Missing 24v on control line


  #1  
Old 05-21-24, 11:43 AM
T
Member
Thread Starter
Join Date: May 2024
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
Missing 24v on control line

My AC unit keeps trying to turn on but turns off after 15 seconds as the Compressor won't kick on. The blower in the furnace kicks on and works fine.

Outside on the Contactor i measured 240v on the in and 0v on the out, but if I push in the Contactor button then I read 240v on the out too and the Compressor starts up. I checked for 24v on the spade connectors and got 0v. I went to the thermostat and checked for volts between R and Y and for 27v. I went to my control board in the furnace area and checked r and y and have 27v there.

The control wires outside did not read any voltage even before going across the Contactor, this made me think that it could be a damaged wire. However the other end of the control wire in the furnace is also reading 0v.

The control wire is red and white. In the furnace the red if connected to the Y and the white is spliced directly with the 24v line from the transformer instead of connecting to the C on the control board. Now if I measure the volts between the splice and Y I'm still getting 0v...which makes me think (make incorrectly) that regardless of whatever is going on outside if I can't get and voltage between the Transformer and Y in the control board I'm never going to get power out to the contractor. I haven't tried disconnecting the control wire from the control boars and measuring the voltage between the splice and Y.

UPDATE: I disconnected the red and white control wires from then transformer splice and Y and I read 27V. So now I know I have power there. So if someone can help me with these basics/rule them out then I would really appreciate it.
1) the red and white low voltage wires are not damaged during the run cause I am reading 24v on them outside when they are disconnected from the Contactor and the low pressure switch
2) the coil on the contactor could be bad and thats preventing the low side from even receiving 24v.
3) the low pressure switch could be open causing the line to not even see 24v. However I bypassed the low pressure switch and I still didn't see 24v when connected to the Contactor

This leads me to believe that at a minimum the contacter is faulty, but it could also be the low pressure switch. Am I on the right track?
 

Last edited by Theone964; 05-21-24 at 12:14 PM.
  #2  
Old 05-22-24, 05:13 AM
Tolyn Ironhand's Avatar
Group Moderator
Join Date: Nov 2007
Location: United States
Posts: 14,428
Received 900 Upvotes on 761 Posts
This is assuming you have a standard outside condenser and an inside furnace/air handler.

When the thermostat is calling for cool you should get 24v between the Y and C (common) on the furnace/air handler terminal board. If you do, then move to the condenser. Make sure the wires are connected well connected to the terminal board. If you don't then you have an issue at the furnace/air handler, possibly a bad control transformer.

At the condenser, when the thermostat is calling for cool, you should have 24v between the two smaller wires going to the contactor. If you don't then you have a bad cable between the furnace/air handler and the condenser. If you do have 24v between the two wires then when you connect them to the contactor coil it should pull in the relay. If not you have a bad relay.



 
  #3  
Old 05-22-24, 12:15 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 65,718
Received 4,111 Upvotes on 3,692 Posts
Welcome to the forums.

I have a sticky at the top of the forum on A/C repair that addresses your problem.
There is a picture there but the board is currently not showing pictures so follow the text.
Please read first for A/C repair

From your description.... you have an open pressure switch = low on refrigerant charge.
 
  #4  
Old 05-22-24, 08:08 PM
T
Member
Thread Starter
Join Date: May 2024
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
Fixed my problem. It was the Contactor. Replaced it and it fired right up.
 
  #5  
Old 05-23-24, 06:08 PM
Tolyn Ironhand's Avatar
Group Moderator
Join Date: Nov 2007
Location: United States
Posts: 14,428
Received 900 Upvotes on 761 Posts
Good deal. Stay cool.
 
 

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