Air Conditioning - Fan doesn't run when AC on?

I have central ac and a gas furnace. When the tstat is set to heat, everything works fine. When I turn it to AC, the compressor turns on but the coils ice up because my fan does not turn on. In addition, the fan will not turn on when I put the switch (on / Auto) on "ON". This is true if the tstat is set on heat or cool. I opened the tstat up and jumpted the R & G but the fan did not kick on. I checked the low voltage wire connections at the furnace and at the compressor and all the connections are fine. I checked the run of the low voltage wire and I did not find any wires chewed/worn through. I believe this narrows my problem to either (and correct me if I'm off base): 1) the blower motor 2) the fan relay (which is integrated to my control module) or 3) the capacitor. Is there any way to determine which one without a service call?

Thanks in advance.

on the furnace, do you hear a click if you jumper R and G?

if yes, then maybe the winding on coooling/fan on is out.. If you know your equipement well, try another speed.

When I jump R & G on the furnace, nothing happens with the fan but there is something happening. It sound like the coolent going through the coils or something but nothing with the fan.

Do you have a meter to see if you are getting power from the cooling speed tap on the board? If you don't, and don't know what to do, you are best to turn of the a/c and call for service.

My meter broke some time ago so I'll have to borrow one from the neighbor tomorrow. You are saying to see if I have power from the control board to the motor on the "cool" wire, correct? If there is no power, does that indicate a relay issue on my board?

Right.

I am guessing you have a multi speed fan.. Cool on most system is wired on black wire, if you have yellow not used, put it on if you are getting power on COOL

I do have a 3 speed fan. Since I don't have a meter tonight, I took your suggestion. I put the wire marked "park" and switched it with the "cool". The fan still does not kick on when I move the fan from "auto" to "on". I put the AC on and it didn't kick on immediatly either. I'll double check it with the power meter tomorrow. Based on our conversation here, it sounds like my fan relay, which is on my control module board which means $300! I'll check the power tomorrow and let you know.

Thanks for the help! I do appreciate it. If I'm off track on my comments above, please let me know.

This just happened to me a couple of weeks ago.

We changed out the t-stat...that wasn't the problem. It turned out to be the circuit boad on the blower unit in the attic.

Neighbor wasn't home to get a tester so I'll try again tomorrow to confirm it is on the relay which is on my board. Thanks for the input RF and Jay. I'll update tomorrow if I get the tester, or I'll just pick up another one since I could use it anyway.

Anyone have a preferred supply place for control boards?

Find out the model number of the unit where the board is hooked up. You should be able to find it that way. I went to Google and pulled up a bunch of places that had the same board.

My a/c guys charged me around $450 for a board I found on the internet for $275.

You should be able to find a good deal out there somewhere.

Put the black wire on the line tap. Moter comes on its the board. If its hot tonight leave it on. and go ahead and use a ac.

I'm not sure where the "line tap" is. I put the black wire that was connected to the board (from the motor) that was plugged into the board and marked "park" on the board and connected it to the spot marked "cool" on the board. The fan still didn't run.

I got my volt meter, set the furnace to heat (which has always been working) and hooked it up between the common and each of the fan speeds (with setting at heat so it would trip) and all 3 (cool, heat, park) have power. I went to the low votage and jumped G to Y and G to W and G to C and all three had power. However, when I jumped G to R there was no power. I had my fan set to "on" at the tstat the whole time. I did replace the tstat but still have the same problem. Should I have power when I jump R to G directly on the circuit board? I thought this would trigger the fan but it didn't. Again, my problem is the fan doesn't run when I put the tstat to "on" and the fan doesn't run when I put on the AC (but the compressor does). What do I check next (capcitor)?

I just got my new control board delivered today and installed it. Everything is working fine now. Thank you to everyone who helped and provided feedback. It helped.