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Old 02-03-2011, 07:16 PM   #1
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where does the 24 volts come from to thermostat

i asked a question earlier but i think im confusing what im looking for lol so ill start a new thread. i dont have the 24 volts going to my thermostat. i have a 66 safari. honeywell thermostat. anyway im trying to figure out wher ethe 24 v wire comes from exactly so i cna figure out why im not getting the needed volts to turn the ac on. all fuses look good and breakers look good. any ideas where to start or where i can find the wire so that i can test it at the sorce to see if maybe there is a break in the wire somewhere?
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Old 02-03-2011, 08:14 PM   #2
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i think you might be looking for 12 volts. i've read that folks have use 24v thermostats in their units with good results.

i again think that the thermostat is a switch and is powered from the ac.

can you post the year and model of the unit in question for the experts? :-)
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Old 02-03-2011, 08:58 PM   #3
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I believe that on the older (1960s) units the wire comes from the air conditioner, where a 24vac transformer is located.

Newer units that have a single thermostat for both the furnace and the air conditioner usually use 12 volts.
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Old 02-03-2011, 10:37 PM   #4
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No 24 volts

You will not have 24 volts at the thermostat. The thermostat is in what is called the switch leg. In the air conditioner unit there is a 24 volt transformer, which puts out 24 volts AC (alternating current). One side of the transformer is connected to the thermostat and the other side of the transformer is connected to the common side of the circuitry within the air conditioner. Don't confuse this with the ground/common side of the DC circuits in your trailer.
If you want to check to see if there is 24 volts at the thermostat; the contacts in the thermostat must be open. Thermostat turned all the way down. When you read across the two wires you may get 24 volts, I'm not sure because there could be other devices in the circuit. But try it.
Do you have a drawing of the circuitry? Usually a label inside one of the covers has a drawing.
What brand of air conditioner is it?
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Old 02-03-2011, 11:15 PM   #5
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Here is a drawing of a control circuit. Don't know if is the same as yours.
In the middle of the drawing down at the bottom you will see the transformer. In series with the transformer there is a cooling coil relay (see terminals 4 & 5). It doesn't appear that there is anything else in the circuit. So my thougt of reading the 24 volts at the thermostat will work. You have to have your meter on AC (alternating current) to be able to read it. Make sure the thermostat is turned all the way down.
This cooling coil relay closes a set of contacts (see terminals 3 & 6) labeled "C1-cooling coil contacts); which in turn causes the start relay (terminals 1 & 2 & 5)to pick up. The first thing that happens is the motor is powered thru the start capacitor; when it reaches a predetermined speed it switches to the run mode and the start capacitor is out of the circuit and the run capacitor is in.
The thermostats contacts pick up and drop out the cooling coil relay; which controls the compressor motor for making cold air.
Does your unit make any noise at all?
I see a switch on the drawing that is not labeled just to the left of where it says "fan cap" above the three position speed control switch. Are you sure you have all of the switches on? With out that switch on, nothing will work.
There are two different control circuits on this drawing but they are basically the same function.
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Old 02-04-2011, 10:52 AM   #6
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nothing comes on at all. no fan no clicks no noise. its like there is no power getting to the system. and the wires run through the walls that si why im just trying to figure out and trace the wires.
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Old 02-04-2011, 11:44 AM   #7
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Can you post a photo or two? Have you taken any covers off of the unit to see if you even have 120 volt AC power to it?
If you can take the cover off of the inside and shoot some pics maybe I can help more.
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Old 02-04-2011, 02:10 PM   #8
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i will try to get pics this weekend i dont have a working camera. i did take teh shroud off and i found the 120 volt wire. i traced it to the low and high speed fan relays. it has the 120v there. but nothing after them. is the fan motors 120v as well? i tried jumping the fan and still got nothing. i mean this unit is very old and has some rust inside and im sure i will need a new unit. but b4 i go and spend 600 bucks i just want to be sure it isnt somethign simple
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Old 02-04-2011, 04:04 PM   #9
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Fan runs on 120VAC and should come on in the fan position without the thermostat so you have other problems.

