[Doug 331]

As of resent my Honeywell Chronotherm IV Plus fails to display, when it does my AC fails to run. I have been able to resolve by removing from the wall and off the pin sets. That said, the problem returns. I donot see a battery within the control wall unit. Any ideas would be appreciated!

Thanks
Doug

[talaniman]

Maybe this PDF will help and the trouble shooting starts at page 18. You don't seem to have a battery backup, and there is a cross referencing guide at the end of the troubleshooting section, but a model number for your unit would help greatly.

http://www.climatesystems.net/wp-con...notherm-iv.pdf

No display seems to be an indication of no power to the unit, either by breaker or loose wire.

[drtom4444]

Any time you are checking ANY appliance with an electrical problem you need to use a meter so you will know what is going on. Without a meter you cannot see very much:Electricity at 24 volts is invisible unless you have a spark. There are so many models of these thermostats that it's impossible to help you without at least knowing if you have a straight cool/heat unit or a heat pump. You have power coming into the thermostat at R (usually red) and C (common) on most digital thermostats. Some thermostats get the power from common though one of the relay coils for the fan, compressor contactor, or another relay; however, most have an R and a C to supply power. The "R" power supply wire supplies the power source that is switched in relays in the thermostat in all thermostats, so you ALWAYS have an "R." Power is then sent to "Y" for the condensing unit contactor, "G" for the fan relay, and "W" for heat on regular units. On heat pumps you will have "O" to pull in the reversing valve (in Florida and most southern states), or "B" for northern states, and "Y" just pulls in the contactor on the outside unit. If the unit uses "O" for the reversing valve then "Y" alone brings heat; if "B" is used then "Y" alone brings on AC. You need to check your wiring carefully with a meter. You can start at the thermostat and then to the inside unit at the transformer where you should have 24-30 volts AC current (It's called control power.) and then trace the wires until you find what is wrong. An animal could have chewed through the wires outside. Sometimes the transformer is located there in the outside unit, but rarely. However, a break could cause a short, as could a tripped safety switch. Here are some manuals to help you: https://dl.dropboxusercontent.com/u/...thermostat.pdf
https://dl.dropboxusercontent.com/u/...e%20Manual.pdf
https://dl.dropboxusercontent.com/u/...20Handbook.pdf
https://dl.dropboxusercontent.com/u/...Electronic.pdf
https://dl.dropboxusercontent.com/u/...%20manuals.zip
Get a good digital meter. The best one I have found for the price can be found for $42.38 at Home Depot: Commercial Electric Digital Clamp Meter-MS2002 - The Home Depot
Attachment 47633This is a clamp meter to measure amperage along with everything else, which you need, especially with HVAC units.