Testing electric trailer brakes is essential to ensuring its safe operation. A
multimeter can help you conduct this essential inspection of the trailer brakes.
Once your multimeter is on amp mode (typically represented as an “A”), connect
one lead of it to both your battery-positive terminal and brake magnet wires.
Resistance Test
Electric brakes are standard on many medium-duty trailers and run off of electrical
current provided by their controller. If this controller becomes defective or there are
wire issues between it and electromagnets in the trailer’s brakes, its effect will not
work correctly and so neither will its brakes.
Utilize a multimeter to test the current flow from your brake controller to each of the
brake magnet lead wires. The recommended current will depend on the size and
specifications of your magnets; refer to your controller’s specifications for more information.
To conduct the test, disconnect one of the brake magnet lead wires and connect it to
both probes of your meter – this will measure resistance between them, providing
insight into any breaks or shorts in wiring systems.
Voltage Test
A voltage test measures the amperage draw on your system using a multimeter with
its amp, or A setting, connected to both battery-positive terminal and one magnet
lead wires. If everything is working as intended, there should be a slight amperage
draw at each brake magnet.
Initial Step: Use the multimeter in Ohms Mode. Probe each magnet wire using its
black (negative) probe until you receive a reading that matches with expected
current range for your brake magnet size. This test should help identify any internal
shorts within wires or magnets themselves and assist in further investigation if there
are any. If no readings appear or have very low readings then chances are the
magnets have gone bad and need replacement immediately.
Amperage Test
Trailer brakes often stop working because they are not receiving enough power, to
determine this you need a multimeter that can measure amperage. Set it to ohms
mode and probe the wire leading to your brake magnet using its negative probe;
touch its positive probe with one magnet wire; if readings fall below specifications
for your size of brakes you must either replace its magnet or its wire.
As part of step three, disconnect one of the magnet lead wires and attach an
ammeter between them, reading its reading on a multimeter screen to compare with
your brake size specifications will allow you to determine whether the controller is
sending enough current through your system.
Magnet Test
If your trailer brakes aren’t functioning as intended, this could be caused by an issue
between the brake controller and magnet wires – you can check this with a multimeter.
To do this, first set your multimeter to ohms mode and place one probe of your
multimeter on both ends of the battery terminal and on your brake magnet wire. If
the reading falls outside of its specified range for your brake magnet size, chances
are good it is likely faulty and should be replaced immediately.
Check if the brake magnet features spots – if so, that indicates good condition and
should continue working as designed; if they have worn off completely then
replacement may be required.
Connector Test
Most electric brake malfunctions can be traced back to wiring issues, whether
caused by corrosion, burnt-out bulbs or a loose ground connection on the connector
plugs. A quick test will quickly identify where there are issues.
Simply connect +12V power to the trailer connector and a multimeter inline with
one of the brake magnet wires to verify if they meet specifications for your type of
trailer brake system and that all is working smoothly with their connections. If that
reading meets or exceeds those specified for that type of trailer brake system then
everything should be working as it should.
Before connecting your multimeter leads to either your vehicle or trailer batteries,
make sure that they have been turned off to reduce the risk of electric shock and
reduce injury risks. It is also advisable to wear safety glasses and insulated gloves as
additional protection measures.