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Multimeters

I'm looking for some responses regarding multimeter basics for the site tag-wiki to point OP's at when we want them to use one.

The question is;

  • What are the basic functions of a multimeter?

  • How would I troubleshoot a headlamp failure with a multimeter?

  • How can I test a fuse with a multimeter?

  • How can I test a battery with a multimeter?

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Basic Functionality

Image In order to illustrate a typical Multimeter

enter image description here

The Multimeter is specifically a multifunction tool. By definition it combines several, well defined instruments and multiplexes the controls for simplicity.

Inputs

Majority of Multimeters will have 3 input terminals. Typically you will only use COM and V/mA/Ohm input (often Black and Red respectively, but not in the sample image). The third input is used for measuring particularly large currents. Some multimeters have 4 inputs for more precise measurements using Kelvin Sensing . In handheld devices this is typically seen on LCR Meters.

Voltmeter Mode

http://sub.allaboutcircuits.com/images/05078.png

In Voltmeter mode you must always perform measurements in parallel with the circuit being measured, the circuit is powered

The basic functionality of the voltmeter is to measure voltages, potential differences across two points in the circuit. Virtualy all DMM's will have voltmeter functionality.

On the representative device shown in the image, the voltmeter modes are selected between 9:00 and 1:00 on the clock face. The flat bar represents DC mode and the Wiggly Line represents AC current. Either mode can safely be used interchangeably (affecting only reading accuracy) provided the voltage ratings are respected. Some multimeters will offer autoranging feature that does not require you to select the range explicitly on the dial.

Some ways you can use the DMM in voltmeter mode for automotive purposes

  • Establishing the presence of a control voltage on a signal line e.g. relay control line, fuel injectors

  • Establishing the presence of a supply voltage on various circuits

  • Measuring the voltage output of the regulator/alternator or battery

Ammeter Mode

In ammeter mode you must always perform measurements in series with the circuit, the circuit is powered

http://www.cdn.sciencebuddies.org/Files/4808/6/fig7_multimeter-series-measure-current.jpg

The basic functionality of the ammeter is to measure electric current. In ammeter mode it will damage the DMM and possibly blow a fuse on the car if you connect your DMM in parallel to a live circuit, that is - in a way that it would measure a voltage were it in voltmeter mode.

On the representative device shown in the image, the ammeter modes are selected between 1:00 and 4:00 on the clock face. Note that the highest range setting on this device (and many others) requires you to use the alternate high current input.

Using an ammeter, when diagnosing a live circuit that should be on (e.g. lamp with switch turned on), the reading you get will inform you of the kind of fault on the circuit, a high current reading means a short circuit, a zero current means an open circuit.

Some ways you can use the DMM in ammeter mode for automotive purposes

  • Testing for leakage - battery current when the car is turned off

Ohmmeter Mode

In ohmmeter mode you should perform measurements in parallel with the circuit, the circuit must be unpowered

http://image.slidesharecdn.com/howtouseadigitalmultimeter-100726083407-phpapp01/95/how-to-use-a-digital-multimeter-14-728.jpg

The ohmmeter measures the resistance between two terminals. The resistance determines the amount of current a device will draw at a fixed supply voltage. Typically "analogue" components on a car like lights, relays, switches, fuses, etc. will have a relatively low resistance, several ohms to 100 or so. Resistance for other components (e.g. sensors, injectors) can be compared against specification to detect failure.

On the representative device shown in the image, the ohmmeter modes are selected between 6:00 and 9:00 on the clock face. Note that on this device, the 2,000 (2k) Ohm setting is multiplexed for continuity readings (more on that later), secondary modes is often selected with the ubiquitous "conspicuous unmarked button". There is no harm (except the risk of getting no reading) in selecting the wrong range on the device

Some ways you can use the DMM in ohmmeter mode for automotive purposes

  • Verify the resistance of electrical components like injectors, lights, relays

  • Identify short circuits and open circuits, sometimes you have very low ohm "not-so-short circuits" that will not trip the continuity meter beep

  • Verify the continuity of a fuse

Additional Modes

Capacitance

Rarely seen on cheap DMM's and never on automotive ones, used to verify the capacitance. Lots of caveats for use and not the most useful feature for automotive repair and diagnostics

Continuity Tester

Somewhat redundant to the ohmmeter mode in terms of diagnostic capability. The primary benefit is the loud BEEEP, that it emits when there is continuity, allowing you to focus your eyes not on the dmm

Diode Testing

Often multiplexed with the continuity mode, the reading it gives you while measuring continuity is the voltage required to make the diode conduct.

Duty Cycle

Useful automotive feature, allows diagnosis of electrical engine components that use Pulse Width Modulation - e.g. injectors

RPM Meter

Useful automotive feature that detects engine RPM from the noise induced on a supply line.

Transistor Characterization

The funny "hfe" mode and three pin connector is for testing transistor beta . Somewhat of a gimmick and not useful for automotive purposes

Temperature Reading

enter image description here

If your DMM has a funny yelow connector with the K symbol, this is for Thermocouples. The DMM should have a "C/F" mode for measuring the temperature. The probe type is important so match the letter type.

  • 1
    That's a fantastic answer. – DucatiKiller Feb 10 '16 at 9:12
  • Picture not correct: combines AC voltage with DC current, VOhmmA label should say <200mA, 10ADC text box should say 'up to 10A' not 'greater than 10A' – philcolbourn Feb 11 '16 at 4:49
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Basic Functions

A multimeter is an electrical device for measuring electrical Voltages (Volts, V), Currents (Amps, I) and Resistances (Ohms, Ω). Most are also capable of measuring continuity, and many have other functions such as testing diodes and transistors/ It is used by setting it to an appropriate range, and using the attached probes to check the component under test - for example, to test a battery voltage, set it to "20V DC" (I.e. measuring a DC voltage in the range -20 to +20 Volts), and touch the black wire to the negative terminal of the battery and the red wire to the positive terminal - a healthy battery with no load should read around 13.5V

Electrical Troubleshooting

See This question and answers that I posted last year...

3

Looks more like 4 questions lol. But here goes. Basic Functions : Multiple Ranges for measuring Volts, Ohms (resistance), and Milliamps. Headlamd failure: Negative or black lead to a good ground (negative battery post,clean spot on the frame etc.) Red or positive lead to the terminals on the headlamp plug (headlight switch on). No voltage? Check for broken wires? Check the switch! Got voltage?? use the Ohms setting, (POWER OFF!!) to check between the terminals. Testing fuse: in place, ground the black lead, touch red lead to first one end of fuse, then the other end.Voltage at both ends, good fuse! Testing loose fuses: Ohms setting, leads on opposite ends of fuse. Auto batteries: best tested with load type battery tester, or hygrometer!

  • Poor man's load test for the battery. Hook red leads to +(positive) and black to -(negative). Disable ignition, will vary on model, check voltage of the battery in volts dc. Attempt to start the car, read the meter while doing this, it should not drop below 9.6. Release the key and leave on, it should read within 1 volt of the original check. – Tobin S Feb 9 '16 at 6:28

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