How to Test Trailer Brakes with a Multimeter (3–Step Guide)

Faulty or worn-out trailer brake magnets can cause serious problems for a trailer to come to an instant stop. Some problems could be noticed by taking a simple look at your brake magnets but sometimes there could be certain electrical problems that will affect the performance of your trailer brake.

A faulty brake magnet can cause weak or surging brakes or cause the brakes to pull to one side. This is a good enough reason to understand the workings of your braking system and how to troubleshoot it if the need arises. The most important step in understanding the workings of trailer brakes is learning how to test trailer brakes with a multimeter.

In general, if you want to test your trailer’s brakes with a multimeter then you need to:

(1) Remove the brake magnets
(2) Place the brake magnet’s base to the negative terminal
(3) Connect the positive and negative leads.

I will explain this 3 – step guide in detail below.

Understanding How a Braking System Functions

The braking system of a trailer comes in two major variations which are the surge trailer brakes and the electric trailer brakes. Before going for a test, you need to know what type of braking system your car has. I will discuss the two types of braking systems below. (1)

trailer brake gain

  • The first type is the surge trailer brakes which contain a surge coupler installed on the tongue of the trailer. The braking is automatic in this type of trailer brake which means there is no requirement for electrical connection between the tow vehicle and trailer except for the lights. There is a connection attached to a hydraulic master cylinder inside. The forward momentum of the trailer drives on the surge coupler whenever the tow vehicle enables its brakes. This causes the vehicle to move towards the rear and apply pleasure over the master cylinder piston rod.
  • The second type of braking system is the electric trailer brakes which are brakes triggered by an electrical connection to the brake pedal or a modifiable dash–installed inertia switch in the trailer. Whenever electric trailer brakes are applied, an electric current that is proportional to the rate of deceleration fuels a magnet inside each brake. This magnet triggers a lever that when activated applies the brakes. This type of controller could be adjusted for different trailer loads.

How to Test Trailer Brakes with a Multimeter

There are 3 specific steps that you need to take if you want to measure your trailer brakes with a multimeter and there are as follows: –

  1. The first step is to remove the brake magnets from the trailer.
  2. The second step is to place the base of the brake magnet to the negative terminal of the battery.
  3. The final step is to connect the positive and negative leads of the multimeter to your battery. You should connect the multimeter in line with the blue wire going to the back of the brake controller and if you happen to notice any current on the multimeter then the brake magnet is dead and it needs to be replaced.

testing trailer brake

I would recommend that you use a 12-volt battery when testing your braking system and you should connect your blue wire which is the wire that controls the brakes to the multimeter and set it to the ammeter setting. You should be getting a reading of maximum amps below.

Brake diameter 10-12

  • 5 – 8.2 amps with 2 brakes
  • 0 – 16.3 amps with 4 brakes
  • 6 – 24.5 amps usage with 6 brakes

Brake diameter 7

  • 3 – 6.8 amps with 2 brakes
  • 6 – 13.7 amps with 4 brakes
  • 0 – 20.6 amps usage with 6 brakes

I also advise you to utilize the use of the ohmmeter function on your multimeter to check the resistance of your brake magnet.

There is a specified range that you are supposed to notice on your brake magnets and this range should be between 3 ohms to 4 ohms depending on the size of your brake magnets if the result is not similar to these then the brake magnet is damaged and it will have to be replaced. (2)

When testing your trailer brakes there are electrical issues that could impact the performance of your brakes and you can do a visual inspection to determine where the fault lies in your braking system.

When performing a visual inspection there are three steps to take to determine if there is a fault.

driver / mechanic testing the trailer brake with multimeter

  1. The first step is to check the center of the trailer brake for signs of any coil. If you find one then that means it is worn out and will need a replacement promptly.
  2. The second step is to take a straight edge tool which you will lay across the top of the magnet. This edge must be paralleled to the straight edge all the way across and if you notice any change or pitting in the surface of the magnet then that indicates abnormal wear and it should be replaced promptly.
  3. The final step is to check the magnet for any grease or oil residue.

Symptoms of a Bad Trailer Brake

There are certain issues that you should be aware of in case you are not a fan of testing your trailer brakes. These issues indicate that you most certainly have a brake problem and you should immediately test your trailer brakes for confirmation. Some of these issues are as follows: –

  • One of these issues is a weak forward electric brake, especially in the case where you have electric brakes installed on the four wheels of your trailer. In a situation where everything is functioning perfectly then the round section of the brake’s actuating arm should point forward for the trailer brakes to work as intended.
  • Another issue is when you notice that your trailer somehow pulls to one side when you hit the brakes. This is an indication that the braking application of your trailer is not balanced.
  • Another significant issue is if you notice that your trailer brake locks near the end of a stop. When you are coming to a stop and your brake locks then the issue stems from the settings of the braking control unit. It could most likely be that the drag on the brakes is set a bit too high which will cause tear and wear on the shoes of the brake.

You may want to check here how to test trailer lights with a multimeter.

Wrapping Up

The thing to always remember is that trailer brakes need frequent regular service and maintenance because of the heavy loads carried by these vehicles so I would advise you to always test your trailer brakes to avoid any sort of mishap or accident on the road due to faulty braking systems.

Issues with wiring shorts also result in significant problems. Frayed or damaged wires could occur because of the wire placement within the axle itself.

If you get an indication on the brake controller screen saying “output shorted” then you should start looking for wiring problems within your axle. You should also be extremely careful when working with wires and electricity to prevent electrical shocks.

Other useful learning guides you can check or bookmark are listed below;

(1) braking system –
(2) magnet –

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About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gCertifications: B.E.E.
Education: University Of Denver - Electric Engineering
Lives In: Denver Colorado

Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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