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Testing Trailer Brake Magnets for Proper Function

There are 4 different tests that can be done to check the brake magnets on your trailer. The first test you should do is to use a multimeter to check the amperage of your trailer braking system. The trailer brake magnets will draw amperage from the brake controller based on the output voltage sent by the brake controller. Performing this test will let you know if the magnets are functioning properly or if further testing is required.If further testing is indicated, you can test the individual brake magnets for proper amperage. Test each magnet for a short and test the magnets for resistance. Below we have outlined each of these tests for you.

Testing with Trailer Connected to the Vehicle

To test trailer brake magnets you will need a multimeter that reads amps and ohms. A good brake controller, like the Tekonsha P3, has test functions built in. To test with a multimeter, first you will connect the ammeter inline with the blue wire exiting the back of the brake controller or use the brake controller's diagnostic readings.Check the amperage. If the amperage is greater than the specified amount (see specifications below), replace the magnet or magnets. If the reading is less than the specified amount, the leads are bad and the magnet or magnets should be replaced.
Illustration of Connecting ammeter to blue wire between brake controller and trailer connector

Electromagnet Specifications

10 and 12 Inch Brake DrumsMax Amps at 12-13 Volts
2 Brakes7.5-8.2
4 Brakes15.0-16.3
6 Brakes22.6-24.5
7 inch Diameter Brake DrumMax Amps at 12-13 Volts
2 Brakes6.3-6.8
4 Brakes12.6-13.7
6 Brakes19.0-20.6

Testing Individual Brake Magnets

Testing individual brake magnets for proper function can be accomplished by severing the magnet wires and connecting the ammeter between the positive terminal of a 12-volt battery and one of the magnet wires; it does not matter which one. Then connect the other magnet wire to the negative battery terminal. Check the amperage. If the amperage is greater than the specified amount (see specifications below), replace the magnet. If the reading is less than the specified amount, the leads are bad and the magnet should be replaced.

Individual Brake Magnet Amperage Specifications

10 and 12 Inch Brake Magnets7 Inch Brake Magnets
3.2-4.0 Max amps at 12-13 volts3.0-3.2 Max amps at 12-13 volts
Connecting Ammeter between positive terminal on 12-volt battery to one of the magnetic wires

Testing Individual Brake Magnet for a Short

To determine if a brake magnet has an internal short, touch the base of the brake magnet to the negative post of a 12-volt battery. Then, connect one of the brake magnet wires to the negative lead of a multimeter and the positive lead of the multimeter to the positive post of the battery. If any amperage is detected, the magnet is shorted and will need to be replaced.
Touching base of brake magnet to negative post on 12-volt battery image

Testing Brake Magnet Resistance

Further testing can be done with an ohmmeter. Using the ohmmeter function on the multimeter, check the resistance between the brake magnet wires. If the resistance is not within the specified range (see specifications below), then the magnet will need to be replaced.

Brake Magnet Resistance Specifications

10 and 12 Inch Brake Magnets7 Inch Brake Magnets
3.0-3.8 Ohms3.8-4.0 Ohms
Testing with an ohmmeter image
Updated By: Dani S.Last Updated: 3/15/2021

Questions and Comments about this Article

Miley

I have not gotten any answers out of Ford: . . . . .So went to a large dealer and talked to the service manager about the warning message that fills the entire dash “WIRING FAULT ON TRAILER”. When this message appears no other information appears on the dash, which requires you to hit the OK button while braking to clear a message as you are driving with no apparent cause or meaning. The dealer has no way to troubleshoot why this warning is coming on. He has no technical detail on how much brake current is too much at any given speed or how much input to the microprocessor is receiving from the other computers on the truck. Since there is no interest from Ford in fixing the Hazard warning label while using the brakes to slow or stop the F-350 truck. The manual under Information Display Warning Messages has this statement: “the trailer brakes may be drawing too much current” what does that mean ? The dealer does know what that number is. I never have that warning light appear when pulling a double axle trailer with 4 brakes. I appears often a lot when pulling a triple axle trailer with 6 brakes. I think the Ford trailer brake system cannot handle the current draws when all 6 brakes are engaged. But with no spec supplied to the dealer and no way to test what that trip point is. The microprocessor controls all of these items yet the dealership cannot check or verify any of these operations. • The trailer brake controller is equipped with a feature that reduces output at vehicle speeds below 11 mph (18 km/h) so trailer and vehicle braking is not jerky or harsh. This feature is only active when applying the brakes using your vehicle's brake pedal, not the controller. • The controller interacts with the brake control system and powertrain control system of your vehicle to provide the best performance on different road conditions. • Your vehicle's brake system and the trailer brake system work independently of each other. Changing the gain setting on the controller does not affect the operation of your vehicle's brakes. What I need Ford to do is help the dealer run checks or simulations to stop the need for the Hazard Notification with no solution to be displayed on the dash that block all normal displays that are shown on the dash. Trailer Fault Indicator on Dash Can you change the parameters on the Ford brake controller ? Can the dealer change the upper current limit on the brake controller when engaging 6 brakes on a triple axle trailer ? I looked in the Manual as to why does the trailer fault display on the dash The dealer tech does not have all the parameters on the brake control That’s what I’m requesting are the parameters that the brake controller sees before the display is shown on the dash I think the random display is caused by the trailer brakes exceeding the current draw limit built into the brake controller. This display warning only occurs with this triple axel trailer toy hauler, no other trailer causes the warming. But the other trailers do not have 6 electric brakes connected to the F-350. When the brakes are applied too hard all 6 brakes draw too much current versus a dual axle trailer with only 4 brakes needing current. The dealer has done the simple test that dealers can do, they find no problems with the very basic test. A trailer repair place said there is no problem with the trailer brakes. And the warning hazard does not occur on an ongoing regular basis only when the brakes are applied too hard. I push the OK button everything is fine until I apply the truck brakes too hard, which causes all 6 brakes on the trailer to engage, this again exceeds the upper limit that the brake controller thinks is excessive and displays the warning on the dash. Most trailers only have two axles or 4 brakes, not 6. This exceeds the controller trip point for a higher than normal current draw. The solution is to program the control to a higher trip point due to 6 trailer brakes drawing higher Amps during a normal braking operation. In the Ford manual on page 296/666 it states this: TRAILER BRAKE MODULE FAULT Displays in response to faults sensed by the trailer brake controller, accompanied by a single tone. If this message appears, contact an authorized dealer as soon as possible for diagnosis and repair. The controller may still function, but with degraded performance. WIRING FAULT ON TRAILER Displays when there is a short circuit on the electric brake output wire. On page 297/666 it states this: If this message displays, with no trailer connected, the problem is with your vehicle wiring or trailer brake controller. Contact an authorized dealer. If the message only displays with a trailer connected, the problem is with the trailer wiring. Consult your trailer dealer for assistance. This can be a short to ground (such as a chaffed wire), short to voltage (such as a pulled pin on trailer emergency breakaway battery) or the trailer brakes may be drawing too much current. Points to Remember Note: Do not attempt removal of the trailer brake controller without consulting the Workshop Manual. Damage to the unit may result. • Adjust the gain setting before using the trailer brake controller. • Adjust gain setting, using the procedure above, whenever road, weather, trailer, or vehicle loading conditions change from when the gain was initially set. • Only use the manual control lever for proper adjustment of the gain during trailer setup. Misuse, such as application during trailer sway, could cause instability of the trailer or tow vehicle. • Avoid towing in adverse weather conditions. The trailer brake controller does not provide anti-lock control of the trailer wheels. Trailer wheels can lock up on slippery surfaces, resulting in reduced stability of trailer and tow vehicle. • The trailer brake controller is equipped with a feature that reduces output at vehicle speeds below 11 mph (18 km/h) so trailer and vehicle braking is not jerky or harsh. This feature is only active when applying the brakes using your vehicle's brake pedal, not the controller. • The controller interacts with the brake control system and powertrain control system of your vehicle to provide the best performance on different road conditions. Thus controller has a microprocessor that handles all of these inputs from different systems on the F-350 truck.

Chris C.

Replaced all 4 trailer brakes with new shoes and magnets etc. after adjusting then very weak brakes. Adjusted them 3-4 times still the same. Tested the truck and it’s giving out 12 volts at trailer plug. Tested wires by brakes and when manual override lever for brakes is applied Im only getting 10 volts at each individual brakes. Tested the resistance of each magnet and it’s 2.7ohms. Range is 3.0 to 3.8 with 12 inch brakes does that mean my magnets are junk? Also I’m not hearing a constant humming noise at the brakes when voltage is applied. Does this mean the magnets aren’t energizing? I think I need new magnets just want to get opinion from experts. Thank you

Etrailer Expert
Reply from David B.

If you have followed the guide and the magnets are out of tolerance then yeah I'd say you have bad magnets

Jim J.

I have a 2016 f 350 diesel and a Legend 24 ft aluminum trailer I just did a full brake job to trailer cause brakes where done and system was giving me a trailer disconnected message. Well everything new and still giving me that message. I have all lights working perfect but no brakes. Any thoughts Thanks Jim

Etrailer Expert
Reply from David B.

Did you check the connectors to make sure there is a solid connection? Some of the pins may be damaged or dirty. I would also check the grounds and go through with a circuit tester to verify everything has the right function going through it. I'll add a couple links to diagrams and install just incase you need them.

Wally

If I disconnect the rear axle brakes will the front axle still work?

Etrailer Expert
Reply from David B.

What are you trying to do Wally? That doesn't sound like a safe thing to try and do.

Reply from Wally

@DavidB I just what to verify that I either have a magnet or wire problem with the rear axle

Etrailer Expert
Reply from David B.

OK, lets test the magnets. To determine if a brake magnet has an internal short, touch the base of the brake magnet to the negative post of a 12-volt battery. Then, connect one of the brake magnet wires to the negative lead of a multimeter and the positive lead of the multimeter to the positive post of the battery. If any amperage is detected, the magnet is shorted and will need to be replaced. That will let you know if it is a magnet issue or a wire issue.

Davis

when I pull emergency brake voltage drops to 10 volts from 12v and brakes very weak I've adjusted brakes and wires look good (trailer is 6 months old)

Etrailer Expert
Reply from David B.

I would test the magnets. Testing individual brake magnets for proper function can be accomplished by severing the magnet wires and connecting the ammeter between the positive terminal of a 12-volt battery and one of the magnet wires; it does not matter which one. Then connect the other magnet wire to the negative battery terminal. Check the amperage. If the amperage is greater than the specified amount (see specifications below), replace the magnet. If the reading is less than the specified amount, the leads are bad and the magnet should be replaced. 10 and 12 inch brake magnets need to be at 3.2-4.0 Max amps at 12-13 volts and 7 inch brake magnets need to be at 3.0-3.2 Max amps at 12-13 volts.

Ed

Just put new brakes on trailer that had minimal braking. install complete brakes with magnets ect. with trailer raised in the air when you apply the brake via the controller all brakes lock up. When trailer is on the ground hooked to truck then you activate controller fully the brakes slow the trailer but not very good. Adjusted brake shoes several times. Any help ?

Etrailer Expert
Reply from David B.

How fast are you going when you use the brakes? If you have a proportional controller it isn't going to "slam" on the brakes if you are goin 10mph.

Paul H.

My 2018 tundra flashes from trailer disconnected to connected This is a brand new axel I put on and wired, this has never happened while towing other trailers with brakes

Etrailer Expert
Reply from David B.

I think you don't have a good ground on your brake magnets. Check both sides and make sure one of the wires is grounded and the other is connected to power. If that doesn't work you need to keep checking connection points from there all the way back to the truck making sure all grounds are secure and there is no damage to wires.

Reply from Ben H.

@DavidB My 5th wheel did this. I had a loose connection between the truck and trailer plug.

Reply from Paul H.

@BenH I brought it to the dealer the factory brake box needed an update, now it works great

Christopher W.

I have tested the resistance on all four of the brake magnets on my trailer. 3.0, 3.2, 3,2, and 3,2. However, when I check the amperage I am only getting 12.8 amps for the 12 in brake drum brakes. Do the brake magnets need to be replaced even though they are within spec? Using a prodigy P3 brake controller to check the amperage.

Etrailer Expert
Reply from David B.

Hey, for 4 twelve inch brake drums, testing at the same time, you need to be at 15-16.3 Max Amps at 12-13 volts. If you are over or under this you have a bad magnet or a bad lead and either way you will need to replace the magnets. Check the link I added to see new magnets

Sparky A.

I used a battery charger as a voltage source. I got a lower current than expected, even though the DC resistance check out. Ohm's law seems to be violated in my garage. But then I realized the "DC" output of a batter charger is not true "DC" but is usually full wave rectifier waveform. Therefore the magnet exerted inductive reactance on my battery charger voltage source, reducing the current. I'm just saying, be advised that a batter charger does not produce a true DC voltage.