Could be the start capacitor , bad relay, bad fan motor or ???

But the fan should run even if the AC compressor will not kicking in.
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Old 02-04-2011, 07:02 PM   #10
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I agree with Garry. Relay, capacitor, bad fan motor. Can you tell if the fan relay picks up when you try to start the fan only? Do the wire connections look corroded? If so clean them up and give it a try. Do you have access to the fan blade where you could give it a spin to see if it's free? If so turn on the fan and try spinning the fan blade by hand to see if it will run. if it runs it is a bad capacitor.
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Old 02-05-2011, 06:43 AM   #11
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the fan spins free by hand. there are 4 wires going to it red white blue and ? ( i forgot) lol. anyway looking at the diagram in the unit it looks like the red wire would be the power wire for the fan. so i unpluged the red wire from the relay and put 120v right to that and still no noise and it didnt turn the fan motor case itself looks kinda surface rusty but the internals inside the motor still look decent ( the red colored coper wires) why are there 4 wires going to the fan? and the diagram really dosent show what wires are what exept the white wire is common. i take it common is like ground or should common have voltage to it at all times to?
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Old 02-05-2011, 08:40 AM   #12
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There are 4 wires going to the motor because it is multiple speeds. When you look at the schematic I sent you can see there is a red (low speed), blue (medium speed), black (high speed) and white.
How do you change speeds of the fan? Is there a series of push buttons or is it a rotary switch?
What are the wires that come from the motor connected to? A terminal strip or what. Are they the connectors that slide over a flat terminal (commonly called spade connectors)?
I still think you have a power problem. It seems odd to me that all three speeds of the fan motor would not work. Either the capacitor is out or you have a bad connection some where.
When you say you are putting 120 volts to it. Exactly how are you doing that?
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Old 02-05-2011, 11:05 AM   #13
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On both models the fan should come on as soon as the switch is made through the on/off 4 position (gang) switch.

Position 1 provides power to the thermostat position and 2,3,4 control fan speed.

The thermostat controls the relays on the compressor side.

The only difference in the 2 diagrams is most likely the size of the compressor and the neutral connection.

You have to have 4 conditions to make the fan come on.

1. 120VAC into the switch at "H" black wire
2. A good switch.
3. A good fan motor start capacitor.
4. A good neutral connection.
On the left diagram model 25-12 the neutral connection comes in from a fused disconnect through the switch terminal G white wire.

On the model 25-14 apparently the neutral comes in terminal G is not fused and does not come in through the switch.

Your comment you pulled a red wire from the relay and applied 120VAC?? There is no red wire from a relay to the fan motor.
A red wire from the switch through an unknown connector (little round circles in the wire diagram denote a connector) between the switch and the motor.

TG Twinkiy can guide you through the troubleshooting but you have to be his eyes that's why you have to be pretty specific about what you have done.

BE VERY CAREFUL applying 120VAC to unknown places will get you into trouble and possibility of injury.
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Old 02-05-2011, 11:32 AM   #14
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AC Unit

I an earlier post you stated that you read 120 volts. Where did you have you meter leads when you read the 120 volts. If you were touching the power lead with one probe of the meter and the skin of the trailer with the other, you would get a false reading. The neutral in your trailer is not grounded (at least it shouldn't be). On the drawing I sent it shows a fused disconnect, but it's probably a circuit breaker in the trailer power panel.
If you are reading voltage by touching one lead of the meter to the chassis (skin), then you may have an open neutral.
Be sure to check the voltage on the black and white wires coming into the unit.
On the left drawing there is a terminal strip.
Just to the left of the Fan motor. Do you see anything like that in your unit? If so make sure the connections are clean and tight. On that terminal strip there are 3 white wires connected to it, one on the bottom of the terminal strip and 2 on the top. There would need to be either a jumper wire between the 2 on the top or a metal buss bar, because all 3 of these wires need to be connected. Again check for corrosion etc.
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