Thermostat display blank, no A/C

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  #1  
Old 07-11-03, 12:42 PM
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Thermostat display blank, no A/C

I noticed that it seemed humid in the house. Not hot. I checked the thermostat to see where the temperature was set. I discovered that the display on the LCD thermostat was blank. Pressing the buttons on the thermostat to change mode, fan, temperature, and reset filter have no effect. Display is still blank. With panel to the thermostat open, using a VOM set to 150 volt range AC, I checked from R to each of the other terminals, in turn, and found no detectable voltages. I tested the outlet in the wall below and found it hot at 115 - 120 volts. (meter works)

Thermostat - Totaline P274-0100
control wiring
red wire - R
yellow wire - Y/Y2
green wire - G
blue wire - C
white wire - W/W1

The thermostat was in the house when we bought it in 11/2000.

Heater - air conditioner - single metal enclosure with everything inside it. It is marked BDP company, on the label it says: Forced air furnace with cooling unit. It was in the house when we bought it in December 2000.

Last serviced in fall 2002. The unit has worked flawlessly since we moved in in December 2000.

I opened the front of the thermostat and closed it to have the prongs of the wiring block go into and out of the receptacle in the front.

I cycled the paired 30 amp breaker arm marked A/C in the electric panel a couple of times. House was completely rewired and the panel replaced in December 2000.

I checked the thermostat wire from where it exits the wall under the house to where it goes into the installation at the outside of the house. It appears to be intact.

I removed and replaced the cutoff plug in the metal box by the unit. I guess this is a disconnect. It is marked that there are no fuses inside. There are 240 volts from one leg to the other of the disconnect.

The unit is electric and natural gas. I am in upstate South Carolina near the mountains.

What should I do next?
 

Last edited by chfite; 07-11-03 at 01:14 PM.
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  #2  
Old 07-11-03, 01:26 PM
Ed G
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Can you check the secondary voltage at the transformer. It should be in your heater. It will have two wires that are most likely black (high voltage 120/240) and two smaller colored wires. You are looking to see if there is any voltage across the two smaller colored wires. there should be 24 volts, if not your transformer is bad or has a blown fuse if it comes with a fuse attached.
 
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Old 07-11-03, 02:48 PM
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The transformer is on a printed circuit board, but the downstream wiring from it according to the schematic shows zero voltage to ground. The schematic shows this as a 24 v circuit.

I guess that now is the time to call someone?
 
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Old 07-13-03, 04:31 PM
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look for

a fuse on the circuit board, probably a blade type. if it is blown, replace it. if it blows again, remove all tstat wires, and try again. then hook up one wire at a time, starting with the red, and when it blows, it will tell us where the short is.
 
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Old 07-14-03, 07:51 AM
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You did check to make sure the breaker is not tripped and the furnace is receiveing power right?
Also, most digital thermostats have batteries, some won't work if the batteries are dead.
 
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Old 07-14-03, 11:22 AM
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Question How are you makin' out?

The place to check for voltage is at R and C. There should be 24 volts. Also, don't check for voltage to ground. Probe both sides of the low voltage cct.
Check the transformer as was suggested and make sure the door swich is good.
Another idea is that if the batteries went dead there is often a reset button on the stat. A Hunter stat I just worked on had a hole in the front that only a needle could fit into.
 
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Old 07-14-03, 12:36 PM
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As Grasshopper returns from the shrine of the air conditioner, he ponders many things. He learned that performing the ritual of removing the screws requires keeping control of the loose screw. The ritual of looking through the grass is long and tempers the soul. Keeping the screws in the nearby rice cup prevents repeating this ritual.

As he recalls his skills at reading the hallowed blueprints and wiring diagrams, he locates the necessary components. Being mindful of the duke power dragon, he checks to find that there is indeed 240 volts on the input and everywhere it should be when the unit is not running.

