accurate to hundredths of a volt.
In the above example, I called it “seeking ground.” Identifying a good location to ground a piece of electronics is only one of a DMM’s many functions in this arena. It can also measure resistance in ohms as well as check diodes for functionality to name a couple. This is how to find ground:
Be sure the probes are inserted as follows:
- Red probe in VΩ
- Black probe in COM
Turn the Selector on your DMM to read continuity— typically labeled with the Ohm symbol or Ω.
Connect the black probe to the negative battery terminal.
Use the red probe to find a low-resistance connection point, anything that reads less than 10 ohms is fine for any low-current device (more on this later).
A more common use of a DMM is measuring continuity when troubleshooting an inoperative circuit. This is explained in Chapter 7.
This is the reason to own a DMM, and it dramatically helps you troubleshoot current draw problems quickly and easily without guess-work. I mean, who can really tell how much current is being passed through a test light given how bright it is? I know I can’t.
Be sure all accessories in the vehicle are turned off, especially the ignition switch, so that you don’t risk the chance of blowing the internal fuse within your DMM.
Disconnect the battery terminals as I outlined earlier and re-connect the negative terminal.(Note: I’ve re-connected my small light bulb.)
Be sure the probes are inserted as follows:
- Red probe in A
- Black probe in COM
Turn the selector on your DMM to read current—typically labeled as A. (Note: If your DMM has two current scales, start with the A scale so that you don’t accidentally blow the internal fuse on the scale with higher resolution by allowing too much current to flow through it.)
Connect the red probe to the positive battery post.
Connect the black probe to the positive battery terminal.
Observe the reading on the DMM.
Note that this reading is well below .4 amps, which is 400mA. Therefore, I can change the selector to the mA scale on my DMM for the higher resolution setting. Additionally, I also have to move the red probe to mA because my meter has a purpose-built location for this scale. When measuring currents below 400mA, this setting is more accurate and gives the most accurate measurement possible with the Fluke 87. Again, this is spot on and not a guess—it is not possible to obtain such information with a test light.
Incidentally, you could then use this information to determine how long this draw takes to drain the battery; you only need to know the actual current draw and the amp hour (AH) rating of the battery to compute this. See the sidebar “Amp Hour Rating” for more details.
At this point you already know far more than any of your fellow car buddies in regards to electrical measuring tools. Continue on and you’ll be able to charge them for your services!
Measuring Voltage Drops: Just as you can measure voltage, you can measure voltage drops. Actually, the practice of measuring voltage drops is measuring voltage across a component in a circuit. Recall, back in Chapter 1, when you learned about Kirchhoff’s Law and Series Circuits (Fig. 1-7).
If this circuit really existed, and you were to take your DMM and measure voltage across any of the lamps, you would measure the voltage dropped through it—in this case 3 volts. Now, what if you really did need to determine why the starter motor on your big-block Chevy was sluggish, you’ve verified the solenoid trigger wire is not the problem, and you didn’t have a DC clamp meter (more on this soon) on hand?Before pulling the starter and taking it to the store for testing, you can determine this the way most mechanics would. That way is to measure the voltage drop across the various components in the circuit to determine if the problem isn’t really the starter motor after all.
So what are those components? Since the starter circuit in most vehicles is incredibly simple, assume that it has the following components to it:
- Starter motor.
- Solenoid on motor excited by ignition switch in the start position.
- Length of cable between the battery (+) and starter motor.
- Length of cable between the battery (-) and the engine block.
- Starter mounted to the engine block.
Figure 2-1 is a diagram of such a circuit:
Finding Voltage Drops
Here’s how you determine the problem:
(Note: Probe locations and selector switch settings are the same as for measuring DC voltage.)
Turn the headlights on for 30 seconds to 1 minute to dissipate any surface charge the battery might have on it.
Measure the voltage across the battery—for the sake of this example, let’s say that was 12.6 VDC.
Disable the vehicle’s ignition circuit—disconnect power to the coil or, better yet, disconnect the coil wire to the distributor.
Measure the voltage between the case of the starter motor and its (+) input terminal (the big wire!) while a helper cranks the motor—let’s say that you measured 10.2 VDC.
