Many vehicles are not designed to allow electrical towing equipment to be directly connected to rear lighting circuits. In these cases a trailer ECU is used to overcome the problems associated with connecting trailer circuits to a vehicle.
Reasons for using a trailer ECU in a trailer wiring system: 1. Many modern vehicles do not provide adequate wiring to handle the additional power required by trailer lamp loads. 2. Vehicle fuse ratings are too low to allow the extra load drawn by the trailer lamps. Using a trailer wiring system without an ECU may result in vehicle fuses blowing, and a loss of vehicle lighting. 3. Some vehicle diagnostic systems may detect the additional load and display errors on the instrument panel. The vehicle computer systems can potentially shut-down the affected circuits. 4. Some vehicles send variable voltage signals to the tail lamp circuits. These variable signals need to be changed to constant voltage signals for trailer lighting.