First It Worked, Then It Won't Start, Has Strange Voltage

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Old 05-17-14, 05:53 AM
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First It Worked, Then It Won't Start, Has Strange Voltage

Hello,

I have a heat pump unit that is old. It was recently turned on for the first time in about four to six months.

It worked fine for three days, coming on, cooling, shutting off. The weather changed and the temps dropped and the unit was shut off at the thermostat.

Two days later when the temps rose again and the thermostat was switched back on the unit would not come back on.

If you switch the fan to ON at the thermostat it does not come on. I removed the thermostat cover and used a digital mutimeter on VAC and probed the terminal strip and get 5.3v on R/RC to W and 8.5v on R/RC to all other wires.

Should there be 24v in there?

I went into the attic where the air handler is and opened up the unit. The service comes in, wire nuts to pigtails to a terminal looking piece with little round contact test points, it is getting 120v to there.

There is also a smaller terminal strip inside there, a transformer, and the colored wires jumped over to an even smaller terminal which looks to be maybe where the thermostat jumper goes out. It also tests the same as the thermostat did.

There is a capacitor on the side of the fan housing, two pole, rusty, took it out to test if needed.

Would the strange voltages point to the transformer or a relay in there somewhere also?

Thanks for any help pointing me in the right direction.
 
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Old 05-17-14, 07:51 AM
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What is your actual problem? What wont come on, outdoor unit or indoor fan?
 
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Old 05-17-14, 08:41 AM
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The G terminal on the thermostat is the one that controls the air handler (blower). If you have a C terminal (not all thermostats do), that is the common (ground) terminal and you should measure ~24VAC between the R/RC and the C terminals.

The transformer that you found inside the air handler is the transformer that supplies the 24VAC to the thermostat. Measure the voltage across both sets of terminals. You should measure 120VAC across one set (input voltage), and 24VAC on the other set (output voltage going to thermostat). If you have 24VAC at the output of the transformer, measure the voltage from the wire that comes from the G terminal on the thermostat (may be green wire, but doesn't have to be green in color), with the fan set to ON. Some control boards have a small (typically 3 amp) fuse to protect the transformer should a short develop. If you see a fuse, check to see that it is good.
 
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Old 05-17-14, 09:14 AM
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The problem is that it does not turn on, nothing comes on.

Even going to the thermostat and moving the fan switch to the ON position does not make the fan come on.
 
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Old 05-19-14, 05:12 AM
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Hi,

I was able to get back into the attic today to check the transformer. (Do A/C co.s have to hire smaller people to work up there all the time or what?)

Coming out of the transformer are two brown wires. One goes to a terminal strip that connects to a jumper that goes down to the thermostat. The other brown comes out of trans and connects to a little round device with several other wires on it (labeled FR on the diagram).

The trans is getting 119v at the input. I get 8.9v from each of the brown to ground and 0v from brown to brown.

Do I need a new transformer?
Are these transformers a universal item?
Should I be able to get one for this 30yr old unit?
Approximate cost?

Thanks for any help.
 
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Old 05-19-14, 06:00 AM
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It sounds like the transformer is bad, but just to be sure it's the transformer and not what's connected to it (load), disconnect one or both of the brown wires (output) and measure between the brown wires. If everything is working OK, you should measure ~24VAC between the brown wires. If the open circuit voltage is OK, then something is loading down the circuit (pulling too much current, thus reducing the voltage). If the open circuit voltage is significantly less than 24VAC, your transformer is bad. They are relatively inexpensive. Below is a link to one from Grainger's to give you an example of the price. They are pretty much a universal product.

WP157598 Class 2 Transformer, 20 VA - Grainger Industrial Supply
 
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Old 05-19-14, 07:16 AM
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I tried to disconnect both brown wires but the one that connects to the FR (fan relay?) was stuck and I didn't want to start yanking too much on the connections. I did disconnected the other brown wire that goes to the terminal strip and jumps to the thermostat.

With the 24v circuit open I get 0vac between the loose brown wire and the brown still connected at FR, so I guess the output of the transformer is definitely bad.

Why did I before get 8.9v and now get 6.8 to 7.9 vac on each brown to ground? Is that a short inside the transformer?

Thanks for the help. I've got a Grainger about 10min. away with that item in stock.
 
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Old 05-19-14, 04:29 PM
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I may have misunderstood you , but do not measure the 2 brown wires to earth ground . Measure from one brown wire to the other brown wire . As has been said , should be 24 VAC , or a few volts higher .

Again , as has been said , look for blown fuses .

If the transformer is bad , some thing could be shorted , causing the transformer to go bad .

The attic would be my last choice of a place to put an air handler / fan coil unit . :-(

God bless
Wyr
 
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