All shop tools need to earn their keep. That multimeter in your tool box probably cost you a pretty good piece of change when you bought it. If your digital multimeter (DMM) isn’t solving problems and making money for you, it should be. Here are some ways to better understand the basic features of your DMM and the advanced features many meters have.
Read the manual
The first step is to read your manual. There are lots of brands and product numbers out there; you need to know what it is exactly that your particular meter can do. If you are more of a visual learner, training videos online and on DVD are usually offered by the manufacturer. Be sure to check the Web site of the company that made your DMM. This is free training that you get just for the asking. There is genuine content in terms of tips and suggestions on how to really use the instrument to solve problems and make money.
These sites are also useful in terms of keeping up with newly issued accessories. As you will see, there are add-ons available that can greatly expand the usefulness of your meter.
A big section of any meter manual is devoted to safety-related cautions. Do please take note of them. The danger in automotive work is not so much about high voltage as it is high current and/or mechanical entanglement. The working tech should consider that the meter will likely be placed on available, more or less flat surfaces with the probes and test wires extended out over or down into the area near the engine. Leads, or even the meter itself, can become entangled in moving things like the accessory belt drive system or in the fan. The point that I’m making here is to be careful.
Understanding meter accuracy
With any brand digital meter there is something called “gauge error” that you need to know about. Gauge error is a catchall term that includes all of those things that affect the accuracy of any type of measurement you might make.
To really understand what the DMM is trying to tell you, you need to know about some of these sources of error and some of the ways that it can make the information on the readout misleading.
Don’t be fooled by too many digits. Typical meters will have a display that could read 5.123 volts, or for that matter 5.123 amps or ohms. If you check the meter data sheet, you will see that when all factors are considered, the basic meter accuracy is probably something like +/-1%. When you add in other likely error factors, +/-2% might be more like it. This means that a reading of 5.123 is really somewhere between 5.0 and 5.2. The last two digits in this reading may not really mean anything and you have to learn not to attach unwarranted value to them.
There is another element to consider. Your meter almost certainly came with at least one set of test leads and possibly many more if you bought a high-end meter. All of these leads have what could be called interface resistance.
This is the slight error that comes from holding the test lead up against some point of contact to make the measurement. If the probe is held against a soft, compliant material like solder or lead, the area of contact will be large and the resistance at that point will be less. If the probe is held against a hard material (like the nickel-plated brass of many electrical terminals), the contact point will be smaller and the reading less accurate and less stable. If the surface of the contact point is oxidized (like a battery terminal) the quality of the connection will be poorer. There is even some resistance in the leads themselves that add to the problem.
There are other sources of inaccuracy. One of them is something called digit rattle. This is especially seen when the meter is set on the low value ranges where the sensitivity is the highest (milliamps, milliohms). What happens is that the digits in the display flicker between one value and another. Our example value of 5.123, for instance, may display as 5.130 one second, and 5.112 the next. The further to the right, the more likely the digit is to flicker. A certain amount of this is normal and should be ignored. Some of it is caused by the meter’s internal electronics, but more of it is caused by variation in the quality of your lead connections. In the real world, it’s not always easy to hold those probes perfectly still.
The temperature of what you are measuring also can be an issue. The secondary resistance of an ignition coil, for example, can vary by hundreds of ohms depending on the temperature of the coil. What may be important to you is to find out that the coil has continuity and is not burned out or open. In that case, the exact value may not be important. If you want to compare the coil’s resistance to a published OE value, then be sure that the coil is soaked out to the same temperature as the OE sample was when it was measured. Don’t be fooled by the outside temperature of the coil. It can take an hour or more for the insides of a hot coil to really return to room temperature.
All measuring systems have some amount of this gauge error. In the typical DMM, these different factors don’t add up to any large amount of error. What is important is for you to realize that these error factors are a part of the meter and the measuring system, including the leads. The solution for it is to understand that not all of the digits shown on the front of your meter really contain real information that you need to act on. The 5.1 is important, not the 5.123.
One of the things you also will need to do is to intelligently choose the right test for what you want to know. The fact that one battery measures 12.35 volts and another measures 12.2 volts does not mean that one is necessarily better than the other. Really this is no different than choosing the right wrench for the task at hand. The DMM by itself is not the right tool to measure the quality or condition of a battery. You will need to learn to use the meter for those measurements and tests that it is capable to make.
