Using Multimeters

We list Radio Shack meters here, because everyone has a Radio Shack store nearby.
If you have a local dealer, you may want to shop around for other brand names and features.

To successfully troubleshoot electrical truck problems, you will eventually need a meter to help you determine what is wrong. Some problems can be found with visual clues, while other problems require a meter. So go ahead and get yourself a multimeter now, so you can do a good job. Multimeter simply means multiple meter functions. For example, most multimeters include resistance measurement (ohm-meter), current measurement (amp-meter), and voltage measurement (volt-meter) capability. Many of the newer digital multimeters also include a temperature measuring mode. Most meters are damaged due to careless use or misunderstanding, so make sure that you understand all sections of this topic before using a meter.

Multimeter Selection

Today there are many different multimeters to chose from. Analog multimeters have physical meters with pointers which deflect to show the value measured. Digital multimeters have digital readouts of the value measured. Most of today's digital multimeters use liquid crystal (LCD) displays. The older digital multimeters used light emitting diode (LED) displays, but they consumed a lot of battery power just to light the displays. The LCD displays require only micro-amps (1/1,000,000 of an amp) to display the results. They operate on such little power because they simply reflect ambient light to make the display work.

The analog and digital multimeters each have their own special advantages. The analog meter works equally well in cold weather, whereas the digital LCD meter works poorly or not at all in severe cold temperatures. The liquid crystal material is really liquid, so it get thicker with cold, and takes longer to change the display results. Storing the LCD in a warm location before using in extreme cold, will greatly increase its usability. The digital meter is much more rugged because it does not have a jeweled meter movement.

Another difference between the analog and digital multimeters is their accuracy. The digital meters are much more accurate for complex voltage measurements because they have a very high input resistance (on the order of 1-million ohms). This eliminates meter loading of the device being measured. The analog meters have a much lower input resistance of 1000 ohms per volt, or 20,000 ohms per volt. This lower input resistance can affect voltage measurements when the device being measured has high internal resistance. Truck devices seldom have high internal resistance, so the 1000 ohms per volt meter will do the job, and costs less than the 20,000 ohms per volt meter.

Another difference between the analog and digital multimeters is their physical size. Analog meters are usually much larger than digital meters. The smaller digital meter is easier to store, and easier to use in crowded areas.

Another difference between the analog and digital multimeters is a function called auto ranging. To use an analog meter, you must have a general knowledge of the expected measurement results. You must first select a scale which will give you the most accurate reading, and then make the reading. If the measured value is greater than the selected scale, the the meter slams into its right hand limit peg. Repeated slamming of the meter pointer will make the meter inaccurate. So you must think before using the analog meter.

The digital multimeters use electronic chips to determine the measured value. These electronic chips are smart enough to auto range to the proper scale without human intervention. When the measurement starts, the meter automatically selects its own scale internally, and displays the digital results.

The last difference between analog and digital multimeters has to do with auto polarity. With the analog meter, you must always observe polarity before connecting the meter. You must connect the positive meter lead to the positive side of the device to be measured, and the negative meter lead to the negative side of the device to be measured. If you do this wrong, the meter will slam into the left hand limit peg. Once again, repeated slamming will make the meter inaccurate.

The digital multimeter also has a positive and negative meter lead. But the digital meter will simply place a minus sign (-) in front of the measured result. Therefore it doesn't matter if you get the polarity mixed up. The reading will still be accurate, and no damage is done to the digital multimeter.

The cheapest analog multimeter at Radio Shack costs about $15 and the cheapest digital multimeter at Radio Shack costs about $25. You can pay extra for better meters with more features, but those extra features have little value when working on trucks. We recommend the $30 meter at the top of this page, for it has the most features for a reasonable price. Of course, if you do a lot of work in cold environments, then an analog meter may be best for you.

One last note about purchasing a multimeter. Take a half-hour and read the booklet which comes with the meter. It is time well spent because you will know how to protect your meter investment, and that is a good thing. You will also know if protection fuses are inside, and you will know what kind of batteries the meter uses. Many people just rip the meter out of the packaging and start jamming leads into electrical circuits. Then after the smoke clears, they wonder why the meter doesn't work.


