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Build a Trailer Wiring (Test) Breakout Box.

There are many types and styles of trailer wiring test boxes on the market. Unfortunately, they all either test the vehicle or trailer, but I have not seen anything that allows testing of the vehicle and trailer together in an operational environment. That realization resulted in my design and construction of this breakout box.




You should realize there are two wiring standards that are typically used for US-built trailers. The more common AWG (American Wire Gauge), and the SAE (Society of Automotive Engineers).

Between the two, SAE wiring is about 11% smaller. For example, 12 AWG stranded wiring has a CM (Circular Mil) measurement of 6,500Mil, while 12 SAE is 5,833Mil.

This means AWG wire will have a slightly higher current carrying capability and a slightly less DC voltage drop per foot.


A breakout box is typically a box wherein the input is passed to the output, with a set of test points available for testing the individual functions (hence the term break out). This breakout simply does that. It provides the ability to test each wire in the 7-Pin wiring harness as it is under operation. Further, the brake line current is monitored.

This provides diagnostic information so you can isolate wiring problems to either the tow vehicle or trailer. In fact, I was having brake problems with my RV, so I built this box so I could not only isolate the problem, but to prove to the Factory and Dealer that there was a problem with the RV.


Breakout Prototype

Breakout Final Project


I have made the breakout box into a project so that if anyone wants to build one, they can do so. This is a fairly simple project electrically speaking, and if you have the wherewithall and a few tools, you can build one.

There are two versions of the breakout box. The prototype - which simply measures the voltage and current of the brake line only will cost just under $100. The fully upgraded breakout box adds the breakout function of all of the wires along with the brake monitoring, and costs anywhere from $120 to $180 depending on how fancy you want the front panel to be.

As I have been doing with recent projects - espeically those that are a bit involved - is a PDF project sheet you can download by clicking on it on the left.

This file includes construction information, electrical schematic, a bill-of-materials, drill templates, and other information. You will want to follow the schematic when constructing the project.



As well, I am providing a template for the front panel should you wish to make one. You can download the template from my Panel Template Page. This webpage includes details on how to order the panel.



When using the breakout box, be mindful there is an insertion loss, due to the voltage drop. This is not severe, but for accurate measurements, you need to realize this fact. The insertion loss is dependant on the wiring harness you use, as well as due to the ammeter shunt.

The insertion loss is dependant on how much brake current is measured, and can be minimized by ensuring the 7-pin wiring harness you purchase uses 12AWG for the brake line and 10AWG for the ground line. Also, it assumes you use no more than 6ft of harness.

For example, if you have a trailer with 4 brakes, and each brake uses 3Amps at 12V, then when you look at the 12Amp line across the bottom, you can see that the insertion loss will be about 0.22VDC.

That means that any voltage measurements you take "downstream" of the breakout box - either by the breakout box's voltmeter or when measuring the voltage at the brake, you should add about 0.22VDC to the reading to obtain the true voltage. This compensates for the voltage drop caused by the breakout box itself.


Typical breakout box use.


As shown here, the voltage at the breakout box is 12.4VDC, and the total brake current is 12A. When looking at the insertion loss chart, we see that 12A is equivalent to a 0.22VDC insertion loss. So we add that loss to the 12.4VDC measured at the breakout box, which gives us a theoretical voltage at the vehicle's connector as 12.62VDC.

We can also calculate the insertion loss at the brakes, by adding the actual voltage reading of 11.4VDC to the same 0.22VDC insertion loss. This can be confirmed by testing the brakes again after removing the breakout box.

Regardless, the voltage drop is less than 10%, which should allow adequate braking.

As noted, you must connect the tow vehicle to the RV via the breakout box, and ideally with the engine running (typically the alternator voltage will produce more voltage than just the battery). Then with an assistant either depressing the brake (or manually actuating the brake controller), you can take a voltage and current reading of the brake line at the breakout box, as well as at each brake with a multimeter.

For obvous reasons, you should not allow the brakes to remain engaged for more than 20 seconds or so - as you would in typical driving.


Notes: The color code for RVs is somewhat different than for other types of trailers. As I constructed the breakout box specifically for RVs, I used the RV color code as follows:


RV Wiring Color Code.


For some reason, and perhaps just to be different, the RV color code standard does NOT follow the SAE standard color-code. At least though the pin assignments are the same. This different color code is important to know as often a manufacturer - in sparing no expense to be cheap - will use a 7 pin RV wiring harness, but then use a flat cable (intended for 4-pin wiring harnesses) to run to the tail-lights. In this example, you might very well find the Left Turn light wire exiting the 7 Pin wiring harness (RED WIRE) and be connected to the YELLOW wire for the 4-pin wiring cable.



In the graphic below, do not be surprised if you run across something like this. You must be mindful of such possibilities.



Analysis. Brakes:

Should you find either find low brake current at the breakout box, or low brake voltage at the brakes (when comparing to the breakout box voltage) check for:

  • Faulty wiring or connectors.
  • Too small gauge wire used to connect the brakes.
  • Faulty brakes.
  • Defective break-away switch.
  • Bad brake magnets.
  • Corrosion.

If you find low brake voltage at the breakout box, check the tow vehicle for:

  • Faulty wiring or connectors.
  • Too small gauge wire used to run the brake lines to the connector.
  • Faulty brake controller.
  • Corrosion.

Of course, these are only a few of the possible causes for low current or voltage at the brakes. Use accepted toubleshooting theory to determine the cause(s).

Turn signal or clearance lights. Similarly to the brakes, look for defective wiring, connections, corrosion, as well as use of insufficient wire gauge when the unit was built.

Simply put - DC wiring (as discovered by Tesla) exhibits a phenomenon that results in significant voltage drop, especially as the current is increased or the wire is lengthened from the source to the load. This occurs even with wiring as short as 1~2ft., and with the typical trailer brake wiring that can be 30ft or longer, a 20% to 40% loss in voltage could be seen.

My personal preference is no more than a 10% voltage drop from the vehicle to the brakes - or lack of "full power" braking - whichever occurs first.


Using the Breakout Box Video.



Project Construction Video Part I - Building the Prototype.



Project Construction Video Part II - Building the Final Project.








Whenever possible, purchase electronc components from electronics suppliers such as:

They are usually less expensive. Still, Amazon as well as eBay sometimes have attractive pricing, especially with Generic items - however, you don't always know what you are getting.



Panel Template


Last reviewed and/or updated June 15, 2017