Low voltage problem.

Reply

  #1  
Old 07-15-14, 10:18 AM
Member
Thread Starter
Join Date: Jul 2014
Location: usa
Posts: 9
Received 0 Votes on 0 Posts
Low voltage problem.

Hey everybody, I'm new to diy and trying to tackle this problem. I've been an auto tech for 16 years so I can tell a flat from a phillips.

Here is the problem. Split system with handler inside and comp/condensor outside. Outside unit quit. I can engage it by manually pressing contactor plunger and comp/condensor fan come on and low side line cools well. I tested voltage across low voltage contactor coil with everything connected and have nothing. Disconnected yellow and brown wires to each side of coil and checked voltage across the 2 and still nothing. Ohmed the coil itself .06 ohms.

Disconnected signal wire coming out of brick wall and hooked jumper from it and connected to contactor coil.....nothing. Pulled jumper wire off coil and checked for voltage to ground 27 volts present. Am i checking this ac voltage correctly? Voltage is also present from brown wire on the other side of coil to ground whether disconnected or not.

Removed thermostat inside and jumped R and Y together to call for cool. Was gonna connect green wire for fan when I noticed the inside blower was still running even though thermostat is removed. Only way to shut off inside blower is to shut off breaker. Something tells me this wasn't wired right.

I opened the handler access panels to check where stat wiring transfers to outside. There is a circuit board of some kind where stat wiring comes in and sends signal outside. I can see low voltage wiring coming from transformer to this board. have 34volts present on low voltage coming out of transformer to ground. checked the diagram on the panel and from the signal to outside unit to ground i have 34volts. there is another labeled "R" next to it and I have 21 Volts there to ground. From the stat I have 34 volts on the "y" signal at this board.

What am I missing? Thanks in advance for the help!
 
Sponsored Links
  #2  
Old 07-15-14, 10:39 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 52,596
Received 337 Votes on 315 Posts
Welcome to the forums.

Ohmed the coil itself .06 ohms
Sounds way too low. Almost a dead short. I'd expect it to be several ohms. Were you on the right ohms scale.

What's the make and model of the system you are working on ?
 
  #3  
Old 07-15-14, 11:25 AM
Member
Join Date: Feb 2010
Location: usa
Posts: 623
Received 0 Votes on 0 Posts
Back at the contactor you measured the voltage between the wires and ground and got 24 volts on each? One wire should have voltage between it and ground and the other wire should have no voltage between it and ground. Those two wires at the circuit board in the furnace are "y" and "c". Common being ground and y being the hot wire. Between y and c you should have between 24 and 30 volts.
 
  #4  
Old 07-15-14, 11:46 AM
Member
Thread Starter
Join Date: Jul 2014
Location: usa
Posts: 9
Received 0 Votes on 0 Posts
On lunch break right now.
Outdoor unit is ruud UAND -60JAZ
Handler inside is ruud UBHK-25J14SFA

Installed new contactor just to eliminate it...my multimeter only has one ohm setting up to 2k ohms. Checked new coil continuity .019 ohms.
Checked coil voltage again....both sides disconnected and checking from each wire end to ground. Yellow 29.3 volts and brown wire 23.1 volts.
 
Attached Images  
  #5  
Old 07-15-14, 11:55 AM
Member
Thread Starter
Join Date: Jul 2014
Location: usa
Posts: 9
Received 0 Votes on 0 Posts
Between y and c on the handler board i've got zero.
 
  #6  
Old 07-15-14, 11:58 AM
Member
Thread Starter
Join Date: Jul 2014
Location: usa
Posts: 9
Received 0 Votes on 0 Posts
Just saw the common coming off the transformer to the board. Checked y to that common and i have 24 volts.
 
  #7  
Old 07-15-14, 12:04 PM
Member
Thread Starter
Join Date: Jul 2014
Location: usa
Posts: 9
Received 0 Votes on 0 Posts
Just jumped the transformer common on the board to the common going out and outside kicked on. Bad board?
 
  #8  
Old 07-15-14, 12:47 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 52,596
Received 337 Votes on 315 Posts
Bad board..... doubt it. Check the circuit. Check from transformer to board to chassis.

The circuit schematic should be right on the equipment cover.
 
  #9  
Old 07-15-14, 01:01 PM
Member
Thread Starter
Join Date: Jul 2014
Location: usa
Posts: 9
Received 0 Votes on 0 Posts
I've got a jumper wire in place from the transformer common on the board to the common going out to the contactor and its working. It loses common on the board from the transformer to the terminal where it goes out to the contactor.

The pic shows how my jumper is in place.
 
Attached Images  

Last edited by outthegate98; 07-15-14 at 01:23 PM.
  #10  
Old 07-15-14, 02:34 PM
Member
Thread Starter
Join Date: Jul 2014
Location: usa
Posts: 9
Received 0 Votes on 0 Posts
Common sense tells me to replace the control board. I looked on the back and the transformer common is shared with the "c" out to the unit. Does this sound right?
 
  #11  
Old 07-15-14, 03:18 PM
Bob14525's Avatar
Member
Join Date: Jun 2013
Location: USA
Posts: 1,470
Received 11 Votes on 8 Posts
Do you have a schematic (circuit diagram)? If so, does it show the transformer common connected to the "C" going to the outside unit? I suspect that the two should be connected, and the fact that the system works when you jumper the two confirms that. Have you removed the circuit board and looked at the back to see if there's a burned trace or a connector that's come unsoldered? If there is something wrong with the board, but it's something you can easily fix, why buy a new board.
 
  #12  
Old 07-15-14, 05:40 PM
Member
Thread Starter
Join Date: Jul 2014
Location: usa
Posts: 9
Received 0 Votes on 0 Posts
Got off work and as soon as i removed the board, sure enough that common circuit was burned. Put in the new one and all is working normally. Thanks for all the help!
 
  #13  
Old 07-15-14, 06:05 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 52,596
Received 337 Votes on 315 Posts
That's a pretty potent transformer to melt a p.c. board foil. You may want to put a 3A fuse in line with the non common side of the transformer for protection.
 
  #14  
Old 07-15-14, 06:52 PM
Member
Thread Starter
Join Date: Jul 2014
Location: usa
Posts: 9
Received 0 Votes on 0 Posts
So on the wire from the transformer to the "r" on the board is where i should add a fuse? Can it be an automotive fuse? Nylon covered female terminals ok? Soldered or crimped?
 
  #15  
Old 07-16-14, 07:43 AM
Bob14525's Avatar
Member
Join Date: Jun 2013
Location: USA
Posts: 1,470
Received 11 Votes on 8 Posts
Yes, you want to add the fuse in series with the line going from the transformer to the R terminal on the board. An automotive fuse is fine (that's what most system that have fuses use). You can use whatever method you want to attach the fuse holder. Crimped connections are acceptable, however if you have a soldering iron, I would solder the connections as well. It just ensures that you won't have any loose connection issues in the future.

I hope the new circuit board wasn't expensive, because you could have easily repaired the old board. The best way would be to solder a piece of wire across the blown trace. If you couldn't do that (no soldering iron), you could have just left your wire connecting the two commons.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


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