AC compressor not starting when thermostat call for cooling


  #1  
Old 05-06-14, 09:45 PM
D
Member
Thread Starter
Join Date: May 2014
Posts: 4
Likes: 0
Received 0 Likes on 0 Posts
AC compressor not starting when thermostat call for cooling

Bryant furnace works fine in heat mode but will not start ac compressor when in cool mode, although the blower fan starts. Verified that temp set on thermostat is 5 or more degrees lower than room temp. Changed out thermostat, same problem. I can get the compressor to start if I jumper from R to Y on furnace control board which will also start the blower motor. Verified that I have 24VAC between R & C and G when the blower fan is running but 0VAC between Y & C when in cool. Is this an indication of a faulty furnace control board?

Thanks in advance.
 
  #2  
Old 05-06-14, 09:58 PM
W
Banned. Rule And/Or Policy Violation
Join Date: Apr 2014
Location: USA
Posts: 41
Likes: 0
Received 0 Likes on 0 Posts
Check y to ground and see if 24v present. Go to outside unit electrical contactor coil is 24 present. If 24 present at coil is it pulled in? Is outside fan running? If nothing is running check contactor could be simple as ants bug lizard or debris blocking contact. Check outside voltage. Do you have a unit disconnect mounted to outside wall powering unit? If so check volts there it may be fused and have a bad fuse there.
 
  #3  
Old 05-06-14, 10:18 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 71,073
Received 3,194 Likes on 2,869 Posts
Welcome to the forums.

Is this an indication of a faulty furnace control board?
No.....the control board does not control power on the Y terminal. That is strictly the job of your thermostat.
 
  #4  
Old 05-06-14, 10:23 PM
D
Member
Thread Starter
Join Date: May 2014
Posts: 4
Likes: 0
Received 0 Likes on 0 Posts
Thanks for reply. I guess I wasn't clear, the compressor and fan both start if I jump from R to Y. But when the thermostat makes the call nothing happens except for the furnace blower fan... Which tells me that the compressor can start and run if it gets the proper signal. But I will make the checks you recommend just to be sure.

thx
 
  #5  
Old 05-06-14, 10:27 PM
D
Member
Thread Starter
Join Date: May 2014
Posts: 4
Likes: 0
Received 0 Likes on 0 Posts
Thats what I thought but wasnt sure...
 
  #6  
Old 05-07-14, 07:41 PM
D
Member
Thread Starter
Join Date: May 2014
Posts: 4
Likes: 0
Received 0 Likes on 0 Posts
Wanted to follow up and let you all know we resolved the problem earlier today. Turned out to be an open in the yellow wire that was tied to Y on both the thermostat and the furnace. The original wire bundle from our thermostat was spliced into a newer bundle a few years back when we converted from the swamp cooler to the refrigerated air. Instead of using wire-nuts, the wires were spliced together with a water proof type of connecter. This bundle was wrapped with tape and tucked back behind a gas line. Somehow overtime that connector failed and caused an open, thereby preventing the signal to trigger the compressor.
We ended up using another thermostat and tied it directly to the furnace control panel with a short section of wiring. Once we realized that we could make everything work it came down to figuring which wire was bad by tying two wires together on one end and ohming out the pair on the other side.

Thanks again for the tips.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: