Help Diagnose Condenser Problem


  #1  
Old 09-15-09, 12:25 PM
T
Member
Thread Starter
Join Date: Sep 2009
Location: Missouri
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
Exclamation Help Diagnose Condenser Problem

The condensing unit will not keep running, but the fan blower runs.

For example, if the actual thermostat temperature is 79F, when I click to 78F the fan blower and condenser fan will operate. But, if I click past 78F to 77F or 76F, the fan blower continues to run and the condenser fan kicks off. In this scenario, if I stop at 78F, the condenser and blower fans run and supply cool air. If I click through 78F to 77F or something less, the condenser fan kicks on as I pass through 78F, but kicks off when I reach 77F. The blower fan continues to run, but obviously no cool air is supplied.

I am trying to determine if the problem is at the thermostat, condenser contactor (maybe a 24V contactor between the 220V supply and the condenser, or if the refrigerant charge is low. I haven’t confirmed if the unit has a low-pressure switch.

Please suggest the steps and how to perform the system checks, so I can accurately diagnose the problem…without getting electrocuted!

Thank you.

Tom
 
  #2  
Old 09-16-09, 05:59 PM
V
Member
Join Date: Sep 2009
Location: Alaska
Posts: 22
Upvotes: 0
Received 0 Upvotes on 0 Posts
I understand your blower fan continues to run normally.......

Does the compressor in the condensing unit keep running when the fan in the condensing unit quits running?
It will normally stop on overload a short while after the condensor fan stops.

Does the compressor AND the fan stop at the same time or does the fan stop then the condensor a short while later.

You need to determine this.... turn on the unit and go to the condensor and listen to see what the situation is.

Do you have a clamp on ammeter to measure stuff with?
 
  #3  
Old 09-17-09, 08:41 AM
T
Member
Thread Starter
Join Date: Sep 2009
Location: Missouri
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
Help Diagnose Condenser Problem

I'm pretty sure both the compressor and the condenser fan stop at the same time.

I do have a snap-around multimeter with one clamp lead (black) and one probe (red).

What would you suggest measuring, without power, with power and when?

I also ran another test at the thermostat. I disconnected the Y terminal wire and touched it to the R (RC) terminal. The compressor and condenser fan instantly kicked on. This leads me to believe it may be a bad thermostat, although I am not certain. Let me know what you think.

If you think it is a bad thermostat, I would still like to hear what you would have tested with the multimeter (to learn and keep in mind for future reference).

Thanks for your help, advice and efforts to keep me from frying myself!
 
  #4  
Old 09-19-09, 09:16 PM
V
Member
Join Date: Sep 2009
Location: Alaska
Posts: 22
Upvotes: 0
Received 0 Upvotes on 0 Posts
Ok Tom, here you go, more info..... hope this helps.

Hmmmm..... not sure what you have going on here but here's how to check it out.

Look on your thermostat and find a wire marked "C". The wire will 'probably' be black or blue.
That's the 'common' wire from the 28 VAC transformer in your air handler (or condensor) could be either or or both, doesn't matter at this point.

Find the wire you just referred to connected to the "Y" terminal.
That's the wire that goes TO the contactor outside in the condensor.

As you have found, when you connect that "Y" wire to the "R" terminal the condensor runs.

THat's a good thing. All the THERMOSTAT has to do is to internally connect the R wire to the Y wire and the condensor will run until the 'set point' is achieved.

One other thing the thermostat has to do is activate the FAN in your AIR HANDLER.
That's done with the "G" teminal.

SOme units will need to activate the "O" terminal. THat's the reversing valve in a heat pump.

So..... connect your multimeter BLACK lead to the "C" terminal on the thermpstat.
Place the meter on an "AC" volt scale HIGHER than 28 VAC, such as 50 VAC or 100 VAC, what have you.

Place the RED multimeter terminal on the "Y" connector on the thermostat.
CLick the thermostat to turn on the unit. The "Y" connector SHOULD have 24~28 VAC ac on it.
Continue to click the thermostat down to a lower temperature while observing the voltage on the Y terminal.

IF you turn the thermostat down lower than the room temperature BY ANY AMOUNT..... you should continue to see the voltage on the Y terminal.
If it goes away or it jumps around (on off) or whatever then the thermostat is very likely bad.

Check the "G" terminal and make sure you see the same 24~28 VAC. The thermostat also makes this internal connection to turn on the air handler fan motor as long as the thermostat 'set point' is not satisfied.

That's about it, pretty simple,

On a call for COOL:
Thermostat connects the R to Y --- Activate contactor in Condensor
Thermostat connects the R to G --- Activate air handler blower motor

A lot of the newer digital thermostats use mechanical relays to make these connections. The relay contacts can become pitted, dirty or punky in some mannor and the connection can become flaky. They are not serviceable so plan on a new thermostat if you find the control voltages flaky.


Understand: We are dealing with THREE 'runable' components here. For clarity:
1) Air Handler BLower
2) Condensor (outside unit) Fan or BLower
3) Compressor(outside unit pump)

Jim
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: