Properly Wiring a Towed Vehicle

From years of experience wiring up towing and braking systems, I can say that brightness of toad bulbs, whether added or existing, is more related to the wiring and its ability to feed power to the bulb, than it is about exact bulb placement in the taillight housing. Before most coach manufacturers switched over to relays and heavy 12 gauge wiring to the 7 pin blade connectors, it was not abnormal to find 10.5Volts arriving at the rear trailer connector. Combine a weak ground, with light gauge wiring or poor diode quality and often the voltage at the toad tail light bulb was 8-9 Volts.  Wiring Adapters are fine as long as the proper pins are feeding power to the appropriate connection.

Even the best diodes are around 90% efficient and the lower quality ones are in the range of 70-80%. Thus there is a significant loss of power going through them. Using a quality 6 pin connector and 16 gauge wiring from front to back in the toad combined with good wiring practices will result in ample power to the Toad taillight, brake and signal bulbs. Grounding the ground wiring at the front and back of the toad to a clean chassis spot is a good practice and shortens the current path providing less loss. When you look at the original wiring use on coaches prior to the widespread use of relays, the circuits were over 100 feet (front of coach to back of toad and return). Combine small gauge wiring with a little corrosion and poor quality connectors and no wonder lights were dim. Some brake systems depend upon the brake signal from the coach to activate and with poor wiring towing safety is compromised.

Coach wiring and extension cables between toad and coach have improved, quality diode packs do exist and quality connectors are available. So where do most problems start? When RVers don’t take the time to understand what should be installed, how it should be installed and are willing to spend the money to ensure we are getting quality components. Dealing with a reputable shop is important; if they cheapen up on the components the day you drive off the lot everything will likely work, however the question is for how long?  If you are doing it yourself then it’s wise to seek good advice. The days of wiring a toad like we did a small boat trailer or cargo trailer are gone. Even quality cargo trailers are using the seven pin connectors.

You can buy cheap diode packs, or expensive high quality ones. The cheap ones not only perform poorly, they degrade rapidly and require replacement frequently. Every towing equipment manufacturer has a “made for a price line” and a “better quality, higher price line”. RVers need to be proactive in maintaining the lighting system by properly preventing corrosion on the connectors.

So what do we recommend? First of all use a six pin round connector on the towed vehicle and a six or seven pin connector on the Coach. This provides a better quality connector and allows for the additional wiring required for braking system purposes. Run the wiring in the toad inside the vehicle along the driver’s side rather than underneath it. Follow the tow system manufacturer’s recommendation regarding whether to use diodes or additional bulbs for the towed vehicle’s lighting.

If you are “rewiring” a coach use a high quality connector. Use Corrosion Block® on each wire connection and pack the wiring area inside the connector with di-electric grease to prevent corrosion. (This applies to the Towed vehicle connector as well). Cover the wiring in split loom for protection and secure it safely up out of the way. Follow the wiring code embossed on the plastic connector housing. The common short forms used, the wiring colors and the associated functions are:

GND    White        Ground
LT    Yellow        Left Turn and Brake
RT    Green        Right Turn and Brake
T/M    Brown        Tail light & Marker
S    Red        Brake or Stop
A    various        +12V Charge power

The Seven Blade “Trailer Connector” found on newer RV’s may be wired with different color wiring and on some older coaches (especially those with separate brake and signal lights) may use the S or Brake wiring for Brake lights and the LT and RT for turn signals. Others may use the Brake wire to provide power for a Trailer Brake system (or it may be used to feed power from a Towed Vehicle’s brake light switch to provide an in the coach warning that the Towed vehicle’s brake system has applied the Brake.
We recommend installing (as the newer coaches have) a signal/brake combiner which takes the three functions (left & right signal and brake) and combines them into a left signal/brake and a right signal/brake function which will allow a single bulb on each side of the Towed vehicle to provide those functions. This combiner would have its output (the two wires) connected to the LT and RT connections on the coach connector. Therefore it is installed before the connector.

Use quality components and a minimum of 16 gauge wiring in the Towed vehicle and the system will perform as expected. It is wise to apply a little Corrosion Block® to each end of the cable extension to ensure good connectivity without corrosion. Check the cable connections daily and always check all lighting functions each day prior to towing the Towed Vehicle.