Cruisair Electronic Controls

R e t r o f i t t i n g
Cruisair Electronic Controls
The SMX II electronic control unit can be added to existing
motorhome air conditioners to take advantage of current
technology and automate several functions.
By Carol F. Maxwell &
E. S. Gurdjian
The SMX II is a microprocessorbased electronic control that is part of
current Cruisair air-conditioning
units. It is of particular interest to
those who have coaches with older
Cruisair versions because it can be
retrofitted with ease.
The SMX II replaces previously
used electromechanical control assemblies and enhances performance
by providing several automatic functions. After the coach owner enters
the desired temperature on the control
unit, the fan can be set for manual
control, ranging from slow to fast, or
on the automatic setting, where fan
speed is controlled by room temperature. The SMX II includes several
programmable parameters, including
a choice of Fahrenheit or Celsius
temperature displays, the range of fan
speed, compressor differential, compressor restart delay, and a dehumidification program. These parameters
are in the program's non-volatile
memory, which retains the settings
even when the power is interrupted.
The retrofit kit consists of a control
panel, a low-pressure switch, a power
logic module, a thermistor, and a cable to connect the control panel to the
power logic module. Solid-state relays became standard in 1989, so an
older air conditioner may also need to
have this relay added. Equalizing
valves became standard in 1988, but it
is estimated that most units that were
not so equipped originally have already been retrofitted. For information about the installation of these
valves, contact AAP Inc. directly.
The installation point of the power
logic module and the solid-state relays is best determined by how each
individual unit is wired to 110-volt
AC. In some installations, these com-
ponents will be more easily installed
near the condenser; in others, near the
evaporator. In our application, the
110-volt AC supply and the electromechanical three-knob control assembly (switch assembly) are in close
proximity to each other near the
evaporator. This position also provided access to the terminal strip for
the original switch assembly. The
new control panel was installed, and
the inside air thermistor was installed
in the return air path. Once the power
logic module was fastened in place, it
was wired to the terminal strip by
removing the wires from the threeknob control and connecting the same
color wires from the power logic
module in their place.
The solid-state relay was easily installed. It was mounted on an aluminum hear sink (6 inches by 12 inches
and .080-inch thick), and then wired
according to the instructions supplied
by Cruisair.
1. White wire to terminal 4.
2. Yellow wire to terminal 3.
3. Black wire to terminal w.
4. A new purple wire that is run
from terminal 1 to the condensing unit.
It is necessary to install a 12-gauge
purple wire and two 20-gauge
paired wires between the power
logic module and the condenser
unit. The 12-gauge wire is used to
operate the compressor condenser
blower. The two paired wires are
used to connect the high and low
pressure switches to the power
logic module. An M.O.V. spike
protector is installed across L1 and
Note: Directions for all wiring are
clearly detailed by Cruisair in the
instructions included in the kit.
The installation is completed by
connecting the cable from the
thermistor to the power logic module, and the interconnect cable
from the control panel to the power
logic module. After installation, the
factory-set parameters can be reprogrammed for individual need if
We used the SMX II unit in our
coach for more than 10,000 miles
during the summer of 1994, a time
when some extremely warm temperatures were recorded. The
automatic variable fan speed notably increased the comfort level in
the coach. We no longer need to
readjust the temperature or fan
speed to maintain the desired room
temperature. This is especially
convenient in the bedroom. No
more waking up in the middle of
the night with chattering teeth and
the feeling of having slept in a
wind runnel. Precise climate control can be attained and maintained
with very low fan speed.
Although we have not specifically
used the dehumidifying program, it
would probably be of value to anyone who stores his or her coach in
humid areas. This feature should
prevent mold and mildew and their
associated odors.
Most appreciated, however, are the
various protection programs that
are built into this control unit. The
value of preventing expensive and
serious damage to the air conditioners as a result of inappropriate
electrical supply and refrigerant
pressures is obvious.
The SMX II cost approximately
$500, and the installation requires
three to five hours.
For more information, contact
AAP. Inc. P.O. Box 430, Milford,
VA 22514; (804) 633-9454
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