bad transformer?

[randytrue]

During the winter, I noticed a loud hum coming from my furnace at all times.
The other day, I tried to turn on the AC and got no response.
Checked the furnace room, and found the noise is gone. No response at all from the thermostat, either heat or AC. Checked power, and there is power at the breaker.
Should I start by replacing the 24V transformer?

Randy

 

[Daddytron]

Megatron was a bad transformer...
But on a serious note, I would check the output voltage on the transformer. What kind of furnace do you have?

 

[user 4883]

Only replace components when they test bad.
Could be the thermostat that is bad but, you would have to test that too.
You will need a volt/ohm meter to perform most tests.

 

[randytrue]

The furnace is a Janitrol AC30-10, SN 9412014 74
I know my way around a VOM meter, so tell me what's next.

Randy

 

[Blue Jay]

The furnace is a Janitrol AC30-10, SN 9412014 74
I know my way around a VOM meter, so tell me what's next.

Randy

At the transformer check both the input and output side, should be 120V on input and 24V on the output. If that checks ok then check for 24V at the thermostat.

 

[randytrue]

At the transformer check both the input and output side, should be 120V on input and 24V on the output. If that checks ok then check for 24V at the thermostat.

The thermostat is easier to check first.
The most voltage I could find was 3V AC.

Randy

 

[randytrue]

At the transformer check both the input and output side, should be 120V on input and 24V on the output. If that checks ok then check for 24V at the thermostat.

Is there a component that will go out between the transformer input and the 120V?

Randy

 

[randytrue]

OK, I'm getting 13V AC at the transformer output.
Can we assume the transformer is bad?

Randy

 

[Blue Jay]

OK, I'm getting 13V AC at the transformer output.
Can we assume the transformer is bad?

Randy

That would be my guess. Only thing that should be ahead of the transformer would be a switch or breaker.

 

[runfast]

13v indicates the contactor on the a/c is partially shorted if the stat was calling for cooling.

 

[Rob-M]

If you have a 110v at the furnace then you have 110v into transformer, which is called the primary, test the secondary side of transformer (24v) by followingthe leads from transformer going to the low voltage connections usually "R" & "C" or "X" thats the 24v connections. Depending on well who has worked on your furnace and the manufacturer there maybe be a fuse once the 24v leaves the transformer going to the low voltage/thermostat connections may be a inline car fuse or colored fuse. If you have 24v there than you will at stat. On a call for cooling your stat will send 24v to the AC unit normally the yellow wire or "y" connection. Your 2 low voltage wires at AC unit are the 24v, so check the voltage there. Then it can go thru safety switches, circuit board then the contactor so follow it. Theres also manual resets at the AC unit to high and low pressure you may try resetting them. Can go on and on but its a start to save u a buck. Make sure your meter is on AC volts and not DC volts too. Good Luck

Rob

 

[Wuzzat?]

OK, I'm getting 13V AC at the transformer output.
Can we assume the transformer is bad?

Randy

A partial short should be pretty difficult to trace down without cutting and jumpering some connections on your controller board.

Taking 40 VA as a generic rating for your transformer, the secondary current should be 40/24 = 1.7A or less. Check it if you can.