All of this said, the DMM is a wonderful tool and should be your most important electrical test tool.
Getting ready to use your meter
The first step in any measurement is what needs to go in your mind based on your knowledge of how the system is supposed to work. You have to first decide what measurement you want to make. Any DMM will measure current, voltage and resistance. More sophisticated meters have become quite common. They may include the ability to test diodes, measure capacitance, temperature, frequency and pulse width/frequency.
Before you can use your DMM effectively, you first need to understand how the part of the electrical system you are working on is supposed to work. Using that knowledge you will need to choose a measurement and then find the appropriate location for that measurement. If you choose wisely, your measurement may take you to a solution or the next appropriate test.
Meter setup and connection configuration
The second step in the measurement process is configuring the meter to set it up for your intended measurement. Most meters use a combination of a central rotary switch and a series of push buttons. Your owner’s manual is of critical importance here. Some setups will require you to select the position of the rotary switch and then follow that with one, or possibly more than one, button push. On one of the meters gathered for this article it was necessary to press one button three times after the rotary switch was selected to get to the desired measurement feature. Your owner’s manual will show you how to do this.
If somehow your meter has become separated from its manual, don’t worry. DMM manufacturers commonly include copies of their manuals on their Web sites. These manuals are easy and free to download. Many manufacturers offer their information in multiple languages, so getting tech literature shouldn’t be a problem.
Meter setup and connection test leads
The start of any measurement is getting access to the terminals in question. The challenge is to choose the right set of test leads that give you the best access. Because there are so many different connection possibilities, you will need a good set of test leads with multiple/different ways of connecting your DMM to the circuit. These are available not only from the maker of the DMM but from other companies like Pomona Electronics, Agilent Technologies, Paladin Tools/Textron and others who specialize in making test leads. The test leads could be divided into “self fixturing” and manual application. Alligator clips not only make the connection, but also hold the connection while the measurement is being made. Hook leads take up less space and can reach into crowded areas to grab and hold the wires and leads that are part of the circuit.
One thing to understand about test leads, especially the clip-on type, is that they can be fragile and will likely need replacement from time to time. They just sort of get used up in day-to-day work.
Manual leads or test prods are simple and reliable. The problem is that you have to hold them to the test points and read the meter at the same time. The best ones have sharp tips that can be pressed through the oxidation or contaminants on the test points. One of my favorite sets of test leads was built with collets on the end of the probe. The collets are just the right size to hold the needles from Victrola-type record players. When the sharp tips get dull, you pull them out, throw them away, put some new phonograph needles in, and go back to work.
Special sensors like the “clamp” used to measure current don’t need to connect directly to the circuit at all. These devices measure current through the use of a Hall Effect sensor that measures the magnetic field surrounding the wire as an indication of what the current is. For automotive work, you will need one that is capable up to 400 amps so that you can measure starter current draw and alternator output. These clamp accessories are available as an add-on to your DMM or as a separate, stand-alone meter.
Back probe connectors are made of a thin, but stiff wire that can be inserted into the back of electrical connectors to make contact where ordinary probes cannot reach.
A wonderfully useful probe is the insulation piercing type. It can poke a tiny hole into the insulation and make direct contact with the wire.
The hole you made has to be covered afterwards, but sometimes it is the only way to gain access.
What should the measurement look like?
Part of the diagnosis process is that any measurement you make has to be compared to a “correct” value. You need to know if your results are right or wrong.
There are at least four good sources of that information: A) OE specs, B) aftermarket information sources such as Identifix, Mitchell 1 and ALLDATA, C) comparative testing (known good system versus possible bad system), and D) recommendations from others including the maker of the meter, fellow techs and instructors. A great resource is the online archives at the International Automotive Technicians Network (IATN) Web site.
Making those profitable measurements
Electrical measurements follow the same basic rules of any kind of diagnosis. First and foremost, you must understand how the system is supposed to work. From this you will know what you want to measure, what the value likely should be. Once you know what you want and where to make the measurement, the next step is to have the meter correctly configured and then connected using the right probes for the job.
Here are some quick, money-making tests you can do. If you have the system knowledge, skill, and a good DMM plus appropriate test leads, you should be ready to rock and roll.
Preliminary battery checks
The function of the vehicle starting system is well known. The first test is a simple measurement of the battery voltage as seen at its terminals. The battery can have a surface charge on it if it has recently been charged by the alternator. The best way to do this test is to configure the meter for DC volts. The black or common lead goes to battery negative and red or positive lead to the positive of the battery. With the meter connected like that, the reading should be about 12.6 volts. With the meter still connected, turn on the headlights. If the voltage falls by more than half a volt it is an indication of a poor state of charge (SOC) or problems with the battery terminals.
Finding poor connections
The second step is to look for poor connections between the battery and the starting system. This is done with a voltage drop test. It is known that resistance in the cables or the connections to either the battery or the starter will reduce power to the starter and may be the source of poor cranking performance. For the purpose of this test, the cables of concern are battery minus to body ground, body ground to engine ground and the positive cable to the starter solenoid. Experience shows that trouble with the connections almost always happens at the ends of the cable where they connect to either the battery or to ground or solenoid. Experience also shows that the most likely cause is a connection that is not “solid or tight” enough or is corroded and thus unable to carry the needed current (it could be as much as 400 amps).
The way to do the test is to connect the voltmeter across each of the connections. For example, place the probes or alligator clips with one on the negative battery terminal and the other on the cable end. The meter should be put on either a low DC volt scale or a millivolt scale. If your meter has the min/max function, press it before starting the engine. After the cranking cycle is complete, shut off the engine. Pushing the min/max button again will show you the maximum voltage drop that occurred across that connection.
You will repeat this sequence for each of the possibly suspect connections. The test must be performed when the starter motor is cranking. You need the heavy current of the starter draw to see the voltage drop if it is there. The value should be less than 0.5 volts across any of the high current connections. You will probably find 0.2 or 0.3V to be a pretty common value. Any more than that and the connection needs to be redone.
Battery test under cranking conditions
It is also possible to do a reasonable test of the battery, using the DMM and the vehicle starter. This is done best by using the min/max feature many meters have. This test is based on the idea that the measured battery voltage will drop under the load of the cranking cycle. It is very much like a resistance load test, only it uses the starter as the load. It is possible to do this test without the min/max feature, but then you have to watch the meter closely during the cranking cycle.
This test is done by configuring the meter for DC volts and then connecting it to the positive and negative battery terminals. The alligator clip lead ends work well for this. With the meter set to DC volts, engage the min/max feature. Now crank the engine, let it run for a second or two and then shut it down. What will happen is that during test sequence the battery voltage will first be at nominal (roughly 12.6 volts). As the starter is engaged, the battery voltage will drop. As the engine starts, the battery voltage will rise to the normal system charging voltage as being supplied by the alternator.
When the start sequence is complete, go back to your meter. When you push the min/max button this will show what the minimum and maximum voltages were. On other meters there may also be an average value feature.
You can get three key pieces of information from this test. The nominal voltage (be sure to have the headlights on so that you are not fooled by a surface charge) will tell you that the battery is not discharged. Typically the reading will be in the 12.0 to 12.6 volt range. The drop in voltage during cranking tells you about the ability of the battery to supply current. Typically, the voltage should not fall below 9.5 volts. The voltage present at the battery after the engine starts can tell you what charging system voltage is that the alternator is applying to the battery.
Typical charging system voltages are in the range of 14.2 to 14.8 volts. Remember that voltage regulators have built-in temperature compensation. They will direct the alternator to increase its voltage when it is cold and reduce it when it is hot. If you bring a car in from a snowy lot, don’t be surprised to see it charging at 15.0 or 15.1. A hot vehicle may be correctly charging even though the voltage is 13.8 or 13.9V.
Beyond the measurement of the voltage at the battery, with the engine running, there are at least two more good ways to get a feel for how well the alternator is performing. The first is to do a current output test. For this you will need your DMM and clamp sensor capable of measuring the full output current of the alternator or the draw of the starter. Normally DMMs are not appropriate for either of these tests because they are limited to a maximum current of about 10 amps. The output of the alternator could be well over 100 amps while the draw of the starter could be 400 amps.
The DMM can be made to work by adding a clamp sensor and its module which measures the magnetic field that surrounds the cable and converts it to a DC voltage the meter can read.
