central air conditioner - fan shuts down

When I turn on my central AC, the furnace fan starts up and runs for a second or two then shuts down. The diagnostic led on the controller indicates Normal - no heat call.
The outside compressor unit continues to run, without the furnace fan blowing.
I turned on the heat, and it works ok.
Does anyone have a suggestion on how to troubleshoot this further?
Thanks, SD.
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SD wrote:

My guess would be a bad starting capacitor on the motor. How old is it?
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CJT wrote:

If the motor works fine for heating, its not the cap.
If you have a switch for the fan on your T-stat, set it to "on" and see if the motor comes on. Then adjust the T-stat so the A/C comes on, leaving the fan switch on. If the fan continues to run, there may be a problem with the wiring of the T-stat which prevents the fan from coming on in the "auto" setting.
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Mikepier wrote:

... except that the two run at different speeds.

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MOST direct drive blower motors don't have a start cap, only a run cap. Split phase motors don't have a run cap at all.
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SD wrote:

Fan relay could be bad, if it uses one. Call an A/C company. - udarrell
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udarrell wrote:

The fan will run if the thermostat switches are: Fan-On+Cool/Heat - Off, Fan-Auto, Cool/Heat-Heat. As soon as I switch the Cool/Heat to Cool, no matter what the Fan setting is at, the Fan shuts down. If I turn the cool/heat Off, and have Fan-On, the fan won't come on until I switch the Cool/Heat to heat. I took the thermostat out of the picture. If I hook Red-Green - Fan comes on, If I hook Yellow-Green - Compressor comes on. As soon as I add Yellow to Red+Green the fan shuts off.
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SD wrote:

My final troubleshooting attempt...I moved the blower fan motor wires around to try and use a different fan speed for the cooling function. No help..it behaves the same on all speed positions. I'm thinking that its the circuit controller.
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The other possibility is that the wiring on the circuit board is wrong. Check there.
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