With more and more vehicle makers requiring some method of maintaining the battery in a towed vehicle while it is being towed, it seemed like it was a good time to bring this topic forward in a TechTip. Towed vehicles have more and more computers running in the background when the vehicle is not being driven. Obviously, the passive entry system that recognize the presence of the "key” when one approaches the vehicle is just one example of a computer running in the background. When one prepares to tow a vehicle, the sequence that the vehicle’s computers expect to go through, is disrupted. Some computers that shut down shortly after the vehicle is entered or started, are suspended in action and do not turn off. This means that these computers or modules are running while the vehicle is being towed. On newer vehicles push button ignition and electric or electronic (steer by wire) power steering systems, many need to be running when towed, draining a towed vehicle battery. The future will provide more of these scenarios.
Sometimes removing a fuse eliminates the drain, however when the start-up sequence may be disrupted, then multiple fuses will need to be pulled. Some makers require that the negative battery cable be removed when preparing to tow, since up to ten Body Control Modules (BCM) or computers can be operating; removing a handful of fuses is impractical. Each approach has advantages and disadvantages to a RVer. Add into this mix, the fact that most vehicle makers are unfamiliar with the systems already available on newer RV chassis’ (such as Ford, Freightliner and Spartan) to meet the maintenance charging need. As a result, they default to recommending a third party Towed Charger or Towed Battery Maintainer which needs to be installed on the RV as well as the towed vehicle, often at great effort or expense. Almost all chassis makers have provided most of what is required as standard equipment on their RV chassis. Simply connecting the Tow Vehicle to that provision is practical and cost effective.
The History of Towing:
A little history will provide some perspective here. During the 80’s and early 90’s when modern RV’s became popular, towing a vehicle was the exception, not the norm. Those RV’s were equipped with a ‘four-wire’ connector designed to power the lighting on a boat or utility trailer. Towing a car was not considered, especially since the frame section to which the hitch receiver was mounted, was often added by the RV body maker. This frame extension was primarily planned and installed to carry the rear bedroom of larger (30-34 foot) RVs mounted on chassis’ which were designed for much shorter delivery trucks. This was the time when there were no specific RV chassis designed for the RV market. Conversion was the only way a chassis could be used to carry RV bodies.
As the market began demanding longer motorhomes, the industry responded by building ‘purpose-built’ RV chassis that were longer, had better powertrains, brakes, suspensions, electrical components and so on to meet the RV maker’s needs. The towed vehicle hitch receiver was installed on the actual vehicle chassis maker’s frame however the wiring continued to be a four or possibly six wires accessed through a matching number of pins in a round electrical connector. As the capabilities to tow larger trailers and vehicles increased, the makers moved to six (6) pin round or the seven (7) pin blade connectors that allowed for a trailer brake controller mounted in the RV to be connected to a trailer. Legislation began to appear that required electric brakes on trailers over a certain weight or carrying capacity and these trailers needed to have a breakaway switch to activate the trailer’s (electrically operated) brakes. This required a source of power (a battery) to be mounted in the trailer to power those brakes when the trailer became separated from the towing vehicle, activating the breakaway switch which applied the trailer brakes.
Around the same time, RV owners began to tow small cars and they were met with the need to comply with the braking and breakaway requirements, which the aftermarket jumped to fulfill. The power source was the vehicle’s own battery which was far larger than those found on trailers. There was little concern about the towed vehicle’s battery running down while the vehicle was being towed.
With the rise of larger tow behind and fifth wheel trailers, the vehicle makers began offering trailer packages on their pickup trucks and these often included a pin in the trailer electrical connector to charge the trailer battery while the trailer was being towed. (See Photos) The trailer and fifth wheel makers started using the black plastic seven pin blade connectors that mated to the standard pickup truck connector. It was a little of the chicken and egg question since the towing vehicle needed to supply the wiring connector and the towed vehicle need to have the mating connector.
Prior to this time, the RV industry had little standardization of the rear connector, however between the economy of scale by RV makers using one standard connector and the RVIA (which sets industry standards and enforces compliance) addressing the issue, the industry adopted a seven (7) pin blade towing connector. This connector had to have, at a minimum, a ground (-) connection, and one for the running/marker lights and both signals which doubled as brake lights. Some had a separate brake light feed as well as a backup light connection. Variations included having separate brake and signal connections. Add in the Electric Brake line and a Charge (+) line and all seven pins were used depending upon the configuration. With this connector standardized, the RV chassis makers could integrate the ‘trailer wiring’ needs into their chassis offerings and then RV makers could choose to simply use the supplied connections and connector or modify to meet their own wiring needs.
This brings us to the present time. Virtually all modern RV chassis makers have a charge line available at the rear towing connector that is integrated into their electrical system. Some have this charge line controlled by the RV ignition so that simply being connected does not supply power, however driving the RV does power the charge terminal in the connector. Often the face of the RV connector cover has the pin layout which shows what connection is in each position on the socket. (See Photo) Confirm with the body builder that there has not been any variation from what is shown on the cover or in their manual.
Properly Wiring a Tow Vehicle
While all makers of flat towing systems have a ‘four (4) wire’ round connector, most can supply a six (6) wire coiled cable with either a six (6) wire connector on either one or both ends. In today’s RV towing environment, four (4) wires are not enough, yet installers and shops are often slow to change. The connector on the RV end of the cable may be a seven (7) pin blade connector with a four (4) or six (6) pin connector on the other (tow vehicle) end. The latter are smaller and are a better fit for the front end of a tow vehicle. The aftermarket has jumped into the void with six (6) pin round to seven (7) pin blade adapters that accept the six (6) pin connection from the coiled cable which is plugged into this adapter that fits the connector on the coach. Some of these adapters have an internal wire that can be moved to provide the battery charge connection on a specific terminal. We highly recommend the six (6) wire cable and connector approach to wiring a towed vehicle for the convenience, safety and flexibility that this alternative provides. An adapter placed on one end of the cable extension becomes the coach end of the cable.
It should be cross wire-tied to the cable as shown in the photo to ensure a tight connection between the two. The RV towing companies have produced cables that have a six (6) pin round connector on one end and a seven (7) pin blade connector on the other end. This is also a suitable alternative, although smaller dealers may not stock this cable. The use of the six (6) pin version and adapter is always a good alternative.
Tow Vehicle Wiring for Battery Charge Maintenance
So where does that leave the RVer which has a seven (7) pin blade connector on their RV?
Note: If the tow vehicle maker requires a fuse (or fuses) to be removed to flat tow, then follow that direction. You can install a RVing FuseSwitch™, (or a similar Bypass Switch or Fusemaster to simplify the fuse removal). A charge line does not eliminate this requirement.
First, review the chassis or RV Maker’s owner’s manual or call their Customer Service team to confirm that the connector has the charging source of power, what the terminal layout is and whether a fuse or circuit breaker needs to be installed to allow the power to flow to the rear connector. Many newer coaches have the connection and on some, it is controlled through the ignition switch.
Wiring the towed vehicle using the six-wire cable connection, requires that the wire from the battery (more about making the connection on the towed vehicle follows) is connected to the ‘A’ or ‘A+’ terminal molded into the back of the connector. Use a 14-gauge (or larger) wire preferably with Red insulation, so that it is easily distinguished from the others and is large enough to carry the necessary charge current (usually a maximum of 15 amps). The other critical connection is the ground (marked G or GND) on the connector and it is the single larger pin of the six (6) pins. Use a 14-gauge (or larger) wire (preferably White or Black in color), then run both wires up to the Underhood Fusebox area of the towed vehicle. Ensure that the Red-wire is protected with plastic loom where it passes near sharp metal and that it is routed safely around moving or hot components. This ground (White or Black) wire should be connected using an appropriately sized ring connector to one of the studs that the vehicle maker provides for grounding under the hood (see photos). The white wire from the flat four wire ribbon cable that is used to provide power to the taillight/marker, left and right signal (& brakes) can be separated from the ribbon, cut to the required length and connected to this stud as well. This saves the challenge of trying to install two ground wires into the tow vehicle connector.
The Red (+12V) wire should be connected to one terminal on a 15 Amp, 12-volt circuit breaker using a suitable ring connector or fuse holder for the plug-in style circuit breaker. Alternatively use a suitable fuse holder that will accept a 15-amp fuse and attach it using a butt connector to the red wire. The advantage of using an auto reset circuit breaker is that if it is over loaded it will reset and if the overload is cleared, it will continue operating, otherwise it will continually cycle on an off until the issue is resolved. A fuse in the same scenario will blow and need to be replaced. It is a personal decision as to which one to use.
If using a fuse holder then install a ring connector on the other of its wires and connect it to one of the battery terminals found in the Underhood Fuse Box (shown circled in the photo). Likewise, if using a standard automotive circuit breaker then put a suitable ring connector on each end of a short red wire, connecting one to the battery terminal in the Underhood Fuse Box and another ring connector to the other remaining circuit breaker terminal.
The RV’s charging system will charge/maintain the tow vehicle battery in the same way as it charges and maintains the chassis and coach batteries while the RV is being driven. It is important to ensure that the tow vehicle battery is not severely discharged since the wiring is designed to maintain a fully charged battery, not act as a boost to this battery.
This approach eliminates the need for an accessory towed charge system and is really all that is required. Using the existing tow cable extension between the vehicles is safer than an additional wire harness. Since this approach uses the chassis maker’s protection system and wiring up to the chassis connector, which greatly reduces or eliminates the work involved on the RV. A RVing Tow Vehicle Battery Maintainer Kit is available from RV Parts Plus, should you prefer to order a Tow Vehicle kit.
If the RV Chassis does not have a provision for battery charging/maintenance then the RVer can decide whether to install a similar circuit breaker, or fuse holder in a new wire run from the chassis battery to the rear towed vehicle connector. Ensure that there is also a good ground connection to the connector. Alternatively use a purpose-built aftermarket maintenance/charger making the same type of connections using their kit.