There are two things to know about using a clamp sensor correctly. The first is to open the jaws and then encircle all of the wires you want to measure the current in. When you want to know what the starter draw is, don’t include the extra smaller ground wire that leads to the rest of the electrical system. The second is to put the clamp on correctly. There is usually an arrow on the clamp that shows the direction of the current flow. If you get it on backwards to the actual flow, it will just tell you that the current is negative. Don’t let that puzzle you. Just ignore the polarity or remove the clamp, rotate it 180 degrees and re-connect it around the red wire coming from the alternator to the battery. The min/max feature can be used here as well. Press it before beginning the start cycle and it will tell you what the peak current draw was during the cranking of the engine.
In looking at the output of the alternator with the clamp sensor it is important to remember two things. One is that the output of the alternator will not exceed the combined demand of the electrical loads plus the battery. The voltage regulator sees to that. You should measure the output and then add loads like the HVAC blower motor and the headlights to make sure the alternator output rises to satisfy those loads. Also you should know that the alternator’s full output is not available at idle. If the voltage at the battery falls under the combined electrical loads, try elevating the engine speed to 2,000 rpm. At that speed the full output of the alternator should be available.
The second thing to remember about alternator output testing is that the cables and connections for the alternator are typically “down in there” near the engine and the moving parts of the engine. It is important to be careful where the sensor, the wires and the meter are located. You want to make sure that the start-up vibration of the engine won’t cause the meter to fall or cause the test lead cables to get tangled. The possibility of danger is there for the meter, the car and possibly even the technician.
Another good, DMM-capable alternator test is to measure the output ripple voltage. This is a great way to spot an alternator that is charging some, but not enough to really keep the battery up. A bad diode will not only reduce the output of the alternator but will actually cause the battery to discharge.
Elevated AC ripple voltage can actually discharge the battery. The way to perform the test is to configure the meter to AC and to then apply the test leads to the battery with the engine running. The AC ripple at the battery should be less than 0.6 volts. Be sure to rev the engine up to around 2,500 rpm to make sure that elevating the rpm doesn’t cause this number to get worse.
Using the DMM to directly measure current
Most meters are also set up to directly measure current without the use of the clamp sensor. Typical ranges are 0 to 10 amps and 0 to 1,000 milliamps. Some meters will even offer to measure micro amps. It is important that you know about how much current you are trying to measure. Current flow is an inline, or series, measurement. Whatever current that flows will flow in one lead of the meter and out the other. If you try to directly measure the starter draw with your DMM, the likely result is a blown fuse, and the possible result is a damaged meter. If you try to measure very small currents on the 10 amp range the result will be a decimal point and a lot of zeros or a reading that is too small to be accurate on that range.
The use of the DMM to measure currents less than 10 amps is simple. Connect the meter in the power line to the load. If you are working on the ground side, the black probe will go to electrical ground while the red probe goes to what was the ground connection for device being tested. The current draw can be read directly from the meter during the time whatever you are testing is turned on. One easy way to use this feature is to remove a fuse from the panel and place the probes on either side of where the fuse plugged in. The meter will read the amount of current the fuse was carrying.
Finding key-off loads
Key-off or parasitic loads are current flows that come from the battery even though the ignition key is off. A certain amount of this is normal. Typically about 100 ma will continue to flow after the key-off event. Usually within a minute, this amount is reduced as the computer shuts down different parts of its processing units. After a short time, the normal key-off load will fall to about 25 ma. This current is needed to keep various volatile memories from forgetting things like fuel trims and other “learned” settings. Most batteries can supply this small “memory keep alive” current for up to a month and still be able to start the car. The whole term “lot rot” is about vehicles that have been left too long without being run.
Eventually, these key-off loads will drain the battery if the vehicle is not started up and run for a while to recharge the battery.
Where the trouble comes in is when unintended loads bleed the battery down much more quickly than expected. Typical to this are glove box and trunk lights that stay on when they are closed due to misaligned switches. Aftermarket audio systems, burglar alarms and remote starters also get accused of this sort of thing.
The DMM is a perfect tool for finding these unexpected key-off loads. The way to do this is to recharge the dead battery or install a new one. To prevent a re-occurrence, the need is to find out what circuit is flowing current when it shouldn’t be. You can often make a direct, logical connection between the amount of time it takes to drain the battery and the likely size of the drain. A battery that goes dead overnight is probably the victim of a large drain. Anything pulling that much current is likely to be warm to the touch. Oftentimes you can find the culprit by simply feeling to see what is warm when it should be cold.
