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Mechanics Corner: Just stop it!

Andy Overbay for Progressive Forage Published on 18 April 2017

Whenever I am around a group of high school-aged boys, eventually the subject of pickup trucks seems to arise. Which one is best? Whose is the stoutest? Which one do I prefer?

After all the cussing and discussing, I take the opportunity to remind them of what I consider to be the cardinal rule of pulling a trailer with a pickup: If you can’t stop it or steer it, you better not be hooked to it.

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All too often, we (old folks like myself included) get caught up in the horsepower and torque wars and forget about the cardinal rule. As a colleague told a young man who was showing off a strong new diesel pickup, “Heck, son, if you wanted a semi, buy a semi.

A good used semi would have been cheaper.” That semi is infinitely safer too because it is able to control a great deal larger load than even the heaviest-duty pickup.

When trailing a heavy load, it is also a good idea to know to what extent your trailer can help you to stop and steer as well. Trailer brake laws vary from state to state. Laws are written such that the total towed weight being pulled determines the need for supplemental braking.

For example, in New York, if your trailer weighs more than 1,000 pounds, you are required to have brakes on your trailer. In Massachusetts and Missouri, in comparison, a towed weight of 9,999 pounds needs no supplemental braking at all.

Other states use stopping distance as a legal determinant. Oregon law requires that any vehicle or combination of vehicles weighing less than 8,000 pounds must be able to stop within a travel lane in 25 feet from 20 miles per hour. If the vehicle or combination of vehicles weighs over 8,000 pounds, the vehicle must be able to stop within a travel lane in 35 feet from 20 miles per hour.

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Kansas, Utah and Wyoming law requires any vehicle or combination to be able to stop from 20 mph within 40 feet or less. It is wise to know the laws of not only the state in which you live but the states in which you are visiting as well.

Keeping working brakes and lights on a farm trailer can be problematic because sometimes the only way you find out you don’t have them is after they fail. With lights that isn’t a great thing, but finding out about a brake failure in mid-failure brings new meaning to the phrase “bad day.”

Magnetic trailer brakes, commonly referred to as electric trailer brakes, are one of the three types of trailer brakes. The other two are hydraulic surge brakes and air brakes. Magnetic trailer brakes are used on all types of trailers except semi trailers and boat trailers. Electric trailer brakes are very simple to retrofit and are the easiest to maintain of all the trailer brakes.

Magnetic trailer brakes work in conjunction with a brake controller – an electronic control box in the tow vehicle. This controller is wired to the brake light switch and to the battery. When the brake pedal in the tow vehicle is depressed, power is supplied to the brake controller, which switches modulated battery power to the magnetic trailer brakes. There are controls on the box to regulate how much battery power the electric brake receives.

This determines how much braking power the trailer has. Without this adjustment, the trailer brakes would either lock up with a light load or not have enough braking power for heavy loads.

The magnetic trailer brake is applied by means of an electric magnet within the brake mechanism. This magnet is attached to the bottom of an arm which pivots at its top end. The arm is also linked to the brake shoes. When the magnet is energized, it is attracted to the spinning brake drum, causing the arm to move forward, applying the brakes.

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The adjustments on the brake controller determine how much current is supplied to the electric magnet, which in turn determines the strength of the attraction to the brake drum. The stronger the attraction, the larger the force applied to the brakes and the harder the shoes contact the brake drum.

There are four different tests that can be done to check the brake magnets on your trailer. The first test 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.

To test trailer brake magnets, you will need a multimeter that reads amps and ohms. A good brake controller has test functions built in. To test with a multimeter, first connect the ammeter in line 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 a specified amount (for a four-brake trailer with 10- to 12-inch drums, the maximum amps at 12 to 13 volts should be 15.0 to 16.3 amps), 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. Specifications can be found either in the controller’s manual or on the internet.

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 a specified amount (for the trailer described above, each magnet should read 3.2 to 4.0 amps maximum), replace the magnet. If the reading is less than the specified amount, the leads are bad and the magnet should be replaced. Again, individual trailer specs can be found online.

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.

Chasing trailer brake gremlins can be a hassle but not nearly as much as dealing with the consequences of a failed system that resulted in a crash or claimed a life.  end mark

Andy Overbay holds a Ph.D. in ag education and has more than 40 years of hands-on dairy and farming experience.

Andy Overbay
  • Andy Overbay

  • Extension Agent
  • Virginia Cooperative Extension
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