Need Help: A/C Blower Not Working

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  #1  
Old 07-29-05, 06:48 AM
Donald100
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Need Help: A/C Blower Not Working

Hi

The blower on our central heat/air conditioner has quit working. I've confirmed the following:
+ does not work in either heat or cool mode
+ does not work with the thermostat set to FAN ON
+ does not work if I manually spin it while having FAN ON
+ the outside condenser unit works normally during the above

It seems that the blower is not getting power (no hum). Will try testing voltages if I can manage to read the wiring diagram (small print, hard to read). Any ideas? Any common problems with controller boards, overrides that might cause this? Can blower start capacitor be ruled out?
Thanks.
 
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  #2  
Old 07-29-05, 01:28 PM
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No fan

Most likely the capacitor can be ruled out since spinning the fan by hand does not help. You could have a bad board or fan motor. Tip: some equipment has a door switch which must be closed (opens when door is removed or opened) for any power to go to the blower section. This is almost always true with a gas furnace.
 
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Old 07-29-05, 08:44 PM
Donald100
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No Fan

Grady

Thanks for the reply. I've done a little more testing since my first post:

+ 120V at access panel switch (the switch you mentioned); I did make sure it was closed in my earlier tests
+ no (or low) voltage at controller board pin for power to fan
+ jumpered 120V to fan, fan seems to run fine (no problem with capacitor)
+ checked water pan switch ok
+ check 5A fuse - ok
+ swapped out my current programmable thermostat with an older White-Rodgers mercury switch type - no change

What to do now?

+ beginning to suspect the control board on the furnace/blower (HW ST9120C, electronic fan timer). Is there a simple way to confirm bad? Do control boards fail frequently?
+ other ideas?
 
  #4  
Old 07-30-05, 03:45 PM
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One more test

On the circuit board, try jumpering R & G low voltage terminals. This problably will do no good but it eliminates the possibility of a wiring problem between the thermostat & unit.
Fan control boards do fail & the bad thing is they are almost all different. After the ST9120C, there should be a 4 digit number. That 4 digit number makes a difference.
 
  #5  
Old 05-28-10, 08:30 PM
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Originally Posted by Donald100 View Post
Grady

Thanks for the reply. I've done a little more testing since my first post:

+ 120V at access panel switch (the switch you mentioned); I did make sure it was closed in my earlier tests
+ no (or low) voltage at controller board pin for power to fan
+ jumpered 120V to fan, fan seems to run fine (no problem with capacitor)
+ checked water pan switch ok
+ check 5A fuse - ok
+ swapped out my current programmable thermostat with an older White-Rodgers mercury switch type - no change

What to do now?

+ beginning to suspect the control board on the furnace/blower (HW ST9120C, electronic fan timer). Is there a simple way to confirm bad? Do control boards fail frequently?
+ other ideas?
I have two 15 year old Heil units with ST9120C fan control boards. I have replaced 6 total boards. The fan on power replay to solder connection on the back of the board has been bad every time. It did not blow a fuse or trip the breaker, the circuit opened up and put a black flash mark on the sheetmetal teh board is attached to. I just changed one last night. I take my circuit boards to a repair shop and they solder the connection back. Takes them 10 minutes. I have seen a replacement cost of $110 to $197. A board repair shop is MUCH cheaper.
 
  #6  
Old 09-13-13, 12:45 PM
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Location: USA
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I had the same problem it seems with my carrier HVAC unit. The fan would not come on in heat or cool mode, I jumpered the R and G terminals at the thermostat and the control board and it still wouldn't come on. The only difference was that, if I gave the blower motor a good spin, and then pushed the door switch, the fan would run, but not at peak speed. I checked the run capacitor (easy to spot, right on the outside of the blower housing) and it passed a continuity test (my voltmeter doesn't have a capacitance test function). The thing that tipped me off though was another website I was checking on referenced what to look for when the run capacitor is bad. It may still show that it has continuity, but the top of the capacitor will bulge out like an over pressurized can of soda. You can see how the terminals on top point out at an angle rather than straight up as they should.

I found a capacitor at a local electrical supply store for about $9 and it fixed the problem. Much cheaper than a new control board or motor and a cheap thing to try first.
 
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