Controlling Trailer Brakes

February 15, 2008
Filed under Towing Guide, Trailer How To

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Electric-brake systems have been employed for several decades on most trailers that have any significant weight. Each trailer with electric brakes, in turn, also requires that the tow vehicle be fitted with a brake controller. For a number of years, brake-controller choices have been extensive — all aftermarket and all electronic — but Ford changed the game in 2005 with the first trailer-brake-control system included as an integral part of the tow vehicle’s design. The trailer-brake controller is part of Ford’s TowCommand System. Today, GM also offers a built-in control on select vehicles. Until the advent of anti-lock brake systems (ABS), most brake controllers were tapped directly into the hydraulic lines of the tow vehicle’s on-board hydraulic-braking system. They were often described as hydraulic/electric brake controllers, as they converted hydraulic pressure in the tow vehicle’s brake system to an electric signal used to activate the trailer’s brakes directly proportional to hydraulic pressure in the vehicle’s braking system.

1452115_Brake-Controller.jpgA significant factor in elimination of this system was the advent of ABS because vehicle manufacturers’ cautioned against tapping into the hydraulic system. The controllers displaced a small amount of brake fluid, and with ABS systems, brake-cylinder volume was more limited, so manufacturers warned not to add controllers that displaced more than 0.02 cubic inches. The tap-in could not occur downstream of the ABS mechanism, so the pressure was on to devise alternate systems. Electronics were the answer, and many different types of aftermarket brake controllers were developed — none tied in with the tow vehicle’s hydraulic system, however. Even in the old days of hydraulic/electric controllers, all were sold in the aftermarket, until Ford became the first to offer a trailer-brake actuation system installed at the factory.

Ford’s Braking System As Ford was first to offer a factory-installed brake controller into the tow vehicle, let’s look at the system. Ford’s integrated controller electronically tracks hydraulic pressure inside the vehicle’s master cylinder, and uses the pressure, along with vehicle speed, to modulate the amount of current produced for trailer brakes. Thus, the system accurately follows tow-vehicle braking — more at high speeds than low — even to the point of utilizing ABS. If the tow-vehicle wheels are slipping, ABS goes into action for the tow vehicle as well as the trailer. The Ford system doesn’t give the trailer the same functions and characteristics of true ABS, but when the truck’s ABS is activated, the trailer-brake application is reduced to avoid wheel lockup — just as with true ABS. A dash monitor indicates the level of trailer braking, and a manual override is provided so the trailer brakes can be applied independently of tow-vehicle brakes.

1452115_HENSLEY.jpgThe system cannot be retrofitted to earlier Ford models, and aftermarket controllers still must be used for most other vehicles, so the demand for aftermarket controllers remains quite large. Accordingly, it’s helpful to understand what’s out there, and how to make a good choice. In essence, all electronic brake controllers fall into two general categories: timer-based brake controllers and inertia-based proportional brake controllers.

All brake controllers generate an output signal to a trailer-brake system when a user first touches the tow vehicle’s brake pedal or activates the brake controller’s manual control — if so equipped. What happens next varies greatly, depending on whether you’re using a timer-based or proportional brake controller. While most manufacturers will identify their proportional brake controllers as such, you usually won’t see the words “timer-based” in the literature for a non-proportional brake controller. Instead, marketers will sometimes advertise timer-based controllers as having the advantage of requiring no leveling, but even that idea is confusing. Several proportional brake controllers do not require leveling. Timer-based controllers are also touted as being microprocessor-operated, but all brake controllers contain a processor of some kind. Even the most sophisticated timer-based microprocessor doesn’t determine how hard you are braking, only how long you’ve been braking. A good rule of thumb is that if it doesn’t say the word “proportional” somewhere in the literature, it’s probably a timer-based brake controller.

Timer-Based vs. Proportional 1452115_P3-Brake-Control.jpgA timer-based brake controller has a timer that generates an output signal for your RV’s brakes that increases with the amount of time you keep your foot on the brake pedal. It doesn’t know whether you’re braking gently on a gradual downhill grade, or if you’re in a panic stop. The rate of increase in output (the slope of the voltage ramp) has no bearing on pedal effort, but can usually be adjusted for braking aggressiveness.

