Not getting 24