If your trailer features electric brakes, it is crucial that you understand how to test
them. In this article, you’ll learn how to use a multimeter to measure current drawn by its controller.
This test can easily be completed with just a multimeter and fully charged battery, and is very safe to do.
Resistance Test
Brake controllers in towing vehicles control the brakes of trailers by sending voltage
through brake magnet wires depending on factors like load capacity and driver
brake pressure settings. A sudden loss of ability to modulate level of application
could indicate problems with the controller.
Testing the current being sent to brake magnet wires with a multimeter is a simple
and straightforward process, provided your vehicle’s battery has been switched off
and that you wear safety glasses and insulated gloves. Connect one probe of the
multimeter to either blue wire coming from the connector plug, or black wire of the
brake magnet; and set your multimeter’s “Ohm” setting; check resistance reading; if
it falls outside manufacturer recommendations then replacement of either controller
or magnet wires may be necessary.
Voltage Test
Most electrical trailer brake problems that cannot be remedied through mechanical
adjustments or reseting are likely caused by low voltage or current. You can use a
multimeter to detect these problems.
Assemble your multimeter by connecting one lead to the “+” terminal on the trailer
battery or power source and placing another lead anywhere along the two wires
leading to your brake magnet (see illustration). Next set your multimeter to Ohms
mode and touch either its negative probe to one of these brake magnet wires while
touching its positive probe to another wire leading into it – if any reading falls
outside acceptable resistance range for that size magnet then its replacement must occur immediately.
Use a metal tool such as a wrench or screwdriver to touch the magnet and see if any
current is flowing from its handle – if this works then your magnet should be in good
working order and your issue lies elsewhere such as with its controller or wires.
Ohm’s Law Test
For this test, a multimeter with an ammeter setting (usually represented by the
letter “A”) will be required. Connect one lead of your multimeter to the positive
terminal on your trailer’s battery or power source while connecting another lead to
any negative connections such as parts of its frame or axle that do not contain
electrical components. With your emergency brake activated and its controller
supplying full battery voltage to its brake magnets, your multimeter should display
readings that match up with expected brake current based on wheel diameter.
If your multimeter fails to produce readings, your system could contain an internal
short or broken wire which should prompt replacement of the brake magnet. Also
make sure that safety glasses and insulated gloves are worn when working with
electrical connections in order to avoid potential shock or damage to tools.
Brake Magnet Test
An effective trailer brake system is essential to its overall safety, particularly if the
trailer features an emergency break-away switch and cable system, enabling full
battery voltage back to the brakes in case it becomes uncoupled from its tow vehicle
and becomes detached from it.
Use a multimeter to test this feature by connecting +12V power directly from a fully
charged battery to either the hot and ground contacts of a trailer plug, or two wires
of each brake assembly’s two-wire assembly. When functioning correctly, brakes
should generate a soft humming noise as they engage their mechanisms.
Set your multimeter to amp (current), as indicated by an “A” symbol on its display.
Touch one probe from your multimeter directly to a blue wire coming from the
trailer’s connector plug and one other probe directly onto a brake magnet wire; aim
for readings within your range, which depends on the size of your trailer’s brake magnets.