June 4, 2017

TT #97 Choosing a New Tow Vehicle: Preparing it to Flat Tow (Part 2)

By: Rob Lowe

 In the last Technical Tip, I discussed the towed vehicle selection process, the installation of the baseplate (bracket) and selecting a tow bar. In this article, we’ll look at the some of the considerations surrounding the selection and installation of tow vehicle wiring. 


New Lighting & Wiring Options:

While it is possible to use an independent taillight, brake light and signal system that does not require much physical installation, most RVers chose to install more permanent rear lighting in the towed vehicle. There are two alternatives: Installing a separate bulb and socket into the existing taillight housing (known as a ‘Bulb & Socket Kit’) or using diodes (known as a ‘Diode Kit’) which utilizes the existing vehicle taillight and brake light to serve as a dual-purpose RV and vehicle powered lighting unit. With the rapid increase in L.E.D. rear lighting on tow vehicles, some challenges arise and I’ll discuss them in this article. 

The towing system makers provide information about which approach to take for a specific towed vehicle, as part of their towing component selection recommendations. This recommendation may vary with the ‘trim level’ of the towed vehicle, since some higher trim models often come with L.E.D. rear light assemblies whereas the lower trim versions may be equipped with conventional incandescent rear lights.  Frequently, a bulb and socket kit is recommended for vehicles that utilize incandescent light bulbs for the vehicle’s taillight, braking and signal light system. These kits may include an incandescent bulb or a smaller L.E.D. bulb that is inserted into a hole drilled into the red lens brake light area of the vehicle’s taillight assembly. These new bulbs are powered exclusively by the towing RV and there is no connection to the towed vehicle’s electrical system. They share the rear red brake and taillight lens assembly so that the towed vehicle complies with some state and all provincial requirements to have a red (not amber) brake light. For vehicles where a separate bulb either will not fit into the rear light assembly or on a vehicle that has a sealed L.E.D. light unit, the Diode kit is the recommended approach to supplying lighting for the RV towed vehicle. 

Installing the wiring:

Running the wiring (usually a ‘flat four wire cable’) from the front to the rear of the towed vehicle is usually not too difficult. There may be additional wiring for a braking system notification light and/or a battery charge line. While some installers simply run the lighting cable along the underside of the vehicle and tie it up at different places along the undercarriage, I strongly recommend routing the wiring along the door sill area from front to back using the location that the vehicle manufacturer uses to route the wiring for the existing lighting. My recommendation is based on safety, since the wiring is protected inside the body from road damage and weather deterioration. If the wiring must be placed under the vehicle ensure that it is enclosed in a suitable wire loom and well secured to the frame assembly away from moving parts and exhaust components as it goes from front to back. Use grommets to feed the wire into the rear taillight areas if the wire needs to enter the rear of the vehicle to make the connections.
Wiring locationAlternative Wiring locations
Since the wiring was roughed in when the baseplate (bracket) was installed at the front of the vehicle, route it over to the driver’s side fender area and locate the large rubber grommet where the vehicle’s electrical wiring passes from the engine to the passenger compartment. Often it is near the power brake booster mounted high up on the firewall near the fender. On the vehicle that we recently were preparing to tow, the grommet was on a secondary ‘firewall’ and barely visible from the engine compartment. The first photo shows the wiring harness location on the first firewall.  The second grommet was just visible when a large corner rubber molding (two arrows in photo to the left) at the back of the hood near the base of the windshield was removed. While it was not possible to take a photo of this grommet, the process for getting the wiring through to the passenger compartment was not too difficult. Often this grommet can be accessed from the passenger compartment, although it may be hidden by sound insulation. Follow the large bundle of wires towards the firewall to find it. The second photo on the right shows us that, some vehicles have multiple locations where wires pass through. Select the best one and using an opened-up coat hanger to carefully hook the wire into a small loop at the end of the hanger, wrap the loop with electrical tape, spray some silicone on the tape and gently pull the coat hanger with the wire through the grommet. Ensure that no other wiring gets caught. Remove the coat hanger and tape.

