O.T. central air conditioner

Last Thursday evening I brought the local granddaughters home. It was close to 8 when we finally got here. It seemed a little warm, so I turned the thermostat down a little. With all the commotion, I didn't notice that the air wasn't working until after they went home. I went downstairs and could hear that the blower fan was not running. The suction line had frost on it both on the furnace and at the condensing unit outside. Saturday afternoon the sons father-in-law came to town for the local festival parade and stopped in to look at it. He used to do heating and cooling as a sideline business. He checked things out and said that there was power going to the motor. He didn't have enough equipment to determine if the motor or capacitor was bad. I took it out last night and tonight I checked it out. The motor works fine with a jumper cord in all three speeds. I don't remember what determines when the motor should run in the air conditioning mode? Is there a relay? By the way, if it matters, it is a Lenox Elite series furnace that was installed as a heating cooling set about 18 years ago.
Any ideas where I should look next? Thanks in advance, Roy

low Freon will ice up also if it is not getting good air flow like a plugged or dirty filter don't ask how I know about the filter BTDT.

Yes, there is a blower relay. Typically a silver metal box, about 1"x1 1/2", black plastic back, 5 or so wires plugged on to it. Just trace the feed to the blower motor back to the relay. Should be a schematic somewhere, on the back side of the cover...

My unit also has a capacitor on the blower motor. I just had to have it replaced to the tune of $74.00. that and other repairs early in June, set me back almost $700.00.

(quoted from post at 03:54:43 07/14/15) low Freon will ice up also if it is not getting good air flow like a plugged or dirty filter don't ask how I know about the filter BTDT.

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id you read or just post???! Yes, when he said blower motor not running, that means no air flow! Duh! :roll:

I would suspect that the unit that controls the blower speeds has failed. Possibly a loss of power from a blown fuse, a relay not functioning, or even a bad thermostat. Low batteries in the newer thermostats can make then do some strange things.
Icing is most likely the result of the fan not coming on.
Without more information, it is difficult to tell, but some of the following may help:
If you are tied into a forced air heating/cooling system, the problem may be in the furnace.
If you have a separate air handler, it may be the problem.
Commonly, there is a 24-volt transformer that provides control voltage. They sometimes burn out, and you lose control voltage.

If its the furnace I think it is, a circuit board controls everything.
Not uncommon for high to go out.
Depending on variation ther are different boards.
Post the complete model # including the # after the- on the end (most importent)
I can direct you to the proper one.
Lennox dealer for 70+ yrs.

(quoted from post at 08:03:24 07/14/15) If its the furnace I think it is, a circuit board controls everything.
Not uncommon for high to go out.
Depending on variation ther are different boards.
Post the complete model # including the # after the- on the end (most importent)
I can direct you to the proper one.
Lennox dealer for 70+ yrs.

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Agree that was the problem with mine earlier this summer.