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m12 Trailer Light and Electric Brake Tester

Trailers are a big part of the DIY and “flipper” life. It’s time to start gearing up to rebuild my own trailer and I have had some people ask me to do some work on their trailers as well.

I’m going to build my own version of these testers.

Also check out my Custom Harnesses.

One of my clients asked me to fix the running lights on her trailer and I don’t have a tester, so I’ll build one.

There are three types of trailers wiring systems:

  • Utility
  • RV
  • Heavy truck

I need some diagrams of standards. I feel like I should have some laminated diagrams stored with my tester.

  1. M12 Battery Adapter – To power the device using a Milwaukee M12 battery.
  2. Fuse Board – A board to hold and organize the fuses that will protect the circuits.
  3. 7-way RV Plug – The connector commonly used for larger trailers which allows the integration of the lights and brakes.
  4. 4-flat Trailer Plug – The connector for simple trailer systems that only need light functions.
  5. Main Power Switch with Fuse – An on/off switch that controls the power to the device with an integrated fuse for circuit protection.
  6. Left and Right Turn Switches – Separate switches to test the left and right turn signals on a trailer.
  7. Flasher Relay – To replicate the flashing signal for the turn indicators.
  8. Running Lights Switch – A switch to test the constant “running” lights of a trailer that are on when the headlights are on.
  9. Electric Brake Switch – A switch to engage and test the electric brakes of the trailer.

What size wire do I need?

Here’s a simplified guideline based on the current that each wire will carry:

  • 20-25 Amps: 12 AWG
  • 15-20 Amps: 14 AWG
  • 10-15 Amps: 16 AWG
  • 5-10 Amps: 18 AWG
  • Under 5 Amps: 20 AWG

The amperage draw of a 10 to 30 foot trailer can vary significantly based on several factors, including the type and number of electrical devices it contains, whether it has electric brakes, the lighting system, and any additional accessories. Here are some typical amperage draws for common trailer components:

  1. Lighting:
    • Tail lights: 1-3 amps per set
    • Brake lights: 1.5-3 amps per set
    • Turn signals: 1.5-3 amps per set
    • Interior lights: It varies widely, LED lights draw much less than incandescent bulbs, but the total depends on the number and type of fixtures.
  2. Electric Brakes: Typically, each electric brake magnet can draw between 3 to 5 amps. So for two axles (4 brakes), the draw could be 12 to 20 amps.
  3. Other Accessories:
    • Electric winches, pumps, or motors: The draw can be substantial, sometimes over 20 amps depending on the device.
    • Refrigerators, air conditioners, and heaters: These are usually found in larger or more specialized trailers and can draw significant power, often well over 10 amps.
    • Battery chargers or inverters: The current draw depends on the charging rate and the power of the inverter.

For basic trailers used for hauling (with only lights and brakes), you could expect a total amperage draw in the range of 10 to 20 amps. However, for RVs or trailers with additional accessories, the draw could easily exceed 30 amps.

When sizing your wires and components, you should plan for the maximum expected load. Often, a 7-way connector wiring setup includes a 10 AWG or 12 AWG wire for the main power and ground due to the potential for high loads, especially if the trailer has electric brakes and multiple lighting systems. For turn signals, brake lights, and running lights, a 14-16 AWG wire is commonly used.

It is essential to calculate the expected amperage draw for your specific trailer and to consult the trailer’s manufacturer specifications or a qualified electrician to ensure that your wiring is adequate for your needs. Always err on the side of safety and use wire gauges that are rated for higher than your calculated maximum amperage to account for any unforeseen power demands and to reduce the risk of overheating or fires.

  • White 10 AWG Ground
  • Black/ Red 10 AWG Auxiliary Power
  • Purple 14 AWG Reverse Light
  • Yellow 14 AWG Left Turn/ Stop
  • Brown 14 AWG Running/ Tail Lights
  • Green 14 AWG Right Turn/ Stop
  • Blue 12 AWG Electric Brakes

Below is a link to a Facebook marketplace listing with a tester I like.

I’m trying to learn Journeyman level electrical in low voltage for communications and 12v and other mid range for vehicles and machines as well as 110v and 220v type systems for residential and light production.