multimeter for cars

How To Use an Automotive Multimeter

What is a multimeter and how is it used?

Multimeters are very common tools in the home and in businesses, particularly automotive organizations.

They are used very frequently for a wide range of tasks when checking your car for problems.

Another name for a multimeter is a volt/ohm meter (VOM). This electronic device is used for measuring. It combines a number of different functions in one easy-to-use handheld piece of kit.

Multimeters measure

  • Amps
  • Volts
  • Resistance

multimeter readings

Almost all models come with a circular dial. This allows you to choose the type of measurement you require. Current, voltage and resistance are the most regular options. Some of the more advanced variants will stretch to measuring continuity and can also be used for testing diodes.

Although there a number of types of multimeter, we can split them in two and think of manual multimeters and auto ranging multimeters. The difference is much like that between manual and automatic cars: the manual requires rather more input from the user.

With the manual version, you choose the range of input. The auto ranging type come with some selections on its dial. Depending on the information received through the equipment’s probes, this machine will guess on your behalf.

4 simple ways to use your multimeter with a car at home

We will look here at some of the most basic ways in which you can use your multimeter for car-related problems.

Remember, opt for a digital model and look out for one which comes with plenty of attachments. They will be useful when working on your car.

multimeter for cars1) Testing a car battery

Sometimes you can guess that your battery is starting to run low on power. With a multimeter you can easily check this suspicion.

Make sure you wear safety glasses and also a sturdy pair of gloves. Car batteries have chemicals which can be extremely dangerous if you are not adequately protected.

Flip open the hood and locate your battery. Refer to the manual and disconnect the cables from the battery.

Use a setting of 20V DC for an accurate reading.

Hook up the multimeter’s red probe with the battery’s positive terminal. Connect the black probe to the negative terminal.

Wait a few seconds and read the meter: it’s as simple as that!

If you have a 12V battery then you’ll need a minimum of 9.6V in order for it to receive a charge. This may not even be enough and you may need a new battery.

Note: A 12V battery which is fully charged and is not connected to the car should show test results of between 12.4V and 12.7V.

2) Testing a car alternator

If all is well with the battery then testing the alternator is a smart move.

Wear gloves and glasses again and keep yourself well clear of moving parts.

Set the multimeter to 20V DC.

Ensure that the battery is connected but the engine switched off.

As when testing the battery, connect red to positive and black to negative.

With the car off the reading should be 12.5 – 12.8V if there’s no issue with the battery. Start up the engine. You should now see the multimeter reading between 13.8V and 14.8V if you the alternator is functioning perfectly.

As a final test, if you flick the heater blower to high and turn on the headlights, you’ll notice a slight drop in voltage before the reading returns to the above range. (This is assuming that the alternator is working OK.)

3) Testing the wires on your speakers

Imagine this situation…

You have the dash open, the stereo removed and also the door panels. The speakers are just about to be replaced but you realize that you are not sure which is positive and which is negative.

Get out your multimeter!

Put your stereo on with the volume low.

Set the multimeter to 20V DC. Place the negative probe on to one of the chassis grounds. Get the speaker and put the positive probe on one of the terminals.

The one with voltage is positive. Job done!

4) Checking the ohms on your subwoofer

 Many people today have car entertainment systems featuring powerful subwoofers.

As these subs are bridged and ready to go, you must be careful. When you want to connect the wires from the sub to amp, it’s crucial that they are correctly wired.

Hook up both ends of the multimeter to the amp’s speaker terminals.

Set the multimeter to 200 ohms (Ω) and you can check that the ohm level of the sub tallies with the rating of the amplifier.

Using a multimeter for car diagnostics

car multimeter

Think of automotive electrical systems as much like a support framework. The diagnosis of electrical problems is helped considerably by a multimeter. It’s absolutely crucial, in fact.

The digital style is much preferred. It works to much lower tolerances than an analogue version. Digital multimeters also have high impedance circuits.

Here are some ways in which a multimeter is used to check up on things…

Airflow meter

  • This is located in the outlet of the air cleaner top cover. Disconnect the wiring and remove it
  • Check the resistance of the airflow meter using your multimeter set to ohms against the specifications for your vehicle

Coolant temperature sensor

  • You’ll find this in the cylinder head
  • As the sensor temperature increases, the resistance values will decrease. The same is true in reverse
  • With the ignition switch off, disconnect the wiring plug from the intake air temperature sensor
  • Check the resistance between the terminals. With a cool engine it should be between 2.0 and 3.0 kohms
  • Warm up the engine and test again. The resistance should now be 400 to 700 ohms
  • Renew the sensor if these results are not correct

Fuel temperature sensor

  • This sensor is another thermistor like that used for a coolant temperature sensor. This just means that the value of resistance changes with the temperature
  • This sensor can be found in the fuel pump body
  • Turn off the car and disconnect the wiring
  • This value has quite a wide range, anywhere between 0.5 and 3.0 ohms is absolutely fine. If you find values outside these parameters you’ll need to replace the fuel pump itself

Camshaft position sensor

  • This is on a support bracket at the right-hand rear of the cylinder head
  • Disconnect the wiring and test for resistance consulting the specs in your manual. These values vary so pay close attention

So

These are just a few of the uses for an automotive multimeter. You can also use one to test fuel pressure and turbo pressure sensors, those sensors found in the accelerator pedal or crankshaft, the fuel pressure limiter, electronic driving units and a wealth of other situations.

Conclusion

With a multimeter, you can very quickly and easily diagnose a wide range of automotive problems.

No commercial garage could do without a multimeter and it also pays to have one stored at home to assess simple problems if things are not working as they should.

Have a look here for some reviews of the best digital multimeters – remember that this type is best for using with cars.

This simple-looking piece of kit is highly versatile. The more you learn about it and the more you use it, the more different ways you will find to put it into action.

About the Author Eleanor W. Herring

Leave a Comment:

Popular posts