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Old 05-27-2009, 11:58 AM   #1
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AC outside fan starts then off, this recurs over and over.


i have a 18 year old Lennox AC unit. (model number:HS18QC-211-C1P)recently The outside fan starts for a few seconds, appears to be spinning fine - with no alarming noise, but then shuts off. This recurs over and over if the AC is left on.

Here's a list of things I've tried to troubleshoot the problem:

-Measure the 24 volts AC control signal on the contactor, when outside fan starts, it has 24 volts AC, but when fan shuts off, it has 0 volts AC.


i'm out of ideas. Any help would be great. please see the attached picture if necessary.
Attached Thumbnails
AC outside fan starts then off, this recurs over and over.-img_4643.jpg   AC outside fan starts then off, this recurs over and over.-img_4650.jpg   AC outside fan starts then off, this recurs over and over.-img_4648.jpg   AC outside fan starts then off, this recurs over and over.-img_4651.jpg   AC outside fan starts then off, this recurs over and over.-img_4642.jpg  


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Old 05-27-2009, 03:30 PM   #2
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AC outside fan starts then off, this recurs over and over.


You may be low on freon, and the ac is short cycling.

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Old 05-27-2009, 04:28 PM   #3
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AC outside fan starts then off, this recurs over and over.


I have tried to bypass DL15 delay-compressor I(see IMG_4643.JPG), the fan is ok now. Is it possible that there is sensor on the DL15 which will detect the low refrigerant please?

Or I should just need to order a new DL15 for replacement?
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Old 05-28-2009, 07:34 AM   #4
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AC outside fan starts then off, this recurs over and over.


I agree, probably low on freon, which means there is a leak somewhere. Not a good idea to disable safety features, might get really expensive.
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Old 05-28-2009, 02:13 PM   #5
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AC outside fan starts then off, this recurs over and over.


Quote:
Originally Posted by Just Bill View Post
I agree, probably low on freon, which means there is a leak somewhere. Not a good idea to disable safety features, might get really expensive.
His thermostat probably has a delay built into it also.

The OP mentioned jumping out the time delay. The only divination I see is he does not mention if the compressor starts too.

There is a low pressure control that will kill the compressor and fan.

The time delay is either good or bad in my experience and won't cause short cycling.

Just to test it jump out the time delay, if you are low on freon the compressor will cut on and off with the fan. If that happens you need freon.

Don't just have a tech fill it up, have him look for leaks. Very possible that you have too small a leak to find.
Luck.
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Old 05-29-2009, 06:57 PM   #6
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AC outside fan starts then off, this recurs over and over.


the compressor starts too.

I just do some research, it seems to me that a low pressure control is not on the DL15.

DL15 is only a time delay, am I right?

I have tested to bypass the DL15, The compressor and fan will not cut on and off. they are running ok, did I just need to order a new DL15.

By the way, so far, I don't see any leaks.
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Old 05-29-2009, 08:51 PM   #7
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AC outside fan starts then off, this recurs over and over.


Is the contactor staying pulled in when you have the problems ??
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Old 05-29-2009, 10:00 PM   #8
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AC outside fan starts then off, this recurs over and over.


Quote:
Originally Posted by timxiao View Post
i have a 18 year old Lennox AC unit. (model number:HS18QC-211-C1P)recently The outside fan starts for a few seconds, appears to be spinning fine - with no alarming noise, but then shuts off. This recurs over and over if the AC is left on.

Here's a list of things I've tried to troubleshoot the problem:

-Measure the 24 volts AC control signal on the contactor, when outside fan starts, it has 24 volts AC, but when fan shuts off, it has 0 volts AC.


i'm out of ideas. Any help would be great. please see the attached picture if necessary.
Also measure the voltage going To your fan from the contactor if measured 240 VAC and the fan motor is not spinning you have a bad fan motor. The capacitor looks OK, but to properly test your capacitor is by using a capacitor tester.
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Old 05-30-2009, 02:43 PM   #9
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AC outside fan starts then off, this recurs over and over.


when the outside fan and compressor off, the contactor pulled out and no 24 volts, however when the outside fan and compressor on, the contractor staying pulled in and has the 24 volts.
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Old 05-30-2009, 05:27 PM   #10
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AC outside fan starts then off, this recurs over and over.


Have you tried jumping out @ the t.stat to see if it will stay on ?? Something is killing 24 v to the contactor... May be t.stat
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Old 05-30-2009, 11:41 PM   #11
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AC outside fan starts then off, this recurs over and over.


Are you saying the S5 which on img_4643.jpg? yes, I have bypassed the S5 limit hi temp compressor as well. AC outside fan starts then off, this recurs over and over. so S5 is not the root cause.
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Old 05-30-2009, 11:43 PM   #12
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AC outside fan starts then off, this recurs over and over.


I have measured the voltage going To your fan from the contactor . No 240 VAC and the fan motor is not spinning . and I have tested my capacitor, it is OK.
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Old 06-01-2009, 01:06 AM   #13
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AC outside fan starts then off, this recurs over and over.


What is S5 on your wiring diagram? It looks like "thermostat r---".

Do you have a steady 24 volts between Y1 and X? If this is cycling on and off the problem is with your thermostat.

Do you have a steady 24 volts between pin 1 and pin2 of the DL15 module? If this is cycling on and off, and
Y1 to X isn't, the problem is that S5 thermostat, whatever that is.

If both of those voltages are steady on, then it looks like the DL15 is bad.

Last edited by jstjohnz; 06-01-2009 at 01:08 AM.
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Old 07-14-2009, 10:40 AM   #14
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AC outside fan starts then off, this recurs over and over.


This case is close. I fixed the DL15, the issue was gone. please see the IMG_4663.JPG

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Last edited by timxiao; 07-14-2009 at 10:41 AM. Reason: attach the picture
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