Outside A/C Unit Not Running


  #1  
Old 08-23-02, 08:37 AM
phillyfan
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Outside A/C Unit Not Running

I have a gas forced-air heating system with central a/c circa 1988. Up until a few days ago the a/c was fine. Now the exterior unit does not run. Came home to a very warm house, 85 degrees. The blower was running on the furnace but the air at the registers was room temp. When I checked the outside unit I just hear an intermittant buz sound on a 5-10 second cycle. I have now shut off the system at the thermostat. It really sounds like the compressor / fan motor is trying to start but can't. I did check that the fan spins freely. I am comfortable with electricity and a VOM and would like to go through the appropriate diagnostic checks. I have a home repair manual that explains in detail how to discharge capacitors, test the motors, contactor, reset switches, etc. The a/c unit is manufactured by Bryant and HAS NOT been serviced in the last 8 years. Obviously that is the source of my problem! Prior to the unit quitting I can also say that the fan was louder than normal (probably needed oil) and when the unit shuts off there is a clunking sound (probably a bad spring or something). If I can get this system running again without a complete replacement I promise to service it regulary. Any thoughts on how to proceed with this diagnostic?
 
  #2  
Old 08-23-02, 12:22 PM
bigjohn
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First thing to do is see if you have voltage at the outdoor unit. You should have 240 volts or so at the line side [power coming in] of the contactor. If so, then set the thermostat for cooling and see if the contactor pulls in. Then check the voltage on the load side of the contactor. Now, do the compressor and fan motor try to start? After the unit has been off for a while, they should both start. If the compressor starts but not the fan motor, the compressor will run for a while before it shuts off on a safety device. If they're both trying to start but the voltage is low on the load side of the contactor, look closely at the contact points. A common failure is the silver burns off or they get clogged with ants or critters. In these situations it's best to replace the contactor. Check these items out and let us know what you find. BTW- if the contactor doesn't pull in, see if you have 24 volts at the magnetic coil.
 
  #3  
Old 08-24-02, 03:25 PM
phillyfan
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Contactor does pull in after switching to cooling mode at the therm. Noticed the fan blade spun about a 1/4 turn then stopped. While in cooling mode hear a steady hum and a second louder intermittant hum. Volts on the load side of contactor tested at 239V. Wire diagram shows a single 3-prong capacitor servicing both motors. If I interpret the diagram correctly both motors will run at the same time unless one of the moters or cap is bad. Decided to check the cap and did a DC volt check. Tested center to each outer and all readings zero. Decided to perform a dischage exercise anyway. Then perfomed a resistance test center to outer leads and got infinity readings. I'm new at this and not sure I performed the tests correctly but I did follow detailed tests from a repair book. At this point can I assume the cap is bad or do I have other tests to peform?
 
  #4  
Old 08-24-02, 06:46 PM
bigjohn
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I assume you took the wires off the cap first? [I trust you made a sketch of where they go?] Make sure you're using the Rx1000 scale on the VOM and not a lower one like Rx1. With an analog meter on a good cap the needle should travel towards 0 resistance and then reverse and travel towards infinity. Staying at infinite resistance means the cap is open whereas 0 means it's shorted. A shorted cap should blow a fuse as would a grounded cap. Look closely at the cap and you should see some markings for the terminals like C, H and F or maybe C, HERM and FAN. C is common, H or HERM is for the compressor and F or FAN is for the fan motor. C is usually the center one, but not always. You want to test from C to the other two individually. The various humming noises you're hearing sounds like both the compressor and fan motors are trying to start. The cap is common to both. Try replacing the capacitor and see what happens. What you have is called a dual capacitor and will have a rating like 35/5 uf- 370 vac. The higher uf rating is the capcitance for the compressor motor and the smaller is for the fan motor. The vac is the voltage rating. The guideline is you want the same uf numbers and the vac has to be equal or greater. There are only two vac ratings- 370 and 440. So, you can replace a 370 with a 440 but not vice versa. Failed capacitors are a common item, so you might get lucky and this is all that's wrong. Let us know how you make out.
 
  #5  
Old 08-25-02, 09:40 AM
rclhvac
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What model #? - Turn unit on & try spinning fan to get it running.
If it runs you have a bad capacitor. If it is hard to turn the motor is bad.
 
  #6  
Old 08-27-02, 05:58 PM
phillyfan
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Thumbs up

Yep, took the wires off the cap and did a discharge exercise before proceeding. As mentioned the readings stayed high indicating an open circuit as you noted. The logical choice was to replace the cap and go from there. You were right on the money, the cap was a dual 35/5 uf 370v and the terms were marked HERM, C and FAN. Called a couple local parts jobbers and they said they had a run on these caps and were out of stock. Was then referred to another jobber and I got one for $15. Was able to install the new one today and hosed out the condensor coils and oiled the fan motor. Put everything back together and it is running great. Actually, the outdoor unit is now much quieter than it every was. Thanks for the assist and keep up the good work. Your forum is greatly appreciated!
 
 

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