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



## jayrod

Its a Carrier split unit. Had the heater on yesterday and at some point when I wasnt around it turned off on its on. Its completely dead at the thermostat, and on the circuit board it doesnt get 24 volts either. So I tested voltage going into the transformer and it has 120 volts on all 3 wires going in but it only has .1 volts on the 24v side. Well thats not right. We have a 2 story house and both units have the exact same transformer so I measured the transformer on the working unit to be sure Im dong it right and it measures 120v on all 3 coming in, and about 27 volts going out. So I go and buy a new transformer, wire it up, and still no 24 volts! Did a resistance check, all wires have some resistance. So now Im stumped. If it has voltage coming in, how is it not coming out!? What should I check next?


----------



## nealtw

Switch transformers from the other unit, maybe you got a bad one.


----------



## kok328

Your on the right track but, nealtw has the most probable answer.  What are the odds?
Unfortunately, most of the time, electronic parts are not refundable as the seller suspects that you did something wrong to burn up the component.


----------



## jayrod

Thanks guys. I have switched transformers this morning. I took the new one I just bought that didnt fix my downstairs unit, and installed it on my working upstairs unit. It didnt work! Same thing, power coming in, but no 24v coming out. So I took it out and put it back like it was and it works still so all I know is I did get a bad transformer or either something shorted it out right when I hook it up. but it doesnt smell or hum or anything. Oe thing I did notice is the working uptairs unit only requires the common and the 240v wires of the transformer to be hooked up while the downstairs unit requires the common, the 208v and the 240v wires to be hooked up. (Common goes to XFORMT1, 208v goes to T2, and 240v goes to T3) But thats how the old transformer was wired so I wired the new one the same. I just didnt think it was very likely to get a bad Transformer.


----------



## jayrod

Wait, I believe I should have been measuring across the terminals to get 240v or 208v. I aint geting either when I measure across. I think there must be a short or a safety switch keeping the 240v from coming in. The contactor outside has 240 going in and 240 going out when I manually press  it in. T2 and L2 have no voltage on the contactor.


----------



## CallMeVilla

Standing by, with rapt attention, to see how this resolves  ....


----------



## kok328

Ok, now you've got me confused.  You originally said that you have a 120V primary and 24V secondary transformer.  Now your saying it's 240V or 208V.  Did you buy the wrong transformer and smoke it when you hooked it up to a higher voltage?
T2 & L2 are opposite sides of the contactor but, are the same leg.
Sounds like you blew a fuse on the L2 phase.


----------



## jayrod

I dont see any fuses on the outside part of the unit. Its the right transfomer, I just didnt undersand it. Its a 240v setup. It says 240vac on the little circuit board inside the handler.


----------



## jayrod

Should T2 and/or L2 have any voltage when its not turned on?


----------



## kok328

is this a heat pump unit?
L1 & L2 is LINE power, T1 & T2 is the LOAD.
On a split system, the air handler/furnace components would be 120V and the compressor unit outside would be 240V.
Where exactly are you working (on the 120V or 240V side of the split system)?
If your working outside on the compressor unit where I would expect to find a contactor, then the fuses would be in a service disconnect box.  Follow the electrical lines from the compressor unit to the side of the house where you will find a metal box that houses the fuses.
You may have to remove a piece of cardboard to see the fuses (make sure you turn the breaker off to the unit before digging into the box).


----------



## jayrod

The metal box has a quick disconnect thats not fused. It says "disconnect not fused" right on it. Its 240 volts outside and inside on the air handler. I measured the main wires going into the handler and they measure 240v. I checked the main breaker and it has 240v.


----------



## keepinitcool

Get a qualified person to service this unit. Electricity is nothing to mess around with and it will save you from damaging other components of your system and possibly costing you more money in the long run.


----------



## nealtw

keepinitcool said:


> Get a qualified person to service this unit. Electricity is nothing to mess around with and it will save you from damaging other components of your system and possibly costing you more money in the long run.


 
This may be good advice but kok328 has already determined that the op has some knowledge and is trying to work thru it with him. It is a diy site.


----------



## wireless

Bad fuse on the load side of the transformer.


----------



## kok328

Can we get a inside pic of the cabinet so I can see what I'm doing?
Being able to see is most benefical.  Blindfold a tech and ahe will at least be able to feel his way around, we can do neither.


----------



## jayrod

Sorry for the late reply It didnt notify me by email like it was and I thought you guys gave up on me. I should be fine working on it. I have fixed it numerous times over the years but this time Im not sure what the problem is. Heres pics of the handler inside the house. If you need more, let me know.


----------



## kok328

Ok, now that I can see what I'm dealing with, I now know that I'm working on an electric furnace not a gas furnance.
Pretty sure you have a multi-tap transformer that takes only 1 of 3 combinations of primary power (120V, 208V or 240V).  The secondary power goes directly to the board.
*What are you using for primary power (120V, 208V or 240V)?*
Also, can't tell if the fan wire is just hanging loose or is attached to the Y2 (second stage cooling).
*Can I get a pic of how the primary wires are connected to power?*
*At this point your saying that you have primary power but, no secondary power and that the transformer from your other unit works in this unit and the unit heats and runs fine with the transformer from the other unit, correct?*


----------



## jayrod

Yellow is connected to Y2. The green one is hanging loose because I tried to jump it with R way earlier.

 I never tried the transformer from my working unit in this unit for fear of shorting it out and then being stuck with no working units! I tried the transformer from the non working unit in the working unit and that made the working unit non-working, if that makes sense. Really you just had it backwards.

By primary you mean the common? The common plugs into the XformT1 terminal in the above pic. It was unhooked in the picture. When I measure across the XforTt1 terminal and one of the main incoming wires, I get 240V. Wouldnt that mean its losing secondary power somewhere along the way in there? Thanks alot! I'll get another pic soon.


----------



## jayrod

If I measure across XformT1 and any of the wires on the front of the heat sequencer I get 240v as well.


----------



## kok328

If the two units are identical, then this will make troubleshooting a breeze.
Are the primary transformer wires in the working unit connected to 120V, 208V or 240V?
What voltage are you readings on the connections on the primary wires for the working unit?
Are the same voltage readings on this unit?


----------



## kok328

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

kok328 said:


> 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

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

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

I'll still try that real quick


----------



## jayrod

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.


----------



## nealtw

You should research the thermostat  and find the amps it requires.


----------



## jayrod

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


----------



## nealtw

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

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?


----------



## jayrod

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

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.


----------



## nealtw

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

nealtw said:


> 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.


----------



## nealtw

With this stuff, I do better asking questions more than answering them.


----------



## Jcastrence

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?

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.


----------



## bud16415

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


----------



## kok328

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?


----------

