Your trailer’s electric brake magnets are connected to two feed wires coming from
its connector that serve both powering and grounding purposes.
Setting your multimeter to the ohm setting and probing both wires that lead to brake magnets should yield an acceptable resistance reading on your multimeter.
Voltage
Electric current from a towing vehicle passes through brake magnets and activates
the trailer’s brakes, with size being the key factor when activating them.
Vanagan standard electric brake magnets require 3.2 amps @ 12v per magnet for
most caravan electric brake systems; for more proportional braking control some
even utilize two magnets per axle.
Proper wiring of brakes is crucial to ensure optimal functioning. Failure to properly
wire them could result in issues, including incorrect connections, broken wires or
blown fuses that lead to incorrect operation of the system.
To check current flow through a brake magnet, set your multimeter to amperage (A)
mode and touch one probe of your multimeter onto one of the wires leading to them. If your reading remains high after disconnecting all magnets from their magnetized ends, there could be an open circuit somewhere within your wiring system causing short circuiting issues.
Amperage
When pressing a trailer brake, electric current from its controller reaches the
magnet and causes it to move closer toward the drum, prompting an inner actuating
arm to rotate and force its way inside to push brake shoes against its inner wall and
stop wheel hubs from turning.
If your electrical system is drawing too many amps, this can damage both the brake
coil and shoes. To assess amperage usage with a multimeter, locate one of the wires
leading to your brake magnet and place its black (negative) probe against one wire
while placing its red (positive) probe onto another wire.
Assuming all brake magnets are correctly wired and functioning correctly, 7-inch
brake magnets should register between 3 and 3.2 amps and 10 and 12-inch brake
magnets should register between 3.3 to 3.4 amps when engaged; their amperage
will increase when engaged; however if its level remains high even after all brakes
have been disconnected then this indicates wiring problems; you can isolate each
magnet to determine its exact cause and isolate its source of problem.
Resistance
When current is applied to a brake magnet, an electromagnetic field is generated,
with metal particles becoming attracted to it. If none of this happens, then that
indicates current is not reaching the brakes – possibly indicative of either an issue
with controller or wires.
Set your multimeter to amps (often represented by an “A”) and connect one probe
from it to a blue trailer pin plug wire and another probe from it directly to a brake
magnet wire. Check your multimeter’s reading against its recommended current
range for your brake magnet size.
If your multimeter reading corresponds with the resistance of your magnet’s base
plate, your magnet is working as intended. Otherwise, there may be an electrical
short in your system or something more serious is amiss. To conduct a more
comprehensive test, perform a voltage drop test, which requires accessing trailer
wiring near wheels – something only experienced mechanics should attempt.
Short Circuit
As soon as a braking magnet malfunctions, its effectiveness in stopping power can
drastically diminish. While many electrical issues can be identified visually, the best
way to determine whether problems with trailer brakes stem from a bad magnet or
short in wiring is using a multimeter to test resistance and amperage levels.
Testing voltage and amperage requires that both vehicles be running, with trailers
connected. A low threshold voltage provides smooth braking; high voltage makes
the brakes grabby and harsh. If system amperage reading remains high after
disconnecting magnet brakes, then a short in the wiring could exist.
Set your multimeter to resistance mode (often indicated with an “O”), and touch
both of its probes against each brake magnet wire while placing the other probe
against the negative post of a 12-volt battery’s negative post. Since wires don’t have
any polarity constraints, probes may be placed anywhere along the circuit path.