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Discussion Starter · #1 ·
These are the symptoms I've had the past few days with my central air conditioner:

1. This unit has been working fine for over 8 years. Even now, most of the time, everything works fine and the house cools nicely. But at least a couple of times a day, the thermostat will turn the unit on, the inside blower will go on, but the outside condenser fan and compressor do not go on. There is no hum or other sound when this happens. Just dead silence.

2. When this happens, if I let the inside blower continue to run and go outside and push on the two pole contactor with a PVC pipe, the fan will go on. But the compressor does not run.

3. If I shut off the thermostat and let the unit just rest for 45 minutes to a couple of hours, and then I turn the thermostat back on, the unit will usually go on normally and cool the house nicely.

4. After a couple of days of this, I noticed that the insulation on the black 220v line going to the L1 lead on the contactor was deteriorating from excess heat. It was getting bubbled and dried up. The copper had also turned a funny pink color, and the plastic on the contactor near the L1 was also deteriorating. I took the wire off, cut it back, cleaned up the screw and copper where the wire goes on, and refastened the wire. Now, a couple of days later, I see that it is again deteriorating from heat. It makes me wonder if this contactor is damaged somewhere where I can't see it.

There are no pressure switches on this unit. The terminals at the compressor are fine. There is a small circuit board that may be some kind of timer to keep the unit from being cycled back on too soon after it's shut off. But I installed a digital thermostat about 8 years ago, so I wonder if this board could be bypassed and how it is done.

Any ideas about what is causing this?
 

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Look carefully at the actual contacts/points of the contactor. If they are badly pitted or burnt then replace it.

Check the voltage across L1 and L2 when it is not running. Should be between 220-240. If it is lower then you got a problem.
 

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If the wires are dis coloring it's caused by heat. That is generally caused by a bad connection. If all your connections are good tgen your contactor is bad. Look at the screws that hold the wires. Is one discolored? If it is you can look at the points like Yuri is telling you and you will see what's going on. If your able you might want to check you run capacitor as well
 

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Discussion Starter · #4 ·
Check the voltage across L1 and L2 when it is not running. Should be between 220-240. If it is lower then you got a problem.
Sound like a good idea, Yuri. I'll do it when I get home.

BayouRunner, yes the L1 screw is discolored.

The points on the contactor aren't terrible, but they are a little blackened, kind of like old car points used to be. I'm going to pick up a new contactor and new capacitor today. Maybe I'll even get a hard start kit. They're not very expensive and worth trying, I think. I'll post how it goes.
 

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I would not add the hard start kit unless it is necessary. They can be hard on the windings and have been known to explode. My Bud in Houston hates them. The only really safe ones are the type that use a potential relay to disconnect a seperate capacitor.

If the screw is discolored than you need to clean it down to bare metal and where the wires attaches. Better to replace the contactor as they are cheap and can stick also.
 

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Discussion Starter · #6 ·
Here's a short update. I installed a new contactor. It was definitely needed (Thanks, Yuri). When I pulled it out, I could see that the contacts were worse than I'd thought. Also, the screw and the metal it screws into were in bad shape. The underside of the screw head was 1/3 totally gone! The new one is much more solid looking with better screws I could really ramp down on to get a good connection, so I'm happy I did that.

I also put in a new capacitor. The old one was very rusty, so I figured I would replace it.

The bad news is, this didn't fix the symptoms. But I did something that I think might have (fingers crossed).

I did some more research and found some technicians on HVAC forums mentioning symptoms like mine and saying they took care of them by bypassing the DOB board. I found out that DOB stands for delay on break. It's a little circuit board that is supposed to delay restart of the compressor by 5 minutes after it shuts down (called a break). It protects the compressor from trying to restart while pressured (such as might happen if the power went down for a few seconds). These boards can go bad and intermittently delay restart for hours instead of minutes. What's more, they mentioned that if you have a digital thermostat, they have DOB built in, so the board in the condenser unit is redundant and can be bypassed. I installed a digital thermostat a few years ago. I checked its manual, and it does have a delay on break of 4 minutes.

I googled the numbers from the board in my condenser unit, and, sure enough, it is a delay on break board. So, I bypassed it. That was several hours ago, and the unit has been cycling on and off normally. I won't know for sure for a couple of days that this is a fix, but so far so good.

I'll let you know. Happy 4th!
 

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Yeah if it is wired into the 24 volt circuit it is redundant.

We had a poster who wanted one to monitor the 240 volt circuit as he gets power bumps and thought they were causing problems. That is a different type of relay. I have had those 24 volt ones die and I bypass them if they have a elec tstat which 99% of people now have.
 

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Discussion Starter · #8 ·
Well, it's been 3 days and the unit's been running perfectly. The fault must definitely have been the DOB board, and bypassing it is an effective fix. Thank you Yuri and BayouRunner for your help. :smile:
 
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