Now that you’ve verified a voltage drop of 2.4 VDC, you need to determine the source of the voltage drop. Obviously, the starter pulls a bunch of current—how much exactly, is unsure. This explains some of the voltage drop, but let’s determine if we have a high-resistance cable or connection (or both) contributing to our problem. Figures 2-2 and 2-3 show how to do it:
First, measure the voltage between the positive battery terminal and the starter motor (+) input terminal as shown while a helper cranks the motor (A).
Next, measure the voltage between the negative battery terminal and the case of the starter motor—let’s say you measured 1.8 volts (B).
The first place to look is the return path, as 1.8 volts seems quite high. Closely inspect the connection between the battery negative and the engine block because this is the return path for the starter motor in most vehicles on the road.(Obviously, you could also measure the voltage drop between the case of the starter motor and the battery (-) connection to the engine block and the voltage drop from that point to the battery (-) terminal to further narrow your search.) What you’re looking for is evidence of resistance:
- Is the connection to the engine block tight?
- Has rust formed between the ring terminal and connection point?
- Has a star washer been used to ensure a good solid trouble-free connection?
- How is the integrity of the connection between the ring terminal and the wire itself?
- How is the integrity of the connection between the battery terminal and the wire itself?
- Is this connection tight?
- Is the wire corroded or oxidized at either end? (You will probably have to pull the insulation back from the connector or slit it with a razor blade to inspect it.)
If you find evidence of any of the above, make the appropriate repairs. This should restore the low resistance return path for the starter. Keep in mind, a complete negative battery cable assembly costs less than $10 at your local auto parts store. If this is your problem, you need to inspect the integrity of the other ground wires because they may have been damaged due to the starter seeking ground through them in the event that they offered a lower resistance return path to the battery negative. They are:
- Battery negative to chassis (typically 10–gauge or larger wire).
- Engine block or bell housing to firewall (typically a braided strap)—there can be many such straps from the engine block to the chassis and firewall in newer vehicles.
In some cases, I’ve seen one of these snapped in half from excessive current flowing through it from the above scenario. Although I’ve never personally seen it, I’ve heard stories of throttle cables or transmission kick-down cables burned in half or melted from exactly this.
This really is the only way to diagnose such problems. Even if you did know the current draw of your starter motor at 12 volts, it wouldn’t do you much good, as only 10.2 volts is present at the starter. Recall, voltage is that which causes current to flow. When voltage is compromised, current flow is reduced.
Grounding problems can cause all kinds of maladies, and you can troubleshoot these issues the same way with a DMM. One accessory exhibiting erratic behavior when other accessories are operated is a tell tale sign of a grounding problem. Using your DMM to measure voltage drops helps you to easily determine the source of even the most difficult of grounding problems. Again, this is something you simply cannot do with a test light.
Advanced Uses of a DMM
Now that the basics are covered, I’ll show you the value of owning a really nice meter, such as my Fluke 87 for example. (At the time of the writing of this book, Fluke offers an updated version of this meter, the Fluke 87V.) Some of the additional functionality the really nice meters offer are:
- Making voltage and current measurements over time—minimums, maximums, and averages.
- Audible continuity checker.
- Measuring very high currents—optional accessory required.
- Diode checker.
- Capacitance checker.
- Measuring A/C frequency.
- Uploading measurements to a PC.
- Measuring temperature.
Obviously, the sky is the limit when it comes to what’s available today, and I didn’t even cover all the additional features that a combination scope (oscilloscope) meter offers! As usual, I’m going to stick to the applications that apply to typical automotive use.
Making Voltage and Current Measurements over Time: Since the idea is the same for both voltage and current, I’ll just provide a single example. Let’s say that I wanted to know the voltage drop caused when both of the 16-inch cooling fans in my Cutlass kick on. Even though they are wired with 10-gauge wiring, these fans consume so much current on turn on, they still cause the analog voltmeter on the dash to bounce radically for a split second. Although this causes no real harm (other than possibly pitting the contacts in the relays over time) it makes for an excellent example of how to record this minimum voltage. It also hap pens so quickly; the naked eye can’t see the actual voltage drop on the dash-mounted voltmeter. Let’s measure it.
