HVAC Low Voltage Problem

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  #1  
Old 04-23-19, 07:49 PM
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HVAC Low Voltage Problem

Hey everyone, I've learned a lot from this forum already, but I'd appreciate some help regarding my A/C issue. I'll try to keep it brief.
  • Thermostat calls for A/C, but neither compressor nor fan engage
  • With thermostat in "off" position, control board has 24V on R terminal and 0V on other thermostat terminals
  • When thermostat calls for "cool," none of the thermostat terminals on the control board hold voltage
  • Control board is fused with a 5A ATO fuse that is not blown
  • Transformer is protected with an attached circuit breaker. Transformer reads 240V and 28V on respective coils
  • Contactor is new. Compressor runs as normal when contactor is manually depressed.
  • Circulating fan operates independently when green and red terminals are jumped
  • Compressor doesn't kick on when yellow and red terminals are jumped
A little extra background: I've been chasing this issue for a while. I'm beginning to think it was, initially, two issues. Now that I've replaced the thermostat (one problem), all signs seem to pointing to a 24V short somewhere; likely between the control board and the thermostat. I've not been able to test end-to-end resistance .The bad news is the routing of the wire makes it inaccessible, so I can really only test at the thermostat and at the control board. I can, at least, verify that the wires don't have continuity when disconnected at each end.
I'd likely just have to snake in a new set of wires rather than seek out the short, but I just wanted to solicit the opinions of those more knowledgeable before I go on a fool's errand. HVAC is a new world for me.

What do y'all think? Am I missing something obvious here?
 
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Old 04-23-19, 07:57 PM
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You don't have a short or you would be blowing fuses.

You need a voltmeter. Set the thermostat on a call for cooling. Allow the 5 minute delay in most thermostats. Go outside and measure the voltage where the thermostat wire connects to the condenser. Typically it will be a wire nutted junction in the main electric area. Don't check anywhere else in the unit. You need to confirm 24vAC TO the unit. If that 24vAC is present..... then it is a problem inside the condenser and my guess would be low on refrigerant. There is a low pressure safety switch inside most condensers.
 
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Old 04-23-19, 08:17 PM
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Thanks Pete; I'll take a look in the morning.
The initial issue was a blown transformer. Once replaced with a protected transformer, it would throw the breaker every time cool was called. It has also blown one 5A fuse, but neither fuse nor transformer have tripped since replacing the t-stat (the old t-stat wasn't switching 24V properly even when removed from the wall). These things made me think I had a short somewhere.
Now I can just hear a "tic" in the control board when cool is called — sounds like an automotive relay trying to engage. Maybe that's the doing of the low pressure switch...it is an old unit.
 
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Old 04-23-19, 08:22 PM
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The low pressure switch will be silent. If you are low on refrigerant.... it will be open.
 
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Old 04-24-19, 05:10 AM
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Hi, with the Stat set for cooling do you have 24 VAC between C &Y on the control board?
Geo
 
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Old 04-24-19, 06:57 AM
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So, unless I'm going crazy, this thing doesn't have high or low pressure switches. I found the product sheet after some googling, and it looks like these switches were "Field-Installed Accessories" -
"• UPGRADE SAFETY PACKAGE - Contains screw in type High pressure, Low Pressure/Loss of Charge switch, freeze protection switch and lockout relay"

I hooked everything back up this morning as normal, and it ran for about 5 minutes. It shut off despite the thermostat still calling for "cool" — indoor temp 73, indoor setting 65. I was outside when it shut off and I heard that same clicking as before; it's the transformer. It seems like the built-in circuit breaker on the transformer is garbage. It sounds like the circuit breaker is trying to trip, but can't quite do it. I can depress the reset button on the circuit breaker and the clicking changes/speeds up. The control board fuse has not blown.

So, it seems like the transformer is the cause of the A/C stoppage, but what's causing the transformer to trip?

Geo - with "cool" called from the thermostat, and with the A/C not operational, neither the Y/C, G/C, nor R/C have 24V at the control board.
 
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Old 04-24-19, 09:34 AM
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No 24VAC between R&C on the control board, where is this transformer with the circuit breaker located, and what is it feeding? Very seldom that a circuit breaker won’t trip.
Geo
 
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Old 04-24-19, 12:45 PM
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Transformer is located a few inches from the control board and appears to feed the control board directly.
 
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Old 04-24-19, 01:09 PM
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And where they attach to the board you do have 24 VAC ,correct?

Maybe some pics of what you have there would help
Geo
 
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Old 04-24-19, 02:32 PM
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Attach your meter to R and C at the control board. Fire up the air. Watch what the voltmeter does. Quite possibly something is drawing too much power and is on the border line amperage of that breaker. If the voltmeter sags from it's normal reading.... something is drawing too much current.

One possibility is the contactor. The coil resistance can go down with age and cause a high draw problem.
 
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Old 04-24-19, 04:44 PM
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Geo - I have 24V at the board until the thermostat calls for cool. Then the breaker begins to trip. With a multimeter on the 24V input of the control board and C, I read 27V in the off position then a small oscilation of 0-0.004V as the breaker trips in the cool position.

Pete - Same thing on the 24V out on the R & C terminals on the control board. It doesn't really sag; voltage cuts pretty quickly as the breaker on the transformer trips. It's just kind of weird; I've had the transformer trip before, and it was a hard trip...meaning there was no clicking and the button had to be depressed before it would output 24V again. Now it just resets itself after the thermostat is reset to off. I think what you said about being on the border of too much amperage is spot on. May be a sign that the compressor or fan motor are on their way out? I determined today the unit is a 99 model.
The contactor is new. I was reading the R/C voltage as my girlfriend switched the thermostat to cool. The contactor appeared to try to kick everything on the first time. It clicked, threw a small spark, then the transformer breaker started doing its thing. We tried again and the contactor was never engaged.
 
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