A multimeter is an electrical troubleshooting tool that typically measures AC and DC volts, amps and ohms.

Basic Functionality

Image in order to illustrate a typical Multimeter

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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

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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. Virtually all DMMs 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

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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

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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 DMMs 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

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If your DMM has a funny yellow 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.

Tools

  • Digital Multimiter

    • Voltage, Resistance, continuity settings

    • Automotive specific DMM's are typically the same device but may have a few extra features (Duty Cycle, RPM)

    • CAUTION: Certain modes on the multimeter can cause damage to the device or your car if improperly hooked up. Never connect the DMM to a circuit in parallel when it is on the ampere setting (take caution when changing setting with probes attached), never use continuity testing on a live circuit unless the device specifically is ok with it. Do not measure Resistance on a live circuit. some tests will require you to have battery on and others will require you to disconnect the battery

  • Test Leads and Test Jigs

    • Minimum set includes basic contact probes (comes with DMM) and Aligator Clips. First thing to get is 20 ft banana plug extension to extend your probes

    • A set of automotive connectors with test leads for easy probing would be the next thing to acquire.

Additional Tools that you May Want:

  • OBDII Reader for pulling codes and getting gross sensor data

  • CAN sniffer for low level communications testing

  • Oscilloscope for thoroughly verifying electrical signals

  • Soldering iron to make electrical jigs

For all testing, make sure to find, print, and understand the wiring diagram for your car, it will make your life much easier if you can use the diagram to determine the expected connections, voltages, and signals at any node where to find them

Testing

Automotive electrics for Conventional (non hybrid combustion) cars can be broken down roughly into three domains

  1. Power Components: Charger, Alternator, Starter, "Power Circuit"
  2. Analog Components: lights, relays, fuses, low current power
  3. Computer Components: ECU's and advanced sensors

The difference between Category 1 and 2 is that 1 is before the fuse box (directly to battery with fewer protective elements) and 2 is generally fused circuits. Greater precautions need to be taken to avoid crowbarring the battery, whereas a fuse will protect you from mistakes. Category 3 will require some additional or specialty tools. However, elements of category 3 will have some overlap with 2, you can verify some very basic functionality without busting out a sniffer (test power supply to components, shorts and breaks)

Power/Analog Components

Here everything is about long and convoluted harnesses, fuses, relays.

With the wiring diagram in hand the First stop is to identify the relays and fuses that are controlling or protecting the circuit believed to be at fault. Verify Continuity between connected nodes (e.g. if the fuse for the headlights connects directly to relay, verify - with the battery disconnected - that there is continuity between the fuse panel and relay panel). Verify/replace fuses and relays. Continuity check everything you can to eliminate harness/wiring damage.

After a basic continuity check we can move on to signal verification tests. With the battery connected you can test voltages at key points on the circuit. Relays that are supposed to be on will have 9-12V on the control pin, verify power at the fuse panel by removing fuses and checking the voltages on related pins. Where you expect a voltage make sure you are reading a voltage. You can compare the voltages on every pin of a relay socket with relay installed and relay removed (this is where connector jigs come in handy) , this will help you further narrow the problem search. You can use the wiring diagram to unambiguously determine which circuits should be on/powered with given switch/key positions. Any signal pins that go to the ECU (related to the faulty circuit) should be noted and tested later.

Functional tests can be performed on most components. Using a 12V supply that is not the car battery you can check relays, pneumatics/valves, lights, fans. etc. by connecting the supply across the relevant pins. Lights will turn on, pneumatics will tick, relays will tock, fans will spin, and pumps will whir.

Computer Components

Verifying ECU and sensor functionality can range from easy to complicated. The most basic check you can do is to verify that the power supply lines to these devices have the required voltage (can typically be done right at the connector) and verifying the relevant fuses and relays supplying power to that circuit.

If the supply is present then you can dive down the rabbit hole of troubleshooting sensors and computer electronics. The first step is to verify that the device is functioning, this will require looking up the part number determining the signal the device supplies to the ECU. Analog signals can be read using a DMM, digital signals will require a sniffer or oscilloscope. You can also test with a known-good device.

If the sensor appears to be functioning but the relevant system is malfunctioning the best bet is to take a CAN sniffer (the majority of intelligent sensing elements send their data to the ECU with a shared bus called CAN) and capture the communications data. It may be difficult to interpret the communications, but in general I would suspect the following . no signal - Broken connection, dead/damaged device or ecu. gibberish data (Look at the data sheet for the device to see how it formats data)- Damaged device, digital noise/damaged wiring , corrupt ECU.

More on using a Multimeter

Sparkfun have a guide on using meters, with loads of pictures:

Guide!

Aimed at hobbyist electrical stuff, but the principles are the same here...

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