The problem could be the thermostat, and any of the following compressor cycling for every few minutes
could be over load for the compressor, dirty condenser coils need
cleaning, To check
properly you should test it with an amp
compressor is rated in running amps ( see model/serial tag or the
on the compressor )...if the compressor is drawing too much current,
may be why it is shutting off. Check the model tag for proper amp
rating. Compressors can
also seize and click on and off or one of the electrical windings inside
could have opened up. You can use a test cord to help
check the compressor and this also will help with the amp test.
Nevertheless, this is the procedure to replace the thermostat on your unit.
- Get the tools together that you will need to do a proper thermostat
installation. You will need:
- A small straight-slot (or flathead) screw driver
- A small Phillips screw driver
- A pair of needle nose pliers
- A utility knife or wire strippers (for small wire)
- Plastic wall anchors (sometimes provided with the thermostat)
- A drill with a bit to make the holes for the plastic wall anchors
- A small level
- Two pencils or pens
- A small paper bag and some masking tape (tape the bag below the
area where the thermostat is so that any trash or dust will
fall into the bag and not onto the floor)
- Some touch up paint
- Clean hands (don't do a great job changing the thermostat and leave
all those prints all over the wall)
- Plenty of light
- Turn the power off to the air conditioning and heating unit at the
circuit breaker or the emergency cutoff switch. This should kill
any power going to the thermostat. After doing that make sure the power
is off by turning the thermostat to the on position and going to
the unit to make sure it is not on. Not all circuit breakers are
labeled correctly and not all emergency switches are hooked up.
Just make double sure that you have killed power to the unit not
only for your safety but also to keep from blowing the
transformer. I get calls all the time to replace transformers because
the homeowner changed the thermostat and didn't kill the power. They
hooked everything up correctly but during the process they
touched the wrong wires together and blew the transformer which
powers the HVAC control circuit including the thermostat.
- Pull the cover off the front of the thermostat. If it is a
mechanical thermostat there should be a little adjuster tab in the
center of it. This is your heat anticipator. It should have numbers
ranging from 1.5 to .1. Take note of this setting and remember to
set the new thermostat to this same setting if you are replacing a
mechanical thermostat with another mechanical thermostat. You
probably want to do this now before you proceed further. If you
are replacing a mechanical thermostat with a digital thermostat, the
digital thermostat should set itself automatically. If not read
the instructions on the new thermostat for instructions on how to
set the anticipator. This is very important. An improperly set
anticipator will cause your furnace or heater to run improperly.
The thermostat is also equipped with a cooling anticipator.
Cooling anticipators are often on the sub-base and are
- Unscrew the thermostat from the sub-base. Take note of each
wire. The following list should match the wires and terminals on
These are the four wires that you need to control the heat,
cooling and the blower or fan. If the colors of the wires do
not match the colors described here make sure you mark the
wires with masking tape. If there are more wires that are not
hooked up don't worry. This is common. Thermostat wire comes
in many different varieties and the contractor who installed
the system probably used 5 wire or 8 wire thermostat wire.
They used what they needed and simply twisted or cut the other
- Red to the thermostat RH or thermostat RC terminal with a jumper
wire between thermostat RH and thermostat RC. Or Red to the
thermostat R terminal which is shared with both the heating
and cooling. It has an internal jumper built in to the
sub-base. The red wire is the source hot wire from the
transformer. All other wires, except the common wire, controls
a specific relay or contactor that energizes the fan, heating, or
cooling depending on the selection. The following is the
common wiring colors but your system may not be common and
different colors could have been used.
- Green to the thermostat G terminal. This is the color that controls
the fan or the relay that control of the blower.
- Yellow to the thermostat Y terminal. This is for control of the air
- White to the thermostat W terminal. This is for control of the
- Remove the wires from the terminals on the sub-base. The power
should be off so you shouldn't have to worry about being shocked.
Be careful not to let the wires fall back into the wall.
Sometimes there is just enough wire to reach the terminals and
that's it. Try pulling the wires a bit to see if there is more
wire behind the wall. Most of the time there is some slack and you
can pull the wire out more. Unscrew the sub-base from the wall
while holding the wires. When you get the sub-base off wrap the
wires around the pencil or pen. This will keep the wires from
falling back into the wall.
- Get the new sub-base and compare it to the old one. Hold it up
to the wall in the position you want it. Is the old paint that
was covered by the old sub-base going to be covered by the new
sub-base? If any of the old paint is going to show you may want to
make some touch ups now. After finishing with that, put the new
sub-base back on the wall in the position you want it. Make sure
it is as level as possible. You can use a level to do this.
(This is very important especially for mechanical thermostats. It
must be level or the mercury switch will not keep the proper
temperature settings in the house. Make sure it is level.) Mark
the new holes through the sub-base where the screws will go into
the wall to fasten the sub-base.
- It is important in this step to have the proper drill bit size
for the size of wall anchors you have. Some wall anchor kits come
with a bit in them. I recommend the wall anchor kits with the
bits in them because it is the perfect size drill bit for the anchors.
The bit should be slightly smaller than the anchor. If the bit is
bigger the wall anchor will not hold and the possibility exists
that the thermostat will fall off the wall. Drill the mounting
holes you made for mounting the sub-base. Insert the wall anchors
and push them hard with your thumb. Approximately 1/16th of an
inch on the lip of the anchor will remain sticking out of the
hole. If it is more than that use the ****-end of the screw driver
and push it in until just the lip of the anchor remains visible.
- Undo the wires from the pencil or pen and run them through the
center of the sub-base. Insert the screws and screw them only
snug tight. Get the level and make sure the sub-base is level.
When you are sure that it is level, tighten the screws. Be careful
not to allow the sub-base to move when you are tightening the
- Using the color code of the wires (or if they didn't match,
the color markings you made with masking tape), attach each wire
to their proper terminal. Some people like to loop the wire around
the terminal screws. This is not necessary. What is necessary is
that the wires are attached to the terminals and they are tight.
Additionally, make sure that none of the bare wire is touching
anything except the terminal. Once the wires are attached you are
almost finished completing the task of installing the thermostat.
The hard part is over!
- Attach the thermostat to the sub-base. The screws for this are
built in the thermostat. Tighten these screws and check to make
sure the heat anticipator is set to the same setting as the old
- Attach the front cover to the thermostat and restore power.
Start and check the heating, air conditioning, and with the heating
and air conditioning off, the fan only sequence. All systems should
be working properly at this time (if you did the task properly) you
are the proud owner of a brand new, properly installed thermostat.