Update: A/C won't start...missing low voltage

To Speedy Jim, Stormin Mormon, mm, Andy & Carol
Back to work on the A/C today... still wouldn't start.
I looked at the Low Voltage xformer.... pretty big sucker...... checked for ac in...was able to measure 120 vac at each leg measured to ground. Strange, I thought that by measuring across the two terminals, I would see 230 but I guess they are both 120volts but out of phase with each other...so there is no voltage differential. Anyway measured 120 vac to ground.
The output of the transformer is listed at 24vac... no voltage at all on the output measured between the legs and each leg to ground. Removed the load to the output, again, no voltage measured.
Removed the transformer and ohmed it out. Primary was 25.6 ohms, secondary was .8 ohms (less than one)... infinite resistance between secondary and primary.
I picked up a transformer from Radio Shack today.... 120 vac in with 25.2 vac out. 2 amp power capacity. I pigtailed it to an extention cord and spliced it into the low voltage circuit. Turned the power on...... moving the thermostat controls, I could hear the relay clicking inside the a/c but it wouldn't start up..... Is a 1amp transformer adequate ?? The existing one is a little larger in size.
I ohmed out the good transformer from Radio Shack....12.8 ohms on the primary... .8 (less than one) on the secondary.... so it looks as the original transformer is ohming out correctly at 230 volts input 25.6 ohms and 25.2 volts output at .8 ohms.
Is it possible to have a bad transformer, even with correct resistance measurements ?? I can think of internal shorting due to arcing across deteriorated oil paper and insulation.
What do you think ???
Thanks in advaince for your help !!!
Peter
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Peter wrote:

Assuming it's a 120V transformer, that means your neutral lead is open. Generally, that translates to "the white wire is broken or disconnected somewhere between the transformer and the breaker box."

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Small hint, transformer resistance is related both to the voltage and current capacity. Just because the existing transformer has twice the resistance as the one you bought may not mean it is for 230V. Just may be a 120 v transformer with lower wattage. From your measurements sounds like it may be a 120 v transformer with the neutral broken somewhere between the transformer and power. Also live in DC and this up coming week looks to be hot. Good luck.

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I would never guess that in a million years. The specific problem that generates your sentence above is that you expected to see 230, so you came up with this idea about both 120 out of phase. You shouldn't see 230. You should see 120. IIRC, only the compressor runs on 230. Everything else is 110/120, and for sure the control unit is initially powered by 110/120. The transformer is meant to lower that voltage to 24.
You never actually say what the voltage was across the primary, but I can tell you from what else you said. It was zero. That means that one of the wires/etc. from the fuse box to the transformer is broken and since you were good enough to measure the voltage to ground and it is 120, that means it is the neutral wire (which should be the white wire, not the black) is open.

It's hard to give a whole lesson here, but the voltage at one end of the primary is 120 because it is connected straight to 120. The voltage at the other end would have been diminished, down to zero, if there were current flowing through the primary. But since there isn't, where E is voltage and I is current, E=IR=0*R=0. That is, the voltage drop across the primary is zero (because the current is zero), and you get the full 120 at the far end of the primary also. If the neutral were good, current would flow and the amount of current would be determined by the resistance of the primary, and that amount * the resistance of the primary would equal the voltage present, 120. So 120 would = IR. But that is in a circuit. You don't have a circuit, because the neutral wire or one of its connections between the primary and the fuse box is open.
Anyway

The primary resistance is another sign the transformer is good. Usually it is either good, zero, or infinity.
(although I have a fan above my bed that smelled like it was burning recently, and now no longer will turn as fast as it did. I think some adjacent windings are shorted to each other, and the resistance is lower than it was, but definitely not zero. I have to smell it. OK, it smells a bit burnt but it might be the oil I was using. A transformer wouldn't have any oil to confuse you.)

Well, if one of the windings were shorted to the case, I guess that would make it "bad", especially if it were the 110 volt winding (dangerous) but I've never heard of that, and that's not what you mean since I don't think you measured that. And I suppose you could have a winding that had the right resistance now and was about to go bad in a moment.
But otherwise no.
Frankly, we already know that the first transformer is good, or at least there is no reason to think it is bad, so you can return the other one. I'm sure it is as good as new. (Well, wait until you fix the whole thing, but don't mess with the new xformer anymore.)

Trace the wires that power the primary back to the circuit breaker. AT the primary, you show zero volts between one wire and the other, and 110 volts between the neutral (one of the two) and ground. When you find the good part, it will be the other way around, 110 between neutral and hot (white and black) and zero between white and ground. Track down where it goes from bad to good, and at the border is where the problem is. An open. Connect them and youre good.

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Seems like the transformer should have read 120 volts to ground one leg, and 0 volts to ground the other leg.
Wonder if your system has an open neutral for the 110 VAC (alternating current) supply?
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