No heat, no a/c, no fan, no power

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Old 08-28-14, 05:54 PM
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No heat, no a/c, no fan, no power

The LED indicators are off in the blower window suggesting no power. I used a DMM and there is 120V going to the control board. The control board fuse is ok. I then checked the line voltage to the transformer and it read 54v??? (Should be 120). What next?
 
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Old 08-28-14, 05:57 PM
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I then checked the line voltage to the transformer and it read 54v??? (Should be 120). What next?
What next ??
You need to follow the incoming power that feeds the transformer to find out where it's getting reduced.
 
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Old 08-28-14, 06:32 PM
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I'm kinda stuck as I am a novice. Bk1 is 120 into the board and bk4 out to transformer is 54volts. I also noted there is zero volts coming out of the transformer. I took it off and tested it and it works just fine. Shouldn't it read 10 volts? (1/5 of 54?)
 
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Old 08-28-14, 06:44 PM
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When you do the voltage testing make sure you measure across the two terminals/leads going into the transformer primary side and the secondary side (24V) output. The primary side should read 120V and the output should read 24V. You can measure across the R and C terminals on the thermostat to see if you have 24V there. If so that indicates your transformer is working properly.
 
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Old 08-28-14, 06:49 PM
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Can you post the part number off that board or make and model of the air handler.
 
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Old 08-28-14, 07:05 PM
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R and C read zero. Primary leads read 54v, secodary leads read zero. I took the transformer out and hooked it directly to 120 and the secondary leads read 24v.


Part 50a65475
 
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Old 08-28-14, 07:17 PM
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Correction: the leads from the board to the transformer read 54v. When plugged into the transformer the primary and secodary both read zero.

Also, when measuring the leads from the board it first reads 121v and then instantly drops to a steady 54v every time.
 
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Old 08-28-14, 07:27 PM
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In the following picture should be your board.
In the black circle you'll see your hot line in and hot to the transformer.
In the white circle you'll see your neutral in and transformer neutral connection.

On the back of that board those pins are solder to the board. In your case one may have broken loose from the board. If you're handy you can resolder the pins. If not.... the board will need to be replaced.

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Old 08-28-14, 07:53 PM
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Ok took all the wires out. How to get board off? It has plastic white clips.
 
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Old 08-28-14, 07:59 PM
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The white pins should pull out. I don't think they're the one way type that will break when you remove them.
 
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Old 08-28-14, 08:48 PM
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Got it off. Nothing loose I can see. Below is close up of area.



Some browning in a couple spots on opposite end of board.

 
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Old 08-28-14, 09:11 PM
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There are five neutral connections in a row all soldered in a common connection setup.
The line H and the xfrmr H are back to back.

I'd resolder all seven connections. I tried magnifying your picture to see a crack but couldn't get the definition I needed. One of those connections would have to be cracked to be losing power. It's not always easy to see the crack.
 
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Old 08-28-14, 09:36 PM
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When I measure the disconnected line in wires (black and white) I get 120v. But once they are connected to line-H and neutral terminals I get zero volts.

I also noticed that after I rewired up the board I can now get the LED to turn on for 2 seconds if I cycle power a few times.

Seems there is a transient power drain.
 
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Old 08-28-14, 09:43 PM
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I'll try the soldering. Could this be a capacitor issue due to the transient power loss?
 
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Old 08-28-14, 09:48 PM
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Ok.... I may have misunderstood you previously. Your saying the line coming into the furnace is dropping in volts. If that is the case..... reconnect the board and check from the black wire to ground. See if the hot is at the correct voltage.

If you are losing power on your incoming power line then the problem is not at the furnace. It can't draw too much power and make the voltage go down. Something is not allowing the power to flow thru. It could be a defective switch or connection.
 
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Old 08-28-14, 10:08 PM
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Not what I meant. The wires that supply power to the board have 120v. I measured the ends of the disconnected wires to be sure.

If I then connect the wires to the board (lineH and neutral terminals) and place my DMM probes on the connected terminals I get zero.

If I only place one DMM probe on the lineH terminal and the other on ground I get 120v.
 
