How Do I…Find Ignition, Constant, and Ground Wires in Any Vehicle?

From the API Signal install team.

The basics chapter covers what a good install looks like. This chapter covers the tricks of the trade that make it faster, cleaner, and less likely to come back. Most of these were learned the hard way.

How do I find the ignition wire and constant 12 volt wire?

Use a meter, not a test light. Older test lights draw enough current to trip a body control module or set a fault code. On anything built in the last 20 years, a digital multimeter or a proper low-current LED probe is the only safe way to test.

Test ignition in all four key positions. OFF, ACCESSORY, RUN, and CRANK. A true ignition wire shows 12 volts in RUN and stays at 12 volts during CRANK. If it drops during crank, keep looking. Many wires that look like ignition are actually accessory.

Test under load when possible. A wire can read 12 volts on a meter and still collapse when the device pulls current. If a device reboots or drops offline when the engine cranks, the power source is weak. Move to a better one.

Ignition switch harness is the best source. For ignition and constant, the harness at the ignition switch or the main fuse panel is more reliable than a random wire in a door or kick panel.

Push-to-start vehicles. There is no key cylinder, so ignition has to be found at the fuse panel or through a fuse tap on a circuit that follows RUN. Test it the same way.

How do I use a fuse tap in a vehicle fuse panel?

• Before pulling a fuse, note which side of the fuse socket is hot with the key off. The tap goes on the load side (the side that is dead with the fuse removed).
• Fuse taps come in four sizes: ATO, Mini, Low-Profile Mini, and Micro2. Bring all four.
• Don't tap airbag, ABS, or engine control fuses. Pick circuits like radio, accessory power outlet, or interior lighting.
• Take a photo of the fuse panel before you start. It saves a lot of time if something needs to go back the way it was.

How do I make a wire connection that lasts?

• A ratcheting crimper makes a better crimp than pliers every time. It won't release until the crimp is complete.
• Strip only what you need. Exposed copper past the connector is where shorts start.
• Heat shrink with adhesive lining seals out moisture. Use it anywhere near a door, floor, or firewall.
• Tin the wire ends before soldering. The joint flows faster and cleaner.
• Stagger your splices. When you have three wires to splice into a harness, space them a few inches apart so the bundle stays thin and no two bare joints can touch.

How do I pull wire through a firewall or behind a dash?

• Tape a wire to a fish rod, a coat hanger, or a piece of stiff wire to pull it through a firewall grommet or behind a panel.
• Wrap the leading end of a wire bundle in tape to a point so it slides through loom and grommets instead of snagging.
• Pull a little extra wire at each end. It's easier to trim than to splice in more later.
• Route power and signal wires on opposite sides of the vehicle from each other when you can. It keeps noise out of camera and data lines.
• Leave enough slack at the module so the harness isn't holding the module's weight.

Where should I mount a GPS tracker in a vehicle?

• Mount the module to something solid. Zip-tie it to a harness or bracket, never let it hang by its own wires.
• Keep GPS units where they have a view of the sky. Under the dash near the windshield is usually fine. Deep inside a metal dash or under a metal seat pan is not.
• Keep the module away from the heater core and blower motor. Heat kills electronics.
• Keep antennas and modules away from the airbag deployment path.
• Make sure nothing can be reached by the driver's feet.

What should I do before leaving a fleet install?

• Label anything the customer or the next installer will need to find. A small tag on the harness saying what it is and when it was installed saves a lot of time.
• Take photos of the finished install: module location, ground point, power source, and the fuse panel. We keep these with the job.
• Put every panel back with every screw. A missing screw is the first thing a fleet manager notices.
• Wipe down the interior where you worked. Grease from a hand on a headliner is a callback.
• Confirm the device is reporting on the customer's platform before you leave. Not "the light is on." Reporting.

What are the most common causes of install callbacks?

1. Tap connectors instead of solder or crimp.
2. Ground on a painted surface.
3. Ignition wire that drops during crank.
4. Module hanging by its harness.
5. Wire routed near a pedal or the steering column.
6. Fuse tap on the wrong side of the fuse.
7. Panels that rattle.

Avoid those seven and most callbacks disappear.

This information is provided as a guide. Verify every circuit with a digital multimeter before connecting to it. Proper installation is the responsibility of the installer.