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Trailer-Brake Control

Chuck Hammock

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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 years, brake-controller choices have been extensive — all aftermarket and all electronic. Ford changed the game in 2005 with the first trailer-brake-control (TBC) system included as a factory-installed integral part of the tow vehicle’s design, and GM and Dodge have since followed suit. The TBC synchronizes vehicle and trailer brakes for seamless braking, even under heavy load, to provide added driving control and confidence.

Until the advent of anti-lock brake systems (ABS), most brake controllers were tapped directly into the hydraulic lines of the tow vehicle’s onboard 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. A significant factor in elimination of this system was the advent of ABS because the tow vehicle manufacturers cautioned against tapping into the hydraulic system.

Ford’s TBC System

As Ford was first to offer a factory-installed brake controller in the tow vehicle, let’s look at its 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 with more at high pedal pressure, less at low, even to the point of using ABS. If the wheels of the tow vehicle 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. TBC-equipped Ford Super Duty trucks also enjoy an added element of safety; if the trailer is swaying, the TBC will actually apply the trailer brakes to help stabilize the trailer.

While the Ford brake control uses brake-pressure sensing, the GM and Dodge integrated controllers use inertia-type sensing to regulate trailer brake action. None of the systems can be retrofitted to earlier truck models, and aftermarket controllers still must be used for most other vehicles, so the demand for those 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 identify their proportional brake controllers as such, you usually won’t see the words “timer-based” in the literature for a nonproportional 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, but 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

A 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 farther 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 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, which 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 the trailer’s 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.

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.

Among more recent developments, solid-state accelerometers are being used to measure braking force. Tekonsha has embraced this technology in its Prodigy P2 brake control. Hayes uses an accelerometer in its G2 Brake Boss, as doesHensley Manufacturing in its high-tech TruControl trailer brake control. Tuson, a newcomer to the brake-controller world, offers the DirecLink, which is a true network-based unit. The DirecLink receives its data from the OBDII diagnostic port and provides proportional control of trailer brakes.

Some brake controllers aren’t compatible with disc-brake systems on trailers; in such cases, the installation of a controller module — such as the Carlisle HydraStar — is necessary on the trailer in order ensure functional braking.

Aftermarket Installation

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.

A reliable power source is a priority, which may include a vehicle’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 connection is made to the brake-light circuit, downstream of the brake-light switch mounted to the brake-pedal assembly, so the controller senses when the brakes have been activated.

Later-model vehicles with towing 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.

Today’s RVer has many choices in brake controllers from many manufacturers. It’s important to carefully select the controller that is right for your needs and capable of handling the weight and number of axles on your trailer.

Be sure to follow the manufacturer’s written instructions about every aspect of the brake controller’s installation, operation, adjustment and maintenance for optimum performance.