Trying to connect a rudd achiever series outside unit to 20 year old colemen thermostat. The rudd unit has 5 termianls on the circuit board,brown (C), purple (D), yellow (Y), blue (B), and red (R). The thermostat has white (IW2), green (G), red (R), blue (X), yellow (Y), and oranghe (O). As you can see the only commonality between the therm and the outside circuit board are the R and Y terminals. On what outside terminals does one connect the W2, G, X, and O connections from the therm? Note also the outside circuit board has only five terminals and the therm has six.

Need more info. If you have a heat pump outside you should have a wire landed on O. What was the external board and what was old tstat. Also is this electric heat or gas inside at furnace? Model numbers of equipment will help, where the wires are landed outside also.

Are you heating or cooling? When the outdoor unit shuts off how does it come back on. Or are you saying that the outdoor unit is short cycling off and on

You may have a variable speed blower that is not wired correctly to the thermostat or the problem could be the ciruit board.

Freezing up is usually from low freon or insufficient air flow across the evaporator coils.

Don't choose a plumber to fix your A/C unit. ;p

Turn the thermostat as low as it will go and have someone stand out side, they should here the condensor fan and compressor come on at the same time. They are two distinctive sounds so you should
be able to tell if the compressor is running.

If nothing happens and you are skilled with electrical
you can turn power off and remove the electrical
cover to the outside unit. Restore power and carefully
push in the contactor, if the fan and compressor
starts you have a thermostat or low voltage problem.

If just the outdoor fan runs on any attempt, more than likely you need a new capacitor.

not all digital thermostats are compatible with the manufacturers circuit control boards in the furnace. Hunter has had this problem several years ago. Other than that you may have miss wired the unit. usually the furnace is used as backup heat (during defrost) or emergency heat if your heat pump konks out. Are you switching to emergency heat for a furnace call?

Sounds like the Unit might be low on charge.But without correct readings not sure.
The new unit with 410 will actually cool better due to higher boiling point temp. of freon type.Poor air circulation could also cause problem.Such as dirty filter, too slow fan speed,dirty coil.Also could have obstruction in freon lines either at pump,in line set,or in coil.
Get someone u trust to check it out and ask for their Rudd Dealership Training Credentials.

The two wires from condensing unit go yellow or y from Tstat and theother wire goes brown if it is a Rudd or Rheem. Red from Air HAndler goes to Tstat R Green to Tstat G white to w. If it is digital tstat brown goes to C.

You should check the terminals where the other end is connected, there is no guarantee that the installer followed a wire color code. There should be a diagram in the HVAC unit. Normally, you have the power wire from the XFMR, a wire for the heat, a wire for the AC, and a wire for the fan.

Set the thermostat four degrees lower then house temperature and fan to on. Go outside and see if fan is running. If not check circuit breaker.