The more difficult drains to find are the smaller current ones. One way to find this sort of a drain is to set the ammeter on the 10 amp scale and connect the leads between the ground terminal of the battery and the cable leading to ground. Remember, the ammeter has to be put “in circuit” to measure current. What you should measure is the key-off loads that are “normal” plus the parasitic one. While 25 ma. is given as the normal key-off drain, the truth is that “normal” varies from vehicle to vehicle and even with trim level and optional equipment.
Once you have determined that you have an abnormal key-off load, the trick is to find what circuit branch is drawing it. One way to do this is to pull fuses, one at a time, until the current you are seeing on your DMM drops to normal. When you have found which fuse stops the abnormal flow, you have narrowed the source of the flow to that branch and the things connected to it. With the fuse back in place, the procedure is to then disconnect the items feeding from that fuse until once again the current flow drops back to normal. The item you disconnect that returns the drain current to normal is the culprit.
One downside to the “pull the fuse” approach is that any memory that was being kept alive will go down as power is disconnected. It may take a while for it to fully relearn what was forgotten by pulling the fuse. Many DMMs have a temperature measurement feature that uses a thermal couple. If the sensing end of the thermal couple is applied to the fuses, it should be able to find the one that is warmer because current is flowing through it. This same trick can be accomplished by IR-based temperature measuring devices. Yes, this is another meter to own, but they are not expensive and they have other uses in A/C and cooling system work.
Another possibility, depending on the fuse block and fuse style being used, is to measure the voltage drop across the fuse. All fuses have some amount of resistance. If current is flowing through the fuse, it makes sense that there will be some sort of voltage drop across it. The exact amount of the voltage drop depends upon the resistance of the fuse. Since that resistance varies with the size and the make of the fuse, the important information to use for diagnosis is the fact that there is a voltage drop (therefore there is current flowing through the fuse). It is not the amount of the voltage drop that is useful, but rather the fact that there is one.
Additional uses for your DMM Testing throttle position sensors
Beneath the main, numerical readout for most DMMs, there are a series of bar-like dots that go from one side to the other. This part of the display represents an analog representation of the readout. Where this is useful is in the analysis of variable resistance throttle position sensors.
In the case of contact-type throttle position sensors, the device is supposed to have a smooth, even, continuous change of resistance from one extreme of rotation to the other. This gives a smooth analog signal to the ECM that indicates throttle position. If there is a point or points of non-contact, the computer will get an indication of open throttle. The test for this is to attach the ohmmeter to the two output pins of the TPS. The sensor is then rotated through its range. The result should be a smooth, linear increase in the bars. A sensor with jumps, skips and misses is easy to find this way.
Another way to do this same test is to leave the TPS installed on the vehicle and measure the same voltage that is being supplied to the ECM. You can use the DC volts feature of the meter to measure the 0-5 volt output of the TPS sensor. You can watch the analog bar graph as you rotate the throttle and watch the voltage change smoothly the same way the resistance test made it change.
Testing fuel injector solenoids
The truth be known, fuel injectors don’t really inject anything. What is called a fuel injector is a really a valve with a spray head that opens in response to a computer signal. Behind the valve is fuel held at a pressure created by the fuel pump and regulated by a bypass valve. The amount of fuel dispensed depends on the size of the valve opening, the amount of pressure and the amount of time the fuel is allowed to flow.
The DMM can be used in a couple of ways to check the performance of the fuel injector. One measurement that can be made is the DC resistance of the fuel injector’s solenoid coil. A common failure mode for injectors is that one or two turns of the coil may have shorted due to overheating. The DC resistance of the coil may show the drop in resistance caused by the shorted turns. Since the two or three shorted turns don’t amount to much in the way of resistance, you will need an especially accurate resistance measurement.
What will improve the accuracy of your measurement is to “zero” the meter. Often when the probes are connected together, the display will show some tenths of an ohm of resistance. This is caused by resistance in the leads and in the connections to the meter. What will help you make a more accurate measurement is to remove this “error offset.” The way to do is to cross the meter leads (short them to each other) and then hit the “rel” button to cancel out the resistance in the leads and make the meter read zero. Now when you measure across the two terminals of the fuel injector, the value you read will reflect the true resistance of the injector’s solenoid.