Before starting, note that some DMMs have a MIN/MAX button that allows you to get into this mode. If this is not clearly labeled, you may have to refer to the manual of your DMM to see how to enter and use this mode.
Making Time Measurements
Make sure probe locations and selector switch settings are the same for measuring DC voltage.
Connect the red probe to the source of power for fans—not the wiring to the fans themselves.
Connect the black probe to the chassis of the vehicle.
Start the vehicle.
The DMM should display a reading in excess of 13.0 VDC at this point.
Press the MIN/MAX button to begin the recording process.(The Fluke 87 can log data over a time span of up to 36 hours!)
Wait for the vehicle to get to operating temperature and for the fans to kick on Immediately after the fans kick on, press and release the MIN/MAX button to stop the recording process.
Press the MIN/MAX button once to get to the MIN voltage recorded—the minimum voltage that occurred when the fans kicked on is displayed.
Audible Continuity Checker: This feature can be very handy when you’re diagnosing an open circuit and you cannot easily see the DMM’s display. Maybe your head is buried up in the dash or you have to troubleshoot an open circuit problem that spans the length of the vehicle. In this case, you probably have to extend one of the meter’s probes. I have a pre-made 10-gauge wire that is terminated with alligators on both ends for just this reason. Simply clip one end to the black probe tip and you now have a very long black lead:
The mechanics of this are the same as outlined earlier in the chapter, but the meter emits a beep on detection of continuity. Imagine how handy this would be if you were trying to diagnose a problem with an inoperative tail light and the cause of it was a broken connection in the driver-side kick panel area. If you didn’t know how to use a DMM to track this problem down, you could burn up an afternoon or even an entire day looking for the cause. Worse yet, you might not even find it.
Here’s how you can go about it, the quick way:
- Be sure the circuit was turned off.
- Place the DMM in the trunk of the vehicle.
- Be sure the probes are inserted as follows: Red probe in VΩ; Black probe in COM.
- Turn the Selector on your DMM to measure continuity—typically labeled with the Ohm symbol or Ω.
- Connect the red probe to the power lead for the inoperative tail light
- Enable the audible continuity function.
- Probe with the black probe into the same color wire in the tail light harness, slowly working your way toward the front of the vehicle.
As long as you have continuity, the meter beeps with every step. It eventually gets to the point where the DMM no longer beeps when you probe the wire. This would pinpoint the source of the open connection between the current probing spot and the prior one, thereby greatly reducing the time necessary to track down this problem.
Measuring Very High Currents: Using a device called a Hall Effect current clamp is the simplest way to do this. Readily available for most of the premium DMMs, it allows you to measure DC current up to 1,000 amps.
Using Current Clamps
This example uses the Fluke Model i410 current clamp to determine how much current the starter motor in my Mustang consumes at start up:
Plug the clamp meter into the COM and VΩ terminals.
Turn the Selector on your DMM to the DC mV scale.
Power the current clamp on and zero the reading via the calibration dial.
Place the clamp around the power wire that goes directly from the battery (+) to the starter motor—be careful not to allow any other power wire to be within the jaws of the current clamp or this could skew the reading.
Have a helper start the vehicle, being sure that your DMM and wiring to the current clamp are out of the way of moving parts.
A Hall Effect current clamp is no meager investment. Although the above example was a quick and easy way to determine the maximum current draw of the starter motor, in this case 290.5 amps, you are able to discern more information about the overall state of the starting system by measuring voltage drops between the starter motor and battery as outlined previously. That being said, a current clamp allows you to quickly and easily measure the current draw of any accessory in the vehicle up to its current rating—and without having to disconnect it and place your meter in series with said accessory! Fluke claims the i410 current clamp is accurate from 1 amp to 400 amps.
Diode Checker: Diodes can be found in numerous locations in the modern vehicle. Most commonly, they are used in charging systems. The diode checker function of a DMM allows you to quickly determine if a diode is good or bad, but the diode has to be removed from the circuit to do so. This function Congratulations—you’re now the car guy who knows how to use a DMM! As you get more comfortable using the DMM, you’ll find more uses for it.
Written by Tony Candela and Posted with Permission of CarTechBooks
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