Intermittent compressor and no 24v to contactor

I've seen similar problems to mine posted, but none that match exactly, so I'm posting this. The past few days, I've had the following problem with my Nordyne split air conditioner (4 ton/10 seer) for a double wide mobile home: Most of the time, it goes on, runs fine, and cools the house. But once or twice a day, I've noticed that the inside blower is running but the house isn't cooling and the air coming out of the registers isn't cool. I will look at the inside unit (A coil, etc.) and everything looks fine except the coil isn't cold. I will look out the window at the condenser unit and the fan isn't running. Going outside, I find that the fan and compressor are totally silent.

I'll try to make this short. While the unit was acting up this way (remember, it usually runs fine), with the electricity off and the panel cover off, I checked/tightened wires, and cleaned things up. I turned the electricity back on, and set the thermostat to cool. The inside blower came on. Outside, there was total silence (no hum). I took a pvc pipe and pushed on the contactor. This only turned the fan on. The compressor did not go on. Releasing pressure on the contactor shut the fan off. So, I assume that the contactor was not getting its 24v juice. But why did the compressor not go on when I pressed the contactor? And what would this have to do with the contactor solenoid not getting electricity? Could the compressor be hard starting and somehow cutting the circuit between the thermostat and the contactor? There is a small circuit board inside the unit near the capacitor. I don't know what it does, but I'm wondering if it is involved, either being faulty or doing what it is supposed to in protecting the compressor. About 45 minutes later, the unit was able to start up normally and run as if nothing was wrong.

By the way, in case somebody is wondering, there is neither a pump nor a float switch on the evaporator coil overflow drain, so no cut off switch there. The condensation runoff just flows out of the pan through a pipe and out of the house by gravity.

I don't know if overheating can be involved because the unit will sometimes run fine through the heat of the day, but not run in the middle of the night.

If the compressor didn't start and hummed, I would suspect hard starting and install a hard start kit (which I may do anyway, since I can't think of anything else), but I don't understand why the 24v circuit to the contactor is also going off. I would appreciate any advice or thoughts you might have.

 

Is there a circuit diagram of the outside unit on the inside of the cover? There are often pressure switches (low and/or high) usually located in series with the 24VAC that activates the contactor. If either is open (you may have only one or even none), you won't get 24VAC at the contactor coil. Also, some units have a time delay circuit that is supposed to prevent the compressor from turning back on if it's been on but then turned off for a short time (usually ~5 min). My outside unit has that TDC and it went bad a few years after the system was installed. If you're using a digital thermostat, the required time delay is built into the thermostat, so I just bypassed the TDC in the outside unit.

Another possibility is a poor connection on the compressor wires where they attach to the compressor. With the power off, pull the connector off the compressor (wear safety glasses when doing so) and check to see that nothing is burned or loose.

 

Thanks. The circuit diagram and all info is weathered and unreadable. I have new info. The unit just acted up again. I went out and checked on it. The insulator on the black main current wire going into L1 on the contactor is fried. Any thoughts?

 

I had another thought. I seems odd that this happened only at L1. The wire at T1 looks fine, and the contactor didn't stick or anything. The screw at L1 seems tight, but the contactor plastic near it is carbonized. Is this just a bad contactor or has my compressor had it?

 

If the charred connector is on the contactor (L1 & L2 are the terminals where the 240VAC connects to the contactor), it's likely not a compressor problem, and there's probably nothing wrong with the contactor (unless the terminal where the wire connects is charred). Turn off the power to the outdoor unit and crimp or solder a new connector onto the wire. You'll have to cut off the burned portion of the wire. If it's too short, you'll have to purchase some new wire to replace it. With any luck, you've found the cause of your intermittent, and when you repair it, the A/C will work correctly. Possibly it was a bad crimp on the L1 terminal, causing high resistance at the connection between the wire and the terminal. When you put a new terminal on, I would suggest that you both crimp and solder the terminal to the wire. That will ensure that you have a good connection and the problem won't happen again.

 

I'll do it. Thank you, Bob!

 

L1 and L2 are the line connections where the power come into the contactor. If you pressed the contactor in and only the fan started then your compressor problem is not there.

Perhaps your contactor is wires backwards where the power comes in on the T1 and T2 terminals.

Posting pictures sure makes things much easier all around. http://www.doityourself.com/forum/el...-pictures.html

 

I'll try to get those pictures, but for now, the black goes into L1 and the white into L2. Black comes out of T1 and yellow out of T2. If it were wired backwards, I don't see how it could ever work, but it's been working fine for nearly 9 years. The same L1 connection has charred again. This time, the L1 screw and brass under it are also damaged. Whether it was the cause of the problem or not, the contactor will now definitely have to be replaced. I'll be driving past a HVAC supply this afternoon, and will likely get a new contactor, new capacitor, and a hard start kit. I don't know if these will fix it or not, but they seem to be worth a try.

By the way, to get this info into the thread, there are no pressure switches on this unit. Also, I checked the compressor terminals yesterday evening. They look perfectly fine, almost brand new.

I'll post how things are after I put the new stuff in.

 

On a 4 ton unit.... there may be no high pressure switch but it's hard to believe there is no low pressure switch. Only low end units come with no sensors and that's rare.


If the L terminals got hot and the line connections were tight.... then the contact(s) are wiped.

 

That makes sense. Hopefully, a new contactor will solve the whole problem.

 

Maybe I've got this licked.

I installed a new contactor. The old one definitely had a bad screw connection at L1. When I tried to tighten it, the plastic housing nearby started breaking away.

I put in a new capacitor, too.

The unit still intermittently wouldn't start.

I found out that the little circuit board in the condenser unit is a time delay and that, if these go bad, they can cause the symptoms I've had. Since I have a digital thermostat, the circuit board isn't needed (digital thermostats have them built in), so I didn't need to replace it, just bypass it. I did that, and the unit has been running totally normally ever since. Since the problem was intermittent, I'll have to wait a couple of days before I can totally rest easy that it's fixed. For now, I'll enjoy the rest of the holiday.

I'll post when I know more.

 

It sounds like you may have found the problem. Good job!

 

Three days later and everything is still running great! The problem must have been the time delay circuit board. If anyone has a similar problem, remember that you can bypass the board if you have a digital thermostat. Otherwise, get a digital thermostat or replace the board.

Thank you, Bob and PJmax for your help!