Using the pulse width measuring feature
Not all meters have a pulse width measuring capability. If your meter does, it can be quite useful in measuring devices that use pulse width modulation (PWM). These devices use electronics to control the amount of time the device is on as a way of controlling how much it is on. PWM devices include fuel injectors, idle air control, and some DC motors like the one used for the fuel pump.
For fuel injectors, the way to do this is to use your back probe accessory probes by slipping them in along the sides of the wire entering the back of the wire harness connector that goes to the injector. You will need to configure your particular meter to the pulse width mode. On some meters this means selecting the DC voltage range and then pushing the Hz range until “Pulse width” is displayed on the screen. It is not the easiest feature to select, but the value is that you can see the amount of time the injector is open as a pulse width time measurement. If you tip the throttle you will see the computer modify the pulse width to enrich the mixture and then lean it back. Your DMM owner’s manual will tell you how to configure your particular meter for measuring pulse width.
Idle air control motors often work on the PWM principle. Using the back probes, configure your meter for the pulse width measurement. You should be able to watch as the ECM varies the pulse width to change the idle speed. Since idle speed problems are often the result of vacuum leaks, you can watch the pulse width as you wiggle vacuum lines to try to make the possible leaks worse. You’ll be able to see the ECM try to adjust the pulse width to compensate for the increase or decrease in the vacuum leak. Other devices such as actuators and solenoids also use PWM techniques and can be tested similarly.
User convenience features
The “hold” feature is somewhat similar to the min/max average function noted earlier. Its main advantage is for the convenience of the technician using it. When this feature is activated, it is possible for the tech to devote his or her attention to getting the probe on to the terminal or test point that is wanted without having to watch the meter readout from the corner of your eye. When the “hold” feature is activated, whatever value the probe detects will be held on the screen until you remove it. You can touch the test point and then look over to the meter to see what was read.
A number of currently available meters have straps with built-in magnets that are included with the device or are available separately. What this lets you do is support the meter and position it for the best advantage for where you are working. The strap attaches to the meter while the magnet grabs on to a fender or wheel well. The result is that the meter is up and away out of possible danger while at the same time being located where it can be easily read. If your meter didn’t come with such a device, don’t worry. They can be purchased from the meter company or you can make one yourself. Appropriate magnets are available at Radio Shack and other similar stores.
Another available feature from at least one supplier is a meter with a detachable face. This meter allows you to connect the body of the meter and the probes to the place where the measurement needs to be made. You can then remove the face of the meter and walk as much as 30 feet away and still be able to know what the meter is reading.
One example would be the troubleshooting of a brake light problem. The meter body could be near the brake lights with probes connected to the brake light wires. The display is carried with you to the passenger compartment. When you touch the brake, you will see the voltage being applied to the brake light that should result in lighting the bulb. In a similar way you could have the meter body under the car with the leads hooked to the oxygen sensor.
Depending on how you have it connected, you could watch the heating element getting power during a cold vehicle start-up sequence. Or you could watch the sensor dither back and forth as the mixture oscillates between rich and lean.
A major benefit of this type of meter is that it allows you to make repairs and diagnoses by yourself without having to ask another tech to interrupt what he’s doing to help you.
Already the DMM has moved a long way down the road from being a simple volts, ohms, amps device. As we have discussed there are features for frequency measurement, pulse width measurement, diode testing, continuity testing and more. Getting the best out your meter depends on you knowing how, when and where to make these measurements.
With that said, there are even more possibilities. Both the meter manufacturers and the aftermarket suppliers have new test devices designed to plug into your DMM. (Some offer an add-on that allows you to measure both vacuum and pressure, and some feature a meter that measures both Celsius and Fahrenheit. Accessory manufacturers make probes to connect the meter for special purposes. You can even get probes designed to clip on to the tiny components used in surface mount electronics. The point is that you don’t need to be limited by what came in the box with your meter. If you have a special test need, be sure to check online or with your meter supplier. Someone else almost certainly has had that same need and there may be a solution already available.
DMMs are simply great tools. They are so versatile and so useful. Just like any other tool, what you get out of it and what it can do for you has a lot to do with the effort you put into it. The information and the resources are out there. It’s up to you to learn about them. ●