Five Multimeter Measurements Every EV DIYer Should Master

Why this happens

A multimeter answers the basic questions in any fault: is there voltage, is the path complete, and is a part within its expected value. Most mistakes come from using the wrong jack or range, such as leaving the lead in the current jack and then measuring voltage, which shorts the source through the meter. Practising on harmless circuits builds the habits that prevent that.

What you need

Step by step

1. 1

   Learn the jacks before the dial

   Look at the meter's sockets. COM is the common (black) lead for every measurement. The V/Ω socket is for voltage, resistance, continuity and diode tests. The mA and 10 A sockets are only for current. Make it a habit to check where the red lead is before every measurement, and to return it to V/Ω as soon as you finish a current reading.

   Tip: Many meter fuses blow because the red lead was left in the current jack.

2. 2

   Measure DC voltage

   Set the dial to DC volts and put the red lead in V/Ω. Measure a 9 V battery or a bench supply set to a low voltage: red to positive, black to negative. Note the reading and its sign; a minus sign simply means the leads are reversed. On a vehicle, you would use this to check accessory supply voltages and the external charge port voltage only.

3. 3

   Check continuity with power off

   Switch to the continuity setting, touch the probes together and confirm the beep. Now test a wire and a fuse: a beep means an unbroken path, silence means open. Continuity and resistance must only be measured on circuits with no power, because the meter supplies its own small test current and outside voltage can give false readings or damage it.

4. 4

   Measure resistance

   Set the dial to resistance and measure a few resistors from your kit, comparing the reading to the colour code. Touch the probes together first to see the lead resistance, which you subtract for small values. Do not hold both probe tips with your fingers, because your body resistance adds a parallel path and lowers the reading on high values.

5. 5

   Use the diode test

   Select the diode symbol. Put red on the anode and black on the cathode (the end with the band) of a diode: you should see a forward voltage, often a few hundred millivolts for a silicon diode. Reverse the probes and you should see an open reading. Try an LED as well; many will glow faintly. This test helps you check diodes and LED lamps off the vehicle.

6. 6

   Measure low-voltage DC current in series

   Current is measured by breaking the circuit and putting the meter in series. Build a bench circuit: a low-voltage supply, a resistor and an LED. Turn the supply off, move the red lead to the mA or 10 A jack as appropriate, set the dial to DC current, and connect the meter in the break. Power on, read, power off, and move the red lead back.

   Tip: If you do not know the size of the current, start in the 10 A jack.

7. 7

   Practise the safe sequence

   Write a short checklist and use it every time: power state checked, lead in the right jack, dial on the right function, probes on, read, probes off, lead back to V/Ω. Repeat each measurement until it is automatic. Only then use the meter on your vehicle's accessory circuits with the key off and the battery disconnected for continuity and resistance work.

How to tell it worked

You can take each of the five measurements on a bench without looking up which jack or setting to use, and your readings on known resistors, diodes and supplies match their expected values.

Questions