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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



Hello, I have been trying to test my 2003 GMC 1500 5500 lb towing vehicle with 5500 lb travel trailer elec brakes. I have noticed during wheel bearing inspection that one one the four trailer Brake drums did not have any marks from the elec magnet. Thus making me think that one magnet was not making contact with drum. This would be the front right side. I have a Tekonsha Prodigy P3 controller. When I jack up the front axle to test the brakes by putting a 1x4 from truck seat With key on trailer plugged into truck both trailer wheels turn, even with break- away key removed wheels will rotate. So I suspected a ground problem.and replaced two ground lugs and cleaned the connection to frame. Now the wheels of the trailer lock up while removing the break away key and while pulling manual over ride lever on controller. But when I place the 1x4 from truck seat to foot brake pedal the wheels on trailer do not lock up. May the foot pedal with not apply until I'm on the road, I don't know. Any ideas? Thanks, Paul



I am not electrician, but on a trailer with four 10" brakes, if min amperage at each magnet is 3.2 amps, how are you going to get the stated min combined 15 amps at the tongue? It seems to me that min combined at the tongue should be 12.8 (3.2*4) amps or each individual magnet min amps should be 3.75 (15/4).

Etrailer Expert

Mike L.


@Jeremy Can you explain what you mean by 'at the tongue'? Do you mean at the trailer connector?



@MikeL I just mean wherever the four magnet wires come together into one. It could be at a junction box at the tongue, at the brake pin on the connector, or a loose wire in your hand. The location is irrelevant. My point is that 3.2 amps * 4 magnets = 12.8 amps, not 15 as the article states. Someone reading the article could be wasting their time tearing their trailer wiring apart trying to achieve 15 amps when the article plainly states that only 12.8 amps is possible if you do the math yourself instead of assuming that whoever wrote the article did the math correctly. Either the min amps per magnet listed is incorrect or the min combined amps listed is. Feel free to email me directly if I haven't explained this clearly enough.
Etrailer Expert

Mike L.


@Jeremy The 3.2 amp figure represents the low end with 4 amps being the maximum. This is the acceptable range, so 15 amps for 4 10 or 12 inch brake assemblies would be in that acceptable range.
See All (5) Replies to Jeremy ∨

Marcus M.


Can you use a multimeter, that's equipped a DC AMP clamp?

Etrailer Expert

Mike L.


@MarcusM You sure can.

Andy W.


@MarcusM Only some really expensive clamp-on Ampe meters will read DC--they need a power source of their own. Most amp clamps read only AC amps. As I said there are some that do, but for most meters the answer is no.



The information contained in the section "Testing with Trailer Connected to the Vehicle" is the same as I see on other websites as well, but when I try to test amperage this way on my Tekonsha Prodigy P2 Trailer Brake Controller it tells me no connection and won't output. It sees the connection fine before adding the multimeter inline to test. How can I test the overall trailer brake amperage if this doesn't work and why would it not see the connection with the multimeter inline? Thanks

Etrailer Expert

Mike L.


@Chad Honestly, the best place to start (if possible) is to try the trailer in question with a different vehicle and the vehicle with another trailer. This will remove all doubt whatsoever as to whether the vehicle or the trailer is the problem. Please try that and let me know what you find. If that's not possible, let me know that as well and we'll dig deeper.



@MikeL I am not trying to determine if it is a truck or trailer issue. I am trying to measure the total amps pulled by the trailer to see if it is correct or not before I begin pulling hubs to check individual magnets. Your answer in no way addresses the question I asked.

The G.


@Chad Use a clamp meter that can measure DC amps. It clamps around the brake wire (blue) and doesn't break or interfere with the brake controller circuit or operation.
See All (4) Replies to Chad ∨



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.



@Miley I just got a new trailer and had this same message pop up on my 2017 super duty. I have a tandem axle trailer (brand new) with lippert 7k axles. My dump trailer with 6k axles has not had an issue but the new one does so I'm wondering if these new brake magnets are intermittently over the current limit you mention. Going to try to do some current draw readings soon but seem to have a similar issue.



@Miley the limit is 25 Amps at the connector from the factory. If you get close to this number, with fluctuations in resistance in the wiring or the brakes, it could be plus minus 2 Amps. Therefore if it crosses the 25Amp limit, it will trigger this fault. The 7 pin connector SAE standard is set to 25 Amps. In reality at some point this old standard needs to be updated, as these trucks can pull larger and larger trailers. However the OEMs don't want to spend the money and nobody wants to tough the old standard. RAM has set the limit in the brake controller higher than FORD, therefore this problem does not happen on RAMs. FORD does not want to change the rating, although the controller could handle more. So this is a FORD specific and very well known problem with Ford!
Etrailer Expert

Mike L.


@R Thanks for jumping in!



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