What is the function of a reverse lockout solenoid on a trailer brake system?

A reverse lockout solenoid prevents hydraulic brake pressure from being applied to the trailer’s brakes when reversing. This is essential for trailers equipped with disc brakes, as disc brakes produce braking force in both directions. Without a lockout system, the trailer brakes would engage during reverse maneuvers. Drum brakes typically do not require a solenoid because they generate little braking force in reverse. Trailers with disc brakes must use either a manual lockout device or an electrically controlled solenoid for proper operation.

Types of Reverse Lockout Solenoids:

1. Stop Flow Solenoid

   • Description: The most commonly used type, compatible with most hydraulic actuators. It has two fluid ports and is installed directly at the master cylinder, with the brake line or hose attached at the rear.

   • Operation: The solenoid is wired to the tow vehicle’s reverse lights. When the vehicle is shifted into reverse, the solenoid activates and blocks brake fluid from reaching the trailer brakes, allowing smooth reversing.

   • Advantages: Simple to install and widely compatible.

   • Disadvantages: On an incline, if the tow vehicle is shifted out of reverse, hydraulic pressure may build against the brakes as the trailer presses forward. This can cause brake lockup if reversing continues.

2. Reverse Flow Solenoid

   • Description: Equipped with three fluid ports, this solenoid requires actuators with a pre-drilled and tapped return port. All Tie Down Engineering actuators are compatible, but actuator compatibility must be confirmed before installation.

   • Operation: Like the stop flow design, it connects to the tow vehicle’s reverse light circuit. When activated, the solenoid redirects brake fluid back to the master cylinder instead of sending it to the brakes.

   • Advantages: Eliminates lockup issues when reversing on an incline. In case of malfunction or clogging, brake fluid typically does not reach the brakes, preventing drag or unintended braking.

   • Disadvantages: Requires actuators designed for a return line, which may limit compatibility.