To help understand truck air brakes a brief run through history. In the 1960’s some trucks had the earliest form of safe braking with the slippery road or wet road valve. This dash control valve cut the air pressure in half for the front brakes. Also maxi chambers appeared. These rear chambers have a spring applied parking brake. Before this, other parking brakes like drive shaft drum etc.
Starting in the 1970’s was the 121 braking system. This system, originally designed for truck tractors to have a complete separate brake system for the trailer. The system, two tanks and a brake treadle valve having two separate valves. This way if the air lines between truck and trailer failed, the tractor still had brakes. The duel system was quickly added to all air brake trucks and the tractor protection valve was added for the trailer supply valve.
In the 1980’s the air systems were revised and experimented with more complicated valves. One example was if the rear air system failed, air from the front system was applied to the rear maxi chambers. Some of this air would be released to apply the rear brakes. Short lived, thankfully.
In the 1990’s, anti-lock brake systems for truck air brakes were added. Required by 1998? This electronic system with wheel speed sensors and electric solenoids dumps the air from the axle that shows a locked wheel. This system when installed on a trailer is powered up by the trailer brake light wire.
We get out of the Way-back machine and now to that 5 volt wire.
@Frankenchevy has the answer. It’s a sensor wire. Since the brake light turns on the anti-lock system, how do we know the system will work? Some of these changes were requested (required) by big brother.
How the 5V check system works. When you turn on the key, the BCM (body control module) or the ECM (engine control module) {where ever they hid it} measures the resistances load on the wire just like most other sensors. If everything checks out, the module blinks the ABS amber light at the rear of the trailer. If there is a problem the ABS amber light stays on and a trailer ABS warning light comes on the dash.
@bucket I checked at work, on the trailer brake factory option, all you get are all the wires pre-installed. As you found out that option provides plenty of wires ready to hook up however needed. Your dealer "cheaped out" with the controller they added.
