Author Topic: A/C Issue  (Read 2563 times)

Offline wazlife

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A/C Issue
« on: July 16, 2013, 07:30:42 PM »
Hey guys,

Hot as heck here in Eastern Ontario. Anyways, I was finally installing the central air unit in my house. Had everything done except for the wiring (24v side not 210). Here's my layout:

RC (no jumper between two)
O/B (Heat Pump application I'm assuming)


So, I installed a jumper between RH/RC and here's how I wired it:

G - G
Y - Y
W - W
C - C
O/B - Unused

Went to turn on the A/C and.....NOTHING! Opened up the panel to get to the board and I could smell a bit of burnt. Went to check the fuse right away and received a shock. lol, Took fuse out with pliers and it was fine. Replaced it as a precaution. No before I energized the furnace again, I removed the C - C connection (told by a colleauge this wasn't necessary connection as it could "power steal" from my digital thermostat). Nothing again. SO i thought maybe a feedback hit the thermostat. So I tried to cross R and G for fan. Nothing. Then I crossed R and W for heat, nothing.

My next guess would be transformer. Or would I have fried the board?

Any tips? Suggestions? Help?

Stay cool!
G2/OBT2 - new to service, worked as a meter installer for a gas company for 16 months.

Offline walker

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Re: A/C Issue
« Reply #1 on: July 16, 2013, 08:51:39 PM »
remove the board from the panel its snapped into and flip it over, if there's a big black burn mark on there, then the boards fried. thats what you're smelling, it could still be the board even if there is no black burnt mark. next put the thermostat into cool mode and remove the blower connection for cooling and check that tab from there to see if you're getting 120V, if not its probably a bad board, also check that the furnace itself is receiving 120V  to the board.

Offline Admin

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Re: A/C Issue
« Reply #2 on: July 16, 2013, 09:18:21 PM »
Test the transformer with the power off.  Set your meter to audible ohms and remove the 4 transformer wires and test between the 2 wires from the primary then the 2 wires from the secondary side of the transformer.  If your meter doesn't beep then that's the side of the transformer that has failed.

I would use the same type of continuity test to verify the thermostat wiring is okay.  Always re-test the wires for no continuity after the continuity test to make sure the wires aren't shorting together.

The 24V fuse should have protected the control board.

Offline Trainerguy

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Re: A/C Issue
« Reply #3 on: July 25, 2013, 09:25:09 AM »
As a community college instructor and former National Training Manager for a large HVAC manufacturer I discourage the use of the "buzzer" portion of the ohms scale as it can lead to improper diagnosis of a problem. Take for example a "thermistor", these are normally in the range of many thousands of ohms resistance and the "buzzer" circuit cannot overcome the high resistance and would indicate an "open circuit", leading to a possible costly misdiagnosis. Also it cannot tell if the "thermistor" is within the resistance/temperature range specified by the manufacturer.
With todays high tech equipment, the use of "thermistors" is becoming more widespread and it is important that techs how they work and how to test them.