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Tow Vehicle Wiring


The popularity of towing an automobile behind a motorhome has increased by leaps and bounds over the last 10-15 years. Unfortunately, the "gremlins" associated with wiring two vehicles together have thrived even more bountifully.

As most of you know, working with automotive wiring, especially the combination of two vehicles, can really test the patience of even the most skilled technicians. Blue Ox has compiled and created some of the most common wiring schematics, and troubleshooting tips concerning wiring a vehicle to be towed four wheels down.

First of all we should briefly visit the legal issues behind wiring a vehicle to be towed. Every state and Canadian province requires lighting on the rear of a towed vehicle that is controlled by the towing vehicle. This includes taillights, brake lights and turn signals. These lighting laws are very similar in nature to trailer towing laws.

There are basically three ways of wiring a vehicle for towing. The first way of accomplishing rear lighting is a light bar (BX8834). This is an apparatus containing lights with red lens' usually strapped or magnetically adhered to the top or trunk of the towed vehicle that is hard wired into the electrical system of the motorhome. This system is seldom used today due to the inconvenient nature of its design. The most popular method utilized over the last few years is to use diodes (BX8847 & BX8811) to interface the two vehicles wiring platforms together. The cleanest, least intrusive and quickest growing method is referred to as the bulb and socket (BX8869).


"Just what the heck IS a diode and why do I have to mess with them," is a very common question. A diode is nothing more than a one-way valve that allows electricity to flow through in one direction. When combining two vehicles' wiring systems together it is very important to protect both vehicles from the possibility of electrical feedback, especially in today's high tech computer controlled and monitored vehicles.

Two basic platforms exist in automotive wiring, the 4-wire system and the 5-wire system. The 4-wire system, most commonly used for years in the United States, has a combined left turn/brake wire, a combined right turn/brake wire and a tail light wire. All rear lights in this system are normally red in color. The 5-wire system, common among imported vehicles, has a left turn wire, right turn wire, tail light wire and a brake wire. The turn signals are normally amber in color. With these two platforms, 4 basic combinations can be encountered as follows:

Motorhome 4-wire system to a towed vehicle 4-wire system.
Motorhome 4-wire system to a towed vehicle 5-wire system.
Motorhome 5-wire system to a towed vehicle 5-wire system.
Motorhome 5-wire system to a towed vehicle 4-wire system.

Choosing the right receptacle

(Refer to figure 1) You must first identify how many wires you need to connect the two vehicles together for proper safe towing. Some vehicles being towed today are towable straight from the factory. The minimum requirement for these is a 4-way receptacle. Those vehicles that are not towable may require a lube pump to circulate the transmission fluid while being towed. Lube pumps require two additional pins on the receptacle. Most supplemental braking systems have an indicator light or switch mounted in the driver's area of the motorhome to alert the driver exactly when the towed vehicle brakes are being applied. This requires another pin in the receptacle. Individually, these are good candidates for a 6-way receptacle. Blue Ox offers a pair of 4 and 6-way receptacles packaged with a coiled electrical cable, BX8861 and BX8862 respectively. If a lube pump and supplemental brakes are both used, you must step up to a 7-way receptacle available at most RV dealerships. The 7-way shown is wired to standard trailer color codes so that the towed vehicle can be disconnected and a boat or other trailer connected without alteration.

With the exception of the 5 to 5 systems, all combinations without lube pumps and supplemental brakes can utilize a 4-way receptacle.

Wiring the vehicle to be towed

Please note that the Blue Ox 9-terminal diode block containing all three diodes (Figure 2) can be substituted for the individual diodes shown in figures 3 through 6. Blue Ox offers the following wiring kits with all necessary wiring and hardware included. BX8811-9-terminal diode block, BX8847-Three individual diodes, BX8869-Bulb & Socket kit.

Motorhome 4-wire system to a towed vehicle 4-wire system.