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Old 08-28-14, 10:13 PM
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Ok... so your black (hot) wire is ok and you are losing the neutral part of the circuit. It could be a loose connection in the safety switch box or further into the house. Next step would be to check the white connection in the switch box.
 
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Old 08-28-14, 10:32 PM
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But both wires are ok. I get 120v when measuring the ends of the black and white wires that connect directly to the board (after the switch box). It is only when I connect them that I get zero when measuring at the terminals.
 
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Old 08-28-14, 11:08 PM
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Are you measuring between Line-H and Line-N terminals for the 120V? Make sure interlock door safety switch is pushed in.

Here are the tests that need to be done with the DMM and door safety switch pushed in:

1. From Line-H to Line-N. Should read 120V + or -
2. From Ground to Line-H. Should read 120V + or -
3. From Ground to Line-N. Should read 0
4. From XFMR-N to XFMR-H. Should read 120V + or -
5. Between two wires coming from low voltage side of transformer or at the 12 pin connector that plugs into the board (make sure they are the right wires coming from the transformer) . Should read 24V + or -
6. Between C and R at the low voltage thermostat wiring terminals on the board. Should read 24V + or -
 

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Old 08-29-14, 07:01 AM
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1 - 0v
2 - 120v
3 - 0v
4 - 54v. (Initially flashes 120 but drops to 54 and steady)
5 - 0v
6 - 0v
 
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Old 08-29-14, 07:24 AM
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Confirm you have the black wire from the door switch connected to Line-H terminal and white wire from power coming in connected to Line-N terminal.
 
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Old 08-29-14, 07:31 AM
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Yes it is hooked up correctly.
 
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Old 08-29-14, 08:31 AM
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As I've mentioned several times previously.... you have a poor connection before the furnace. A digital meter will read voltage right thru a poor connection.

Did you pull out the safety service switch and check from neutral/white to black ?
The board needs to stay connected. Everything needs to be connected when you test. You need to find out where you are losing your neutral connection.

The problem is between the furnace and the circuit breaker panel.
 
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Old 08-29-14, 08:33 AM
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Sorry for the delay. My next suggestion is to reset the circuit breaker to the furnace and wait at least 30 seconds. Turn circuit breaker back on and perform test 1 again. If you still don't get a proper reading shut power off to the furnace.
Remove the white wire from the Line-N and place it on one of the other empty "N" terminals adjacent to it. These N terminals are all tied together. Turn power back onto the furnace and perform test 1 again.
 
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Old 08-30-14, 09:20 PM
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Thanks for all the help everyone. I'm on a mini vacation and will start troubleshooting again on Monday.

I already tried putting the white wire on all adjacent terminals with no result. I also hooked the black and white up to a lamp to see if a load would drain the power and it did (no light). Yet I still measure 120v across the same black and white wires using a DMM.

I'll start looking at the door switch, junction box and master switch more closely.
 
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Old 08-30-14, 09:32 PM
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I'll start looking at the door switch, junction box and master switch more closely.
As you work your way backwards...... leave the lamp connected as a load.
There won't be enough current flowing thru the power connection which should be helpful in finding the problem.

With the lamp connected...... check from the black/hot wire to ground. If you measure 120vac then the problem is in the white/neutral line which would eliminate the service and door switches. You'd then focus on a neutral connection.
 
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Old 08-30-14, 09:39 PM
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Thanks! I'll let you know what I find in a couple days.
 
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Old 08-31-14, 11:22 PM
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Fixed it! I got home early and it only took me 15 min to fix it. I used the lamp as a load and first checked the furnace junction box wires. I removed all the wire nuts and checked the main B/W wires which showed power. I rewired all the nuts and then checked the safety switch which also showed powere ( lit lamp). I then checked the B/W on the board and the lamp lit!!. I then hooked the board back up and the furnace started. Looks like a bad neutral in the junction box. I have A/C again!!!

Can't thank you guys enough. Especially PJ.

Cheers everyone!!
 
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Old 09-01-14, 04:52 PM
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Awesome...... glad you got it running.
 
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