Brake Controller Calculator

Do I need a brake controller?

If your trailer has electric brakes, yes: without a controller in the truck they don't work at all, and Ford's towing guide warns against towing a trailer fitted with electric brakes unless the truck has a compatible controller. If it has surge brakes, no: those are worked by the trailer pushing on its own coupler, and a controller has nothing to drive. So the question that comes first is whether the trailer needs brakes at all, and that has two answers that often disagree. The state sets a legal floor, usually by loaded weight. The truck's maker sets its own, and it's frequently lower: Ford's 2026 guide calls for separate trailer brakes above 1,500 lb, Ram's manual above 2,000. Almost every travel trailer and fifth wheel crosses both, which is why they come with electric brakes from the factory. The tool runs all three questions against your trailer, your state and your truck.

Does a 3,000 lb trailer need brakes?

It depends on one word. Of the 32 codes that draw the line at 3,000 lb, 15 say "3,000 pounds or more", Arizona's § 28-952 among them, so a trailer at exactly 3,000 needs brakes there; the other 17 say "over" or "not exceeding", as Washington's RCW 46.37.340 does, so it doesn't. Add the states with lower lines and the ones that weigh the trailer empty, and a 3,000 lb trailer that weighs 2,400 empty meets the published threshold in 23 of the 51 codes. The line is measured loaded in most of them, and a camper sold at "2,200 lb dry" can be 2,900 lb with water, propane, batteries and a week of gear. At 2,900 lb loaded (2,300 empty), the published thresholds in 8 places already call for brakes: California, Idaho, Mississippi, Nevada, New Hampshire, New York, North Carolina, Ohio. Behind a small SUV the 40% rule adds more: in the states that use it, a 2,900 lb trailer needs brakes behind anything lighter than 7,250 lb.

How do you set brake controller gain?

Tekonsha's Prodigy P3 instructions give the road test. On a dry, level, paved surface with no traffic, warm the brakes first, a quarter mile with light manual braking at about 45 mph, because warm trailer brakes respond more strongly than cold ones. Then at 25 mph, apply the controller's manual knob fully, on its own. If the trailer wheels lock, turn the power down; if the stop is soft, turn it up; repeat until a full manual application stops the trailer firmly, just short of lock-up. Dexter's axle service manual gives its own version: several hard stops from 20 mph, gain down if the brakes lock and slide, up slightly if they don't. Either way the setting changes with the load, so reset it when you go from empty to loaded. On a proportional controller the boost settings change how early the trailer brakes come in; they don't replace getting the gain right, or adjusting brakes that need it.

Proportional or time-delay?

A proportional controller has an accelerometer and applies the trailer brakes as hard as the truck is slowing: a gentle stop is gentle, a panic stop is hard. A time-delay controller ramps up to the gain you set after you touch the pedal, however hard you're braking, which makes light stops grabby and hard stops late. For a travel trailer or fifth wheel the proportional unit is the one to buy. Either way it has to handle your trailer's brakes: two axles of 10-inch magnets draw 12 A (4 × 3.0 A, Dexter's figure), three axles of 12-inch draw 18 A, and the controller's maximum axles and output current are on its box.

Does a car towed behind a motorhome need brakes?

In most states it does, and a brake controller isn't how you give it them. A towed car ("toad") is a trailer as far as most state codes are concerned, and it's measured by its own weight: a 3,500 lb car is over the published line in 40 of the 51 codes in the table. Its brakes are its own, so the motorhome needs a supplemental braking system, either a portable unit that sits on the car's driver's floor and presses the pedal or a permanently installed one that works the car's brake system directly, and the car's maker usually says which kinds it allows for flat towing. Motorhome makers add their own limits on towed weight, on the chassis data plate and in the manual; the towing-capacity calculator runs the motorhome-and-toad checks.

Does a trailer need a breakaway switch?

Any trailer with brakes should have one. 32 of the 51 codes in the table set a breakaway requirement, most from about the same weight as the brakes, and several (Washington, Florida, California and Texas among them) say the brakes must hold for 15 minutes; every electric-brake trailer is built with one. The switch is a pin on a lanyard clipped to the truck: if the trailer separates, the pin pulls and a small battery on the trailer applies the brakes full-on. Test it every trip. Pull the pin and try to roll the trailer forward with the truck, and the brakes should hold it; then put the pin back, because a pin left out drains the breakaway battery flat. The battery charges from the truck's 7-pin plug, so a trailer that sat all winter may have a dead one; the battery-voltage page reads it.