Voltage Measurements (volt-meter)

Measuring voltage is one of the easier things you can do with a multimeter. When you measure voltage, you are simply placing the meter across the voltage to be measured as shown above. First, ensure that the meter is in the voltage mode. If you try to measure voltage while in the current or resistance mode, damage to the meter will most certainly occur. Then select the correct voltage scale, which on trucks will usually be the 20 or 50 volts scale (the lowest scale which is greater than the maximum expected voltage of 14 volts). With auto-ranging digital meters, don't worry about voltage scales.

Now figure out which side of the device to be measured is the positive voltage side, and connect the positive meter lead. Connect the other meter lead (the negative lead) to the other side of the device to be measured. With a digital meter, the positive side of the device doesn't matter since the meter will auto-polarity adjust the readout accordingly.

Most truck electrical devices use the truck chassis as the negative lead. For those devices, simply measure the voltage by connecting the positive lead to the device, and the negative lead to the truck chassis ground.

Current Measurements (amp-meter)

Measuring current is another quick way to damage your meter. So pay attention while make current measurements. Fortunately, there are very few situations where current measurements are important in fixing truck problems.

When measuring current, the amp-meter must be placed in series with the device to be measured just like in the diagram above. Just like a device fuse, the amp-meter must be in-line with the power source. Since most truck devices use the truck chassis for the negative return path (chassis ground), the amp-meter must be placed into the supply (positive) side of the device.

Selecting the proper current scale is very important. You must have an idea of what current is expected, then you can select a current scale which wont damage your meter. Remember, all the current the measured device draws, will pass right through the amp-meter. If the measured device draws 20 amps and your amp-meter is set to a 10-amp scale, then the meter will probably be damaged. Always ensure that the selected current scale is greater than the current draw required by the device.

Once you have selected the proper current scale, you must be certain to observe polarity of the meter leads. The electron current flow must enter into the negative lead of the amp-meter, and exit the positive lead of the amp-meter. Do this backwards, and the analog meter will probably be damaged. Most all trucks connect the negative battery terminal to chassis ground. For those truck devices, the negative amp-meter leads connects to the device, and the positive amp-meter lead connects to the positive battery supply voltage.

When you have finished your measurement, remove the meter from the circuit and restore normal circuit connections. DO NOT leave the meter in current mode. Place the meter in its off mode or its voltage mode as soon as the meter is out of the circuit. If you later try to measure voltage while in current mode, damage to the meter will most certainly take place.

Resistance Measurements (ohm-meter)

Measuring resistance has one important requirement. The device being measured must have NO power supplied to it. Trying to measure the resistance of a powered device, will probably damage the ohm-meter portion of your multimeter. Disconnect the device from all power sources before you attempt to measure resistance of the device.

Next, select the resistance range that will give you approximate mid scale meter deflection on an analog meter. Notice that the meter scale is non-linear. This means that a 100-ohm meter scale, will NOT read 50 in the mid-scale position. Instead, the mid-scale position would read more like 4-ohms. At the very left end of the ohms scale you will see a symbol which looks like the number 8 laying on its side. This is called the epsilon symbol, and it represents infinite resistance, which we call an open circuit. The right side of the ohms scale represents zero resistance, and we call this a short. On a digital meter, simply select a range which gives you a displayed reading. Many digital meters which don't have auto-ranging, will display "OV" (over range) when the resistance exceeds the selected resistance scale. When this happens, just select a lower resistance scale. For auto-range meters, the range selection is done automatically.

With analog meters, you must now calibrate the resistance scale. To do this, just short the meter leads together, and then adjust the zero ohms adjust knob on the meter for full scale deflection of the meter. Adjust until the meter pointer is pointing to the zero ohms mark of the scale (full deflection to the right). Since your meter leads are shorted together, there is zero resistance between them. Now the ohm-meter has been compensated to read zero ohms while using the ohm-meter battery voltage inside your multimeter. As the ohmmeter internal battery gets used up over time from resistance measurements, the battery voltage will decrease and require a new zero adjust as described above, to compensate for the weaker battery voltage. When you can no longer adjust for zero ohms, it is time to replace that battery.