An inertia-based proportional controller generates an output that is, as the name suggests, directly proportional to your braking needs. Most proportional brake controllers measure the tow vehicle’s rate of deceleration by means of a pendulum. The quicker you slow, the further the pendulum is displaced, via inertia, from its at-rest position. This creates an electric signal to your trailer’s brakes that is proportional to your deceleration rate. Tekonsha and its sister companies use a series of LEDs and photoelectric detectors in their proportional controllers to determine the position of the displaced pendulum, and therefore determine the deceleration rate. Hayes-Lemmerz uses the Hall effect, a well-known physics principal involving moving magnets, to determine the pendulum’s position. All pendulum controllers are subject to some inaccuracies, as the pendulum can tilt slightly forward or backward on steep grades. Most such controllers employ a damping device to stabilize the sensor against vibrations, and the damper helps reduce the effect of the fore-or-aft-tilt problem. The pendulum’s position is adjusted through the level-control knob. This allows the pendulum to be oriented to a true vertical resting position to compensate for the angle of the brake-controller body (the “leveling” referred to in some timer-based-controller advertising). The level adjustment also allows the driver to pitch the pendulum slightly forward or aft of its normal resting position to set up the trailer so its braking is aggressive or delayed.

Most drivers prefer some braking effect from the trailer’s brakes on the initial touch of the brake pedal, and adjust the brake controller accordingly to provide this so-called threshold voltage. This adjustment produces some output (typically 2 volts), without the initial time period having passed or deceleration having occurred in the two electric brake-controller types. This time period, or deceleration event, is normally needed to activate the timer-based controller or the proportional pendulum-based controller, respectively.

Which is Which? With dozens of different models of brake controllers on the market, many RVers may find the selection of the right controller difficult without some form of guidance. While budgetary considerations may force a user into one category of controller, this is a critical system where pinching pennies may not be advisable. When faced with the need for a panic stop, most drivers want a controller that will respond in proportion to their braking needs at that moment. Keep in mind that a timer-based controller can’t respond in this manner, as its output is fixed for a certain timed duration.

Wire Away Critical to every successful brake-controller installation is the proper tow-vehicle wiring. While you should carefully follow the manufacturer’s recommendations, there are certain common elements to almost every controller. Among priorities is a reliable power source, which may include a vehicle manufacturer’s built-in circuitry for an aftermarket brake controller. The main power line should be at least a 10 AWG wire from a circuit breaker at the fuse box, or the battery, to the brake controller’s power-input connection (follow specific instructions for your controller). The circuit should continue from the controller, without any splices, and terminate at the seven-pin connector found at the rear of your tow vehicle. Most manufacturers advise against grounding the controller to the vehicle’s firewall, but recommend routing a 10 AWG (or larger) separate ground wire directly to the battery; the controller may not function properly if the ground connection is not made directly to the battery terminal. The other wiring connection is made to the brakelight circuit, downstream of the brakelight switch mounted to the brake-pedal assembly, so the controller senses when you’ve activated the brakes. Later-model vehicles with tow packages have brake-control wiring bundled under the dash with a plug-in connector attached. A pigtail with the mating part of the connector is supplied with the vehicle, and is ready for connection to the brake control wiring. This setup avoids most of the complexity and possible mistakes with locating the correct wiring among that maze under the dash.

1452115_Wiring-Diagram_450.jpgAmong more recent developments, accelerometers are being used to measure braking force. Tekonsha has embraced this technology in its Prodigy brake controller, and U.S. Gear uses it in the company’s system intended for vehicles towed behind motorhomes (dinghy vehicles). Likewise, Hensley Manufacturing uses accelerometers in its new high-tech TruControl trailer brake control. Today’s RVer has many choices in brake controllers from many manufacturers. It’s important to be careful in choosing the controller that is right for your needs, and that’s capable of handling the weight and number of axles on your trailer. Be sure to follow your manufacturer’s written instructions about every aspect of the brake controller’s installation, operation, adjustment and maintenance for optimal performance.

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One Response to “Controlling Trailer Brakes”

  1. terry majors on November 7th, 2012 9:46 pm

    to increase the efficiency of my fridge in hot weather, I installed a very small 12 vdc fan out of a discarded computer, into and under the cooling coil of the fridge. Then ran a hot wire thru a switch mounted on dthe wall. When needed , I run the fan to push the hot air out and bingo, the fridge works better.

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