There are 3 basic types of brake controllers. Going up in technical sophistication (and price), these are:
1. Ramped time base - these controllers can only sense that the tow vehicle brakes have been applied via a signal from the tow vehicle brake lights. When they receive this signal, the ramped time base controller (as its name implies) ramps up the voltage to the trailer brakes over a time period. The ultimate voltage applied and the speed with which it is applied depend on the user settings that are entered into the controller. The problem with these controllers (as you are experiencing) is they do not take into account vehicle speed, road conditions (high traction surface or slick with rain), the amount of stopping power needed (are you making a gradual approach to a stop light or is this an OMIGOSH situation where someone has just pulled out in front of you?) The result is that, if you have the controller set up for typical low speed in-town conditions, you won't have enough braking power on the highway. Conversely, if you have the controller set up for highway conditions, it will be grabby and tend to lock the trailer brakes in town.
2. Inertial - these controllers use an accelerometer (the old ones actually used a pendulum and rheostat!) to sense the amount of deceleration the tow vehicle is undergoing and, if they sense that the tow vehicle brakes are being applied (again, by the tow vehicle's brake light wire), they send an output signal to the trailer brakes that's based on the rate of deceleration they sense. These are a step up from the ramped time base controllers, but they can have their problems as well. For instance, we had a Tekonsha Voyager set up for optimum stopping under dry road conditions when we encountered a sudden Texas Gulf Coast downpour. This resulted in extremely slick road conditions as the oil on the road floated on top of the water as it typically does when the rain first begins. Naturally, Murphy's law being what it is, that's the exact time we encountered one of those OMIGOSH moments when a driver ran a stop sign at an intersection and we were forced to make a heavy brake application. The truck brakes were trying to lock up (thank goodness for anti-lock brakes), yet the truck was not generating enough deceleration due to the slick conditions to energize the trailer brakes - in other words, the trailer was pushing the truck into a jack-knife attitude and trying to shove the truck into a deep ditch. We managed to save it only by getting off the brakes, countersteering and accelerating hard out of the situation.
3. Brake-sensing proportional controllers - these controllers actually sense what the driver is doing with the tow vehicle brakes and directly translate that into a proportional output to the trailer brakes. This sensing can be via direct mechanical linkage to the tow vehicle's brake pedal (e.g., Jordan Optima 2020, no longer in production) or by a hydraulic pressure sensor in the tow vehicle's braking system (e.g., MaxBrake). This is the optimum approach as the controller output to the trailer brakes directly mimics what the driver's right foot is doing on the brake pedal. The tow vehicle and trailer feel like they stop as a single unit. Once set up for a particular trailer's brake sensitivity and weight, you probably will never have to touch this controller's settings again.
With brake controllers, as with many other things in life, ya pays yore money and ya takes yore choice. To me, however, spending $400 or so on a top-of-the-line brake controller to protect an $80,000 to $100,000+ tow vehicle and RV combination (not to mention your family's safety) is a no-brainer. Unfortunately, many RV dealers will only install the cheapest brake controller they can to make the sale and get the unwary consumer off the lot.
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