I have a heat pump controlled by T87 thermostat. Attached to the thermostat is a S.P. double throw switch that allows me to switch from the heat pump to an evaporative cooler.My question is can I keep the switching arrangement and install a 6300B progammable thermostat.

If you have recently changed your thermostat to this one, It may not be compatible with your system. If you have a heat pump system, your thermostat will not work with a heat pump. You will have to install a thermostat that is compatible with a heat pump.
-Said that, assuming this has worked in the past during the heating season, some times the switch slides on these thermostats are fairly tempermental in that the switch slide contacts inside the control board sometimes do not make contact and if jostle around a bit, will maybe make contact. You can also remove the front plate from the base plate and check the wire are properly attached and not loose. While you have it off, check the hg he switch to make sure it is selected properly. If you have a gas or oil furnace it should be in hg. If you have electric heat it should be in he.
Depending on the age of the thermostat, you may need to get a new one because of the slides do wear out and tend to give unreliable performance. If you replace the thermostat, may I recommend you get a Honeywell built thermostat. Many of the cheap thermostats are cheap and poorly constructed and become unreliable. On cold days or hot days air conditioning is essential for comfort, and the money spent for a good thermostat is money well spent.
Hope this helps.

The two wire Honeywell T87 and CT87 is nothing but a switch in a control circuit. Wire color is not important at all. What is important is the TERMINAL DESIGNATION those wires are hooked to. Soooo....take the wire off of the sub base of the old stat terminal marked 'R' and attach it to the new sub base terminal marked 'R' and so on. Wire COLOR means nothing at all in your case.

You could have a problem with your controller

You could have a Wiring Issue that is not allowing the switch

You could have a Valve Issue....

check for voltage then check connections sometimes the wire breaks and no power gets to the stat

Defrost cycle is to allow frosy or ice to melt down and not restrict the output air. The thermostat is set too low and in high humidity ice forms quickly restricting the air flow. tur up the thermostat and let the room cool gradually.

Have to know what kind of heat it is, Gas, Electric or Electric Heat Pump.

Heat pumps, when air over air type, have an extra setting on the thermostat labeled Emergency Heat, Although these thermostats can work on the other types of heating as well.

Both modern Gas and electric furnaces have the blower controlled by a blower board. The flame and or heating elements can also be controlled by the same or a separate board. SO you could have a board problem, or a safety switch or other symptom causing a safety switch to trip off on you.

Consult with your diagram and find something you can recognize then trace the wires down and follow to where you get the power to turn on the heat (usually always R feeds thermostat and W feeds Heat). Colors of the attached wires means nothing at this point. Look for the terminal designations not colors.

Hope this helps you. Please grade me accordingly and GOOD LUCK. :)

how old is unit? Sounds like it may be short cycling which could be do to it going off on limit.

The switch on the right side of the T87 is for the fan. Down is the auto position which will cause the fan to run only when needed. Up is the always-on position, used to circulate the air constantly. If you want to shut down all heating cooling associated with this thermostat you need to look at the switch on the left side of the stat. To the right is heating, center is OFF, and left is ac.

Hey John, man that ole T87 has been around for a hundred years and I have installed many of them, but I cant remember the letters on the wiring. Seems to me that it is R-W-Y, any way you will use the R and W. I have a t87 at home still in the box and I could wait till I got home to help you but Im sure its the R and W terminal. Dont forget to set the heat anticipator. Set it the same as the current value of your gas or oil valve.

t87 usually are about 5 degrees off. sometimes more because of dirt and dust.get a digital