A/C low voltage control issue


  #1  
Old 07-20-15, 02:04 PM
M
Member
Thread Starter
Join Date: Jul 2015
Location: United States
Posts: 4
Upvotes: 0
Received 0 Upvotes on 0 Posts
Question A/C low voltage control issue

Hi all,

Iím sitting here sweltering with A/C issues, and am hoping someone has run across this before and can provide guidance!

The initial symptom was the A/C fan was running, but no cooling. The outside unit was not running. This was initially intermittent (only when over 100 degrees outside), but has since become permanent.

After lots of trips to the attic and to the backyard, I finally found that when the contactor was connected, and the A/C was commanded on, the 24vac appears to drop to 0v! When the contactor is disconnected, we still have 24v. The even stranger part is the house fan continues to run! If it really went to 0v, I was expect the house fan to stop.

So, to me, it appears the coil in the contactor appears to be shorted, but it still has about 12.6 ohms. Low, since any reference I could find says about 20 ohms is normal(ish)!

In any case, I went out and got a new contactor. And, in an unusual case of thoughtfulness, I tested it out at the thermostat (we have a Wi-Fi thermostat, so we have the 24v at the t-stat). Connecting the contactor between the blue (C) and red (R) wires should have activated it, and pulled down the coil, right? It didn't. It just showed the same issue of pulling the voltage down to 0. I checked and the meter is indicating that itís pulling just 0.25ma (yes ľ of a milliamp).

Granted most of my experience has been with DC and not AC wiring, but I'm obviously missing something! What is it? Can it be the transformer?

Thanks again for any guidance or suggestions you might have.

Mark
 
  #2  
Old 07-20-15, 02:50 PM
Bob14525's Avatar
Member
Join Date: Jun 2013
Location: USA
Posts: 1,554
Received 59 Upvotes on 50 Posts
Does the control board on your furnace or air handler have a fuse (typically 3A)? If so, check to see if it's good. If it is, then your transformer may be bad, although it's odd that it would output 24VAC yet not supply any reasonable current. If you have some clip leads, you could try temporarily connecting the contactor coil directly to the output of the 24V transformer and see if the contactor pulls in.
 
  #3  
Old 07-20-15, 03:08 PM
M
Member
Thread Starter
Join Date: Jul 2015
Location: United States
Posts: 4
Upvotes: 0
Received 0 Upvotes on 0 Posts
Fuse is good (Replaced it to be sure).

Good idea of going direct to the transformer, leaving everything else out of the picture. I'll try that this evening/tomorrow morning.

What I hadn't mentioned was that for a while I thought it was the t-stat wiring. So, I replaced the wire from the control box to the t-stat. I had it coming out the attic access connected to the t-stat sitting on a chair for a few days, and it worked fine (but the temp was a little lower then). But, after I ran it through the wall and the temp jumped back into the 100's, it started acting up again. One thing to note, and I doubt it has anything to do with this, was there is a power wire going through the same hole as the t-stat wire. Doubt there could be any sort of cross-talk, but has anyone heard of that before?

Thanks,

Mark
 
  #4  
Old 07-20-15, 03:33 PM
Bob14525's Avatar
Member
Join Date: Jun 2013
Location: USA
Posts: 1,554
Received 59 Upvotes on 50 Posts
Mark, is your air handler in the attic? If so, this is a longshot, but I'm wondering if you might have a bad transformer, one that has a high resistance secondary winding. When the system was intermittent, the high temperatures would have made the transformer core & windings expand, and if you had a small break in the wires, it could conceivably create a high resistance connection. Enough to supply 24V open circuit, but drop to 0 when a load is connected.

Also, I don't think your 0.25ma measurement is correct. Don't forget, this is AC voltage, not DC voltage. Most multimeters that I've seen only measure DC current, not AC current. If you assume a DC resistance of 20 ohms, 24VAC would give an inrush current of 24/20 or 1.2 amps. The steady state current will be somewhat lower because the AC impedance is higher than the DC resistance (function of the inductance and frequency {60Hz}).
 
  #5  
Old 07-20-15, 04:22 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 62,067
Received 3,422 Upvotes on 3,068 Posts
The even stranger part is the house fan continues to run!
The house fan won't run if the transformer is bad or the fuse is blown. You need to check between Y and C at the air handler and again at the condensor. It sounds like you have a partial or corroded connection.
 
  #6  
Old 07-20-15, 05:11 PM
M
Member
Thread Starter
Join Date: Jul 2015
Location: United States
Posts: 4
Upvotes: 0
Received 0 Upvotes on 0 Posts
I just got down from the attic and am sitting under the ceiling fan trying to dry off.

Thanks Bob, for your suggestion of jumping the contactor directly to the transformer. I did that (disconnecting the lead to the board first), and the contactor worked. Ok, so it's not the transformer. Next, I disconnected the Red wire from the board, and jumped the contactor directly between the red wire and the blue (C) wire. Nothing.

To me, that sounds like something in the board is gone. Triple checked the fuse and that's fine.

Anyone know where I can find a schematic of the board, rather than the connection diagram? It's for a York Diamond 80 (couldn't find a more specific model number, anyone know where it lives up there?).

Thanks for all your help!
 
  #7  
Old 07-20-15, 06:21 PM
Houston204's Avatar
Forum Topic Moderator
Join Date: Oct 2009
Location: Texas
Posts: 6,420
Received 75 Upvotes on 68 Posts
Limit switches can be located between the transformer and the furnace R or C terminal.
Does the fan turn off from the thermostat?
If a limit switch is tripped the fan would probably be stuck on.

That connection diagram may help determine if you have a limit switch in the circuit.

We will need a model number to see a diagram.
 
  #8  
Old 07-20-15, 08:05 PM
M
Member
Thread Starter
Join Date: Jul 2015
Location: United States
Posts: 4
Upvotes: 0
Received 0 Upvotes on 0 Posts
I think I got it!

When I was in the attic earlier with the contactor, I only disconnected the R wire when I tested it at the board. I later realized that I should have at least disconnected the Blue (C) wire, since that runs out to the condensor with the Yellow wire.

So I went back up, and tested it with the C wire disconnected and it worked! The pair of wires heading out to the condensor had about 750K resistance (when disconnected at both ends). I switched to a different pair of wires (the wire going out is a 5 conductor t-stat wire). I knew I was on the right track when I started feeling shocks from the wire

So it's all put back together and running. Since the unit is undersized, it will take a while to be sure, but it does feel like cool air coming out!

Thanks again for all your suggestions and ideas. They really did help point me in the right direction!

Mark
 

Last edited by merlin309; 07-20-15 at 08:07 PM. Reason: Fixed some grammar errors!
 

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