Direct the wiring down the driver’s side kick panel, which may be loosened and removed if required, by pulling it away from the sidewall. Lift the sill plates at each door opening by grasping the plate on each side and pulling it up and away from the carpet, metal sill and the adjoining plastic pieces. The left photo below shows the area between the carpet and the door sill with the factory wiring harness just under the carpet, while the second photo shows the factory wiring harness as it enters the plastic covering. Notice the new wire that has been run beside it. The track provides a guide for the new wiring as it is directed to the rear area of the vehicle.  
Wire TrackPlastic Wire Channel in Wiring Track




















L.E.D. Lighting on Late Model Vehicles:   

If the vehicle has L.E.D. rear lights, then there are couple of ways of that they may operate. On some vehicles, the same L.E.D. ‘bulb’ glows dim when in the taillight mode and brighter when in the brake and signal light mode. If the control board is located within the taillight housing then two separate wires come into the board from the vehicle. The L.E.D. light shines bright for brake/signal if there is 12 Volt power on the brake/signal wire and dim for taillight if the 12Volt power is on the other wire. The other approach uses the vehicles body control module to control the brightness. That module is in another location within the vehicle and does not function if the ignition is off. These light units will have single power wire connected to the L.E.D. ‘bulb’. If the wiring from the RV for the taillight and signal/brake wires are connected to this single wire, the ‘bulb’ will shine at full brightness all the time. In this scenario, the light mimics the ‘brakes being on all the time’ even when the taillights are on. This creates an unsafe situation, since applying the brake or turning on the turn signal does not alter the brightness. The interim solution has been to add a resistor inline with the RV taillight power feed to reduce the voltage to the ‘bulb’ (and therefore the brightness). The vagaries of this design for adding an aftermarket part is in trying to determine what resistance to use. It is more complex to handle the necessary lighting needs on these vehicles and beyond the scope of this article. Fortunately, a great number of vehicles have been designed with two separate light connections with the discrimination circuits contained within the L.E.D. light assembly. Properly connected Diodes, (See Blue Arrows below) will provide the correct operation of the vehicle rear lights when the vehicle is towed behind a RV. The baseplate (bracket) supplier should be able to confirm which lighting kit is required and what is the best method to use to connect to the rear lighting in the tow vehicle. Look at the prospective vehicle and trim level to determine whether it has L.E.D. rear lighting so that you can be certain what to expect. Follow the light kit installation instructions.

Most rear light fixtures are secured by screws, often with Torx® heads, sometimes covered by removable caps in the trunk or liftgate opening. (See the green arrows for locations in the photo below). With the screws out, the assemblies are withdrawn straight back on two or three pins that fit into the vehicle body to properly locate and secure them. (Yellow arrows in photos below).
Rear of Taillight
At the front of the vehicle, I recommend using a six (6) pin round connector mounted directly on the baseplate posts provided. This type of connector will not only provide all lighting connections, it has pins for a ‘Brake On’ indication in the coach as well as one for a battery charge line from the coach to the towed vehicle. The connecting extension cable should be a coiled with either an end suitable to fit the coach seven (7) pin blade connector or an adaptor that adapts the six (6) pin round to the seven (7) pin blade connector on the coach. It is far more secure than a connector dangling on the wiring.
Finished Installation showing wiring connector
Recently wireless remote RV lighting bars have been introduced. A transmitter is plugged into the rear RV connector and a self-contained lighting bar  is magnetically mounted on the rear roof or tailgate of the tow vehicle. This lighting unit has a long life (12 hours) battery which is charged by connecting an adaptor into the power outlet in the towed vehicle or RV when not towing. The shortcoming of this system is with the transmitter installed it prohibits a charge line or brake controller/warning light.

Hopefully this overview will assist you as you make decisions about the next vehicle to tow behind your motorhome. 



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