1. Install the standard 4-wire harness in the towed vehicle from the front bumper area to the rear of the vehicle where the existing wire loom feeds the taillights and turn signals. On most vehicles, this will be inside the trunk area on the driver side. Be certain to protect the harness by going through the frame, under the kick plates or by using a plastic wiring loom.
2. Install a 4, 6 or 7-way receptacle on the front of the towing vehicle as chosen previously. The main thing to remember is that the receptacle on the towed vehicle will be wired exactly identical to the receptacle on the motorhome. Be certain to locate a good solid ground, as this is usually the gremlin behind 90% of all wiring problems experienced. Attach the white wire here using a crimp on round terminal.
3. Molded diode modules, such as those illustrated from Blue Ox, have two input terminals and one output terminal. Identify the towed vehicle's existing right turn/brake, left turn/brake, and tail light wires in the trunk using a continuity tester. Each of these will be cut, one at a time, spade terminals installed and connected to the diode blocks as shown in figure 3. Be sure the wires you cut do not also feed the center third brake light. If you tie into these it will blink with one of the turn signals.
4. The 3 remaining wires from the 4-wire harness previously installed will then need to be connected to each diode block with spade terminals as shown.

Motorhome 4-wire system to a towed vehicle 5-wire system.

Follow the steps outlined in the 4-wire to 4-wire instructions with the following exceptions. In step three you must identify two different wires as well as the tail light wire, the left brake and right brake, in the trunk using a continuity tester. Each of these will be cut, one at a time, spade terminals installed and connected to the diode blocks as shown in figure 4. Sometimes it is necessary to use extra wire to extend existing wires to facilitate a better location of the diode block and easier installation. The amber turn signals are separate from the brake light signals and will not be used. NOTE: It is not legally mandated to use the rear amber turn signals on the towed vehicle if so equipped.

Motorhome 5-wire system to a towed vehicle 5-wire system.

Our recommendation is to install a Blue Ox Max-Lite tail light converter to change the motorhome from a 5 to a 4-wire system as shown in figure 6. Then it's as easy as wiring it like the 4-wire to 5-wire schematic, ignoring the separate amber turn signals on the towed vehicle.

If you choose to wire this system as is (5-wire to 5-wire) you once again follow the same steps for the 4-wire to 4-wire with the following exception. In step three you must identify four wires in the towed vehicle. These are the right turn, left turn, tail light and brake light wires in the trunk using a continuity tester. When identifying the brake wire be certain it is the wire that feeds both the left and the right brake light.

Each of these will be cut, one at a time, spade terminals installed and connected to the diode blocks as shown in figure 4. An extra wire must be added for the ground, as there are only four wires in the wiring harness. NOTE: If using the 9-terminal diode block, an additional diode must be added for this configuration. The left, right and taillights can go through the 9-terminal block and the brake will go through the extra diode.

Installing the Max-Lite Tail light converter. (BX88163)

1 The Max-Lite is used to convert a motorhome's existing 5-wire system to a 4-wire system. Test each wire in the rear receptacle of the motorhome to determine which wire carries each signal. Mark each of the five wires as Left, Right, Tail, Brake and Ground. Detach the wires from the receptacle and attach them to the 5 wires going into the tail light converter following the drawing in figure 6.
2 Locate a suitable spot to anchor the Max-Lite converter, hidden from sight, at the rear of the motorhome so the four wires coming out are within reach of the towing receptacle. Attach these four wires according to one of the drawings shown in figure 1 depending on which receptacle you have on the motorhome.

Installing a Bulb and Socket Wiring Kit. (BX8869)

This method is far and away the simplest and fastest growing way to wire a vehicle to tow. The bulb and socket was designed by Blue Ox for the Jeep Grand Cherokee because it has a VIC (vehicle information center) that lets the driver know when tail lights or turn signals are not operating properly. Adding diodes into this system will give false bulb out indications. Since it's development the bulb and socket's popularity has grown, as technicians are putting them into everything they can fit them into due to their simplistic installation procedure. I've seen some pretty inventive Technicians when it comes to avoiding diode installations.

To install a bulb and socket you must first determine if there is room in the existing taillight housing for the extra socket by taking the taillight assembly out of the vehicle and inspecting it. Figure 7 shows Blue Ox's bulb and socket. It requires about 3/4" of clearance for installation. Things to look for and avoid are inner reflective material, existing wiring and printed circuit boards that are molded into the tail light housing. Once your location has been determined, drill a one-inch hole in the housing using a hole saw and insert the socket with bulb.
Follow steps 1 and 2 in the diode section for 4 to 4 systems. This will get your 4-wire harness and receptacle installed on the towed vehicle. Using figure 8, connect the sockets to the 4-wire harness and ground them properly using the self-drilling screws provided.

"Gremlin Hunting" (Trouble Shooting)

They say cute little fuzzy gremlins do not like water or bright light and they should not be fed after midnight or they turn into nasty ferocious little critters as proven in the 1984 movie. Keep these valuable hints in mind when hunting gremlins in your electrical system.

Water. It couldn't be truer as water can really enhance electrical problems through short circuits and corrosion, especially salt water. During the wiring process you should take special care not to nick or cut into any of the wire harness that may be exposed to the elements. All electrical connections should be kept free from moisture by either using a heat shrink covering or dielectric grease. Electrical tape is an option, but should be inspected often as it does not last forever. 3M offers a very resilient electrical tape that lasts longer than most and is weather proof. It is called 3M 33+.

Bright Light. Yep, it's a must. When working on wiring, a bright work light will chase away many gremlins. Technicians must be able to clearly make out wiring color schemes, and possible exposed wires. Identifying these things right at the time of installation could save you hours of painful searching for the source of electrical problems.

As for the third issue……..I would expect that if your still up after midnight; your wiring project is not going so well. Don't give in to the little critters. Get a good nights sleep and try out some of the following gremlin hunting tips in the morning……Happy hunting.

Troubleshooting Approaches

There are two approaches to troubleshooting. The first is based on knowing what is wrong. This can be describe as the "I've seen this problem before" approach. Finding the most likely problem is the easiest way to troubleshoot, but it doesn't always work. What would happen if you changed a bulb, and the problem didn't go away? Would you change the socket, the light fixture, or redo the wiring entirely? It's often this next step that gets us into trouble when the easiest way doesn't work. We start "Flock-Shooting" or just changing stuff. This can waste a lot of time and cost a little money. It can also be the source of induced failures, which really compound the problem. In case you're not familiar with the term, an induced failure is when you break something while trying to fix something else. We've all been there before.

The second approach is based on knowing what is good. For example, the right rear turn signal stops working on the towed vehicle. You check for a signal coming out of the motorhome's receptacle and discover it isn't there. I prefer to use a lighted continuity tester with a needle point prod and a wire lead for ground. If the bulb comes on when its connected, you have voltage and current, and current is what makes a bulb glow.
Therefore, you've eliminated the towed vehicle as being the problem. Next you check the motorhome's turn signal and discover it works correctly. Therefore, you conclude the motorhome's basic wiring is good. You remember the other lights on the towed vehicle were working correctly. Therefore, you conclude, the ground wire is good. At this stage, you conclude there is only one wire left to check and that's probably where the problem lies, the wire supplying the turn signal to the receptacle.

This second approach to troubleshooting is the best approach when you have a difficult problem. Systematically go at things and keep narrowing the area where the problem might be - eliminate what is known to be good. Don't assume anything; prove that everything is good. Too often, it's the stuff we assume is good that comes around and bites us. Yes, gremlins do have teeth!!

Motorhome Wiring

When something goes wrong with the lights on a towed vehicle, people automatically assume the problem is in the towed vehicle. However, reality sometimes tells us something different. A lot of the time, the problem is in the motorhome. If the "I've seen this problem before" approach doesn't fix the problem, start with the motorhome. Make sure all the lights are working correctly on the motorhome. Once this checks out, test the receptacle at the rear of the motorhome. Prove to yourself that the motorhome is putting out the correct signals on the correct connector pins. Again, I prefer to use a lighted continuity tester with a needle point prod and a wire lead for ground.

To test the signals at the receptacle, turn on the taillights and then connect your test light between the white wire pin and the brown wire pin. The test light should come on. Touch the other pins in the receptacle, one at a time and verify there is no signal on any other pin. Turn off the taillights, and then repeat this basic test approach for each turn signal and the brake lights. Remember, always connect the test lead to the white wire pin (ground) and see what is on the other pins. The test light should only come on when you are on the correct pin. Also keep in mind, when testing the brakes, the test light should come on at both the yellow and green wire pins.

Common Motorhome Problems

If the problem at the receptacle is that the test light never comes on, then the problem is probably a broken ground wire or a bad ground connection. Trace the white wire from the receptacle back to where it connects to the vehicle frame. If it doesn't connect to the vehicle frame, then connect it there. A good connection requires a crimp termination (round lug) on the end of the wire, a metal screw or a bolt and a self-locking nut, and a star washer. The frame connection point should be bare shiny metal. To test this connection point, turn on the vehicle's running lights and connect the test bulb between this point on the frame and the brown wire pin on the vehicle jack. If this connection point is good, then check for an open or broken ground wire.

If the test light fails to come on at one pin, then you probably have an open wire going to the receptacle. Retrace the wire back to it's connect point and ensure the wire is not broken. Also check to see if it has a clean; mechanical sound connection that is protected from the weather elements. If in doubt, redo the connection.

If the test light comes on at too many pins on the vehicle's jack, then the wires are probably shorted together somewhere. Retrace the wires back to their connection points and look for places where two or more wires are pinched together. If the wires are good, make sure the connection points are not shorted. If the connections are physically right next to each other, separate them a couple of inches to avoid the potential for a short circuit. If the test light comes on at the wrong pin, then the wires are probably connected to the wrong connection points. Retrace the wires to the connection points and reconnect them correctly.

Towed Vehicle Troubleshooting Basics

The most important part of troubleshooting a towed vehicle problem is a good visual inspection. Check all ground connections and make sure they are clean, mechanical sound connections that are protected from the elements. Examine all bulbs and light fixtures up close. One of the most common problems with towed vehicle wiring is a bad ground connection. When troubleshooting the towed vehicle, do not connect the tow bar between the two vehicles and do not connect the safety chains or cables. These can provide false grounds to the towed vehicle. The only connection between the two vehicles should be the electrical plug connection.

Strange Towed Vehicle Problems

It's very difficult to get people to check the ground connections, even though bad grounds probably cause most wiring problems. Some of the strange symptoms of bad ground connections are things like; the lights on the wrong side of the towed vehicle come on; the lights on one side are brighter; the lights are on, but they're dim. Here's what's happening when these type of symptoms show up.

Some of the bulbs on the towed vehicle have two filaments in them. One filament is for the taillight; the other is for the turn/brake light. Each filament has a wire going to it. Both filaments use the same return, the base that is connected to the vehicle's frame or ground. Under normal situations, current flows from the positive terminal of the battery, through the wire to the filament, through the filament, through the base, through the ground, through the vehicle frame back to the battery negative terminal.

What happens if the ground connection is bad? Well, current flows from the positive terminal of the battery, through the wire to the filament, through the filament, through the base. Okay, it can't go out the ground because the ground is bad. So, what happens? Well, it goes back from the base through the other filament, through the wire to the bulb on the other side of the towed vehicle, through that filament, through that base, through that ground, through the vehicle frame back to the battery negative terminal. When all this happens, the filaments won't have the full voltage across them, so they will be dimmer, but on one side two filaments could be lit and that should appear brighter unless the other side filament is the brake filament. Okay, you get the picture. When things start appearing to be real strange, start looking at those ground connections.

In summary, wiring problems can appear to be complicated. However, a systematic approach based on eliminating things that are proven good can help you focus in on the real problem. If you remember anything from this article, please remember, when strange things start happening, check those grounds!

By Mark Penlerick
Engineering Team Leader
Blue Ox Towing Products


Submitted by Mike Sundberg - 3/28/06

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