Matthew K.

I have two trailers and a brand new silverado 3500 hd work truck, On my dump trailer, I get " brakes connected" several moments later check trailer wiring. This goes back and forth on its own even with the truck just sitting there. Its been in the dealership twice but all the found was a blown trailer break fuse in the fuse box and a rubbed out nuetral between front of truck and connector. Need help on how to test the trailer for some mysterious issue. All trailer brakes appear to work.

Reply from Mike R.

@MatthewK - I had the same issue as you are describing with my RV. I too have a new truck but its 2021 Silverado 2500HD. After researching online for troubleshooting I had added 2 new grounds in addition to a new trailer 7 pin cable. What I found was not the trailer that was giving me the issues but when I finally check my trailer connection on my truck. Mine corrected itself after pushing on the wires inward on the back side of the connector. I have not had any other issues. Hope this help with yours.

Etrailer Expert
Reply from Les D.

Excellent troublshooting!

John

In your picture for Individual Brake Magnet Amperage Specifications, you have the negative of the multi meter connected to the magnet and negative battery terminal. But in your dictation you say to connect the positive of the multi-meter to the magnet and positive battery terminal. Which is it? I connected the positive of the multi meter in line with the battery and brake magnet, then connected the negative of the brake magnet to negative on the battery. I got no reading on a brand new brake magnet, what is the problem?

Etrailer Expert
Reply from Victoria B.

That section states than an individual magnet can be tested by "connecting the ammeter between the positive terminal of a 12-volt battery and one of the magnet wires." The positive of the multi-meter will be on the positive battery terminal. The negative of the multi-meter will be connected to one of the brake magnet wires. The other brake magnet wire will be on the negative batter terminal. It sounds like this is how you wired up your brand new brake magnet for testing. Since you didn't get any readout at all on the multi-meter, I would start by checking the battery's charge and that the multi-meter is working correctly.

Stephen B.

Some great testing ideas and specs from a company that I have purchased many repair parts for my fifth wheel.

Al

Got brake problems,magnets are humming,shoes are adjusted with some drag,still no brakes,these are new brake assemblies,whats going on?

Etrailer Expert
Reply from Victoria B.

I have linked another help article that will walk you through a process of testing your braking system to help determine where the issue is originating. This is where I would start troubleshooting.

Reply from Glynn J.

@Al the magnet must make full contact with the built in rotor to create enough friction to move the lever that actuated the brakes. Also the lever must be easily moved by hand

Etrailer Expert
Reply from Les D.

@AL I think the lever that the commenters are referring to is on the brake controller at the drivers seat dash. Since your brakes are humming, you are getting some amount of power to them. I suggest you take another look at the article that Victoria sent you and report back your voltages. Thanks,

John

I just recently picked up a 16 foot flatbed trailer with brakes. Someone has removed the brake harness from the axle forward. Both wires coming out of the brakes are green. There’s one of these go to ground and the other one to the power source on my seven wire plug? With both of them being green does it matter which one goes where?

Etrailer Expert
Reply from Victoria B.

You are correct that either wire can be used to connect to either function. You will want to ground one wire and the other wire should be connected to the brake output wire on the 7-Way connector. You would not want to connect the brakes to the 12v power wire as that would cause the brakes to always be engaged.

David G.

i have 12volts going to front axle black and white wire but no brakes

Etrailer Expert
Reply from Victoria B.

Do you have signal on one of the wires going to the brakes at all times, or only when the brake pedal is pressed? Is there signal on both wires, or just one?

Mac E.

What if the code S.H. stays on when the trailer is disconnected? Does this indicate that the problem is with the truck wiring?

Etrailer Expert
Reply from Jon G.

Yeah that's what it sounds like to me. Look for any places where the wires might be damaged or exposed and fix them.

Reply from Mac E.

@JonG - Thanks for the prompt response, the hunt continues!

Jeff K.

The directions for determining the amperage across the blue wire does not apply to any brake controller that requires resistance (a load) to function. Example: Curt Triflex requires an electrical load be sensed before the controller will send a signal in the blue wire (signal = power) . A simple 12V test light shining brightly is a suitable indicator of signal.

Kenneth C.

Best troubleshooting guide on the web. Thx



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