For most truck devices, lead polarity doesn't matter. The meter has its own internal battery which performs the resistance measurement. So polarity is not normally an issue. One truck device, the alternator, has devices called diodes (or rectifiers) inside which can affect resistance readings if the meter leads are not connected properly. Measuring diodes is covered in the next section of this topic.

DO NOT leave the meter in the ohm-meter mode. Place the meter in its off mode or its voltage mode as soon as the resistance measurement is completed. If you later try to measure voltage while in the ohm-meter mode, damage to the meter will most certainly take place.

Diode Check & Beep Indicators

Many of the newer digital meters include diode check and beeper modes. The beeper mode is easy to understand, so we will discuss that first.

When you are checking for continuity, this means that you expect a very low resistance to be measured. For example, if you are testing a fuse (out of the circuit, no voltage present) then you would expect about zero ohms. If you were checking the continuity of a wire, then you would also expect about zero ohms. The beeper mode places the meter in the ohm-meter mode, but instead of displaying the resistance value measured, it simply beeps when a low resistance is detected. Some meters display the resistance value and beep at the same time. In any case, the beep tone is a quick and dirty indication of continuity. Just touch the meter leads to the device to be tested, and the audible tone confirms continuity.

Diode checks is another whole ball of wax. If you really need to do this, then read on. If you don't need to perform diode checks, then skip to the next section.

A diode is a special device which allows current to flow in only one direction. Alternating current (AC) such as in your home, reverses direction 60 times per second. This alternating current has less power loss during transmission through those long distance power lines you see along the highways. Less loss to the power company means more profit for them, and smaller electric bills for you (yeah right)! Since the truck does not have to move electricity very far, the higher losses of direct current (DC) are negligable, and the DC devices are cheaper to make than AC devices.

Sometimes diodes are used for idiot lights in cars, and they may also be found in some newer truck circuits. When a meter specifies diode mode, this just means that the meter circuits will consider the requirements to make valid diode checks. A good diode will test with low resistance in one direction, and much higher resistance when the meter leads are reversed.

The alternator in your truck also generates AC because AC can generate more current at slower engine speeds. The AC generated by the alternator must be converted to direct current (DC) in order to charge your truck battery. Inside the alternator are groups of diodes which rectify (allow current to flow in one direction only) the alternator AC and convert it into DC. The DC is what your truck battery and electrical devices need.

The alternator is actually 3-AC generators in one. This is called 3-phase, and the diode clusters add all 3-phases together and rectify the current into a single DC output. The 3-phases is one of the reasons that an alternator can put out more current at lower engine speeds than a generator.

When one or more of these alternator diodes shorts or opens, then the current generating capacity of the alternator is reduced, and it can not keep the truck battery fully charged. Sometimes, shorted diodes inside the alternator may cause discharge of the truck's battery.

To test the alternator diodes, the alternator must first be disassembled. Then the manufacturer's test procedures must be followed to determine if the diodes test good or bad. Many repair manuals devote much attention to this subject and should be referenced when testing alternator diodes.

Battery & Fuse Replacement

All multimeters have batteries inside, and many of them have fuses inside. The battery powers the ohm-meter circuits of the multimeter, and for the digital meters, the battery also powers the meter circuits. For the analog meters, a weak battery will prevent zero ohms calibration while the meter leads are shorted together.

For digital meters, the digital display will not work when the battery is weak. Some digital meters have a separate battery for the resistance measurement circuits.

Fuses are inside some meters to protect the meter from stupid mistakes. When the mistake is executed, the protective fuse blows, and must be replaced before that mode can be used again. Some meters have two or more fuses inside. The more the meter costs, the more fuse protection it has.

Some stupid actions which blow these fuses are:

  • Current measurements exceeding the meter capacity
  • Voltage measurements exceeding the selected scale
  • Measuring voltage while in the resistance mode
  • Measuring voltage while in the current mode

This concludes our multimeter topic. Your multimeter should serve you for a very long time if you follow the procedures described above.

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