Cant figure this out! Not getting 24v at the board or thermostat

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kok328

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I'm pretty sure you've lost a leg of power on the 240V circuit and your meter is reading the backfeed from the remaining leg.
 

jayrod

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I'm pretty sure you've lost a leg of power on the 240V circuit and your meter is reading the backfeed from the remaining leg.
That would make the transformer short out or go bad or whatever happened? Because Ive gone through 2 now. Original one and a new one.
 

kok328

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It won't make it short out but, it would make it not produce 24V on the secondary.
Take either transformer (old or new) and hook it up to 120V using the white (neutral) and black (hot/120V) and test the secondary for 24V output. Chances are they both still work.
 

jayrod

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Transformer doesnt work because I tried it on the working unit, and then that unit didnt work. No 24V. So either the Transformer was sold to me bad or I blew it.
 

jayrod

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Alright I still haven't resolved this and winter is coming fast! I haven't even touched it since. I do not want to blow another transformer so I would like to fuse the next one. Do I just fuse the red 24v wire coming out (the one going to the thermostat board) with a 3A or 5A fuse? Will that atleast keep me from blowing the transformer and just blow the fuse instead? Also, I was going to replace the contactor outside since its cheap. See if that fixes it.
 

jayrod

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Yes but is that the correct spot to wire the fuse so it doesnt blow again?
 

nealtw

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You would only have to fuse the wire carrying the voltage, if you have a short to ground a fuse on the other wire would not help.
 

jayrod

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Okay I made some progress. I unscrewed the thermostat board and looked at the solder points on the back and one of the main ones that feeds the Transformer is burned up. The solder point is burned, looks like its not making a connection. The wire that goes to the burned solder point is the black main power wire coming into the house that goes to the heat sequencer, and then goes to the board labeled HEATER, and that's where the burned solder point is.

More general pics of what the unit looks like are on page 2.

The first pic is where the main power (black) wire comes into the house and splits into 3 wires going into the sequencer.

The 2nd pic shows the front of the board, and the HEATER connection. That's where the black wire leads to at this point. I had the whole connector unplugged at that point.

The 3rd pic shows the back of the board, and the burned connection. That connection feeds the T3 terminal that the Transformer plugs into to get 240 Volts, and also looks to feed the M2 part of the motor which I assume is where it gets its 240 volts. So what would cause the burned solder point?? Do I need a whole new board or can I just clean and resolder?

The 4th pic just shows the whole back view of the board.

Edit - Just had a thought. Since the blown point only leads to 2 connectors (T3 and M2), why cant I just bypass the board all together and connect the black wire to T3 and M2 at the same time? That would bypass the board and the burned solder point. Wouldn't that work?

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jayrod

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Connected the wires together to bypass that burned solder point and it works! Sort of. The fan runs in all modes now but the outside unit doesn't want to come on. Usually after I set it to heat the outside unit comes on after a few minutes and that's when it actually starts heating. Now its not coming on. Uughhhh!
 

jayrod

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Okay it finally came on outside but didn't stay on long enough to heat up anything. It kept getting louder and louder outside and then it just shut off. Doesn't sound good. The compressor felt very hot.

Edit - It seems to be working like it always has over the years. This thing has always taken a while to heat this house but then again its 3400 sq. ft.

Its just so loud outside! Sounds like a train.
 
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nealtw

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Are you sure the load unit hasn't got some other problem that would cause it to draw more current than it should and maybe play havick on other components. Perhaps it has a shut off for hi heat.
 

jayrod

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Are you sure the load unit hasn't got some other problem that would cause it to draw more current than it should and maybe play havick on other components. Perhaps it has a shut off for hi heat.
The unit is pretty darn old, and that's about all I can tell you. Once it actually gets cold around here, Ill see how it does. Its still in the 80's here in south Ga. We only use this unit in the winter time because its downstairs, and heat rises. It had one of those thick jackets around the compressor. Should I remove that? It hardly ever freezes here.
 

Jcastrence

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Hey guys, I had a similar problem, the board is not getting 24v out from the transformer, bit the transformer is getting its 120v. So far I had the luck of buying two bad transformers not putting out 24v bit 11v. What are the odds of that right! So my question is there another problem why the transformers are going bad, the board is getting its efficient 120v to and out from the board and to the transformer. Help please.
 

Wuzzat?

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There's a partial short/overload in the circuitry fed by the transformers that is pulling enough amps (maybe around 3A) so that your transformers can only supply 11v and then they fail due to overheating, not from some fault within themselves.

Check the current draw with a clamp-on ammeter; voltage is only half the story.
 
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bud16415

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Are you checking voltage on the transformer secondary with it open (not connected)?
 

kok328

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Do yourself a favor and replace it with one that has a manual overload reset built in.
Where are you checking for voltage on the secondary of the transformer?
Is there a fuse on the circuit board that is bad?
 
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