Your MeterThe tool that tells the truth, and the wiring you never probe

Built by Installers, for Installers

A digital multimeter is the cheapest way to look smart. It is also the way you avoid a callback. This lesson covers the three readings every install needs and the one place on every vehicle where you leave the meter in the bag.

Three meter tests, every install

1. Constant: key out, black on ground, red on the wire. Battery voltage means a candidate.
2. Ignition: same leads, watch OFF, ACC, RUN and START. 0, 0, battery, battery.
3. Ground: device powered, one lead on the ground point, one on battery negative. Small number is good.

That is it. The whole course so far fits in three tests. If you do these on every vehicle, you catch the faults before the customer does.

Meter settings that trip people up

• Set the dial to DC volts, the straight line with dashes symbol, not AC.
• Red lead in the volts jack, black lead in common. Leads in the amps jack are a classic way to blow a meter fuse.
• On a 24 volt truck the logic is identical and the numbers double. Read the key-out voltage first so you know what normal looks like on that vehicle.
• Back-probe a connector when you can instead of piercing the wire insulation, so you do not leave a hole that corrodes.

Why a digital meter beats a test light

A test light only tells you there is something there. It cannot tell you 12.6 volts from 8, and it can draw enough current to upset a control module on circuits that were meant to carry almost none. A digital meter reads the number and draws very little.

The one place the meter never goes

Airbag wiring. It is called the SRS, short for supplemental restraint system, and its wires are wrapped in a bright yellow outer sleeve. The trouble is simple: SRS circuits are built to fire a squib that deploys a bag, and a wrong probe or a wrong current in the wrong place is how somebody gets hurt. Logistimatics' hardwire guide tells installers to leave yellow connectors alone. Tesla's service manual SRS precautions go further, saying not to use a multimeter or other general-purpose test equipment on SRS components.

The rule is easy and has no exceptions: if the sleeve is yellow, you do not probe it, tap it, tape to it or pull it. A good source of power is never under a yellow sleeve. Find it at the fuse panel or a power distribution point.

Your turn

Quick answers

What meter setting do I use for car wiring?

DC volts on a digital multimeter. For a voltage drop test the circuit has to be powered, so use DC volts rather than the ohms setting.

How can I tell airbag wiring?

SRS (airbag) wiring uses a yellow outer sleeve and often yellow connectors. Do not probe, tap or cut it.

Is a test light safe to use on a tracker install?

Use a digital meter instead. A test light draws more current and can upset sensitive circuits. Never use one on SRS wiring.

Built by Installers, for Installers.
Constant. Ignition. Ground. API Signal.