Alas, the output of the 24v transformer is naught. After having put the duke power dragon at bay by pulling the disconnect, he examines the blade fuse to learn that it is indeed intact. The blade fuse is replaced. But he ponders the significance of the internal protection symbol superimposed on the transformer symbol.

Unleashing the duke power dragon anew, he checks voltage again on the 24v circuit. Alas, nothing.

Reinstalling the access panels, he finds no door switch to further cloud his vision. The ritual of looking for dropped screws is not repeated.

As he returns from his sojourn, he ponders the clouds in the sky and considers what lightning might have done.

Anyway, still nothing. The thermostat does not have any batteries. It has the 5 wire cable running to it. According to the wiring diagram, there should be 24v across R and C as you said.

The input wiring has 120v to ground from each leg but nothing from leg to leg where it mounts on the main block in the unit. It is hot on the other side of the contactor as well.

Getting a lot of mileage from the attic fan.
 
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Old 07-14-03, 04:05 PM
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120 volts to ground

but no 220 means you have lost a leg of power, grasshopper. snatch the fuses and ohm them out, if there are some at the disconnect. if not, go to the breaker and check there. one leg is present, but you are reading feedback on the other.
 
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Old 07-14-03, 04:18 PM
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As blueprints and schematics help form the building block of technlogy, you are correct in considering them hallowed.

I am not clear on the locations and the results of your probing.
You have 240 volts everywhere it should be but then have 120 volts to ground and further on have nothing from leg to leg???

You have to be careful when testing a three wire 240/120 volt circuit.
You cannot test using ground or neutral when testing the 240 volt side. Because if one leg were open you would still get a voltage reading to ground or neutral because the current would travel through the load and back to the open leg.
You must always test from leg to leg.
Kinda looks like you may have dropped a line. (Lost electromotive force on one of the hot legs. )

The fact that you have a C terminal at the thermostat says that it derives it's power from the furnace transformer.
I'ts possible that it uses an internal battery for memory or I
wonder if it's old enough that it doesn't have battery back-up.
Post the mod of the stat.

Don't give up yet I think were close.
 
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Old 07-14-03, 07:46 PM
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Thermostat - Totaline P274-0100. Unless there is a small flat battery between the case and the circuit board which is riveted into place, there is no apparent battery.

I checked from leg to ground to determine an energized circuit, because there was no reading from leg to leg. I thought it odd, but went on. There is 120v from leg to ground. I will check in the morning when there is light to see about the breaker. Where the panel is located is poorly lit, and I want to avoid becoming fried grasshopper.

The disconnect is marked that there are no fuses inside of it.

Even a new breaker is cheaper than a service call.

Thanks for hanging in there with me.
 
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Old 07-15-03, 09:03 AM
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Checking further

Back into the fray.

Checked the input voltage at the main wiring block in the unit where the main wiring comes through the unit from the inaccessible back. 120 v leg to ground on both sides. Nothing from leg to leg.

Cut the power to the house, removed the circuit breaker. When closed, both sides are 0 resistance. When open, infinite resistance. It appears to operate smoothly. It is new as of December 2000. Wires were tightly screwed into place. Nothing inside the box appears to be out of place or loose or burnt.

Reassembled panel and restored power to house. Everything is flashing.

Went to the disconnect and measured the power from leg to leg. All that is in the box that can be seen without disassembling it are the two contacts that the pull out fits into. 240 v from leg to leg.

I have 240 v from leg to leg in the disconnect, but nothing from leg to leg in the main wiring block in the unit.
 
  #12  
Old 07-15-03, 02:03 PM
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Found at the shrine of the AC. Somehow one of the burnt offerings must have gotten out of hand.

http://www.chris.fite.com/fried_wire.jpg

I cut out the burnt wiring and cleaned everything up and put it back together. Now the display on the thermostat works.

In closing from the air conditioned house in Taylors, SC, I want to thank all of you who stayed with me on this and kept the focus on locating the problem.
 
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Old 07-15-03, 02:18 PM
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grasshopper

glad to help, seems you helped me with my computer in the past...what goes aroud comes around....
 
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