By
Mark Penlerick
Engineering Team Leader
Blue Ox Towing Products
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).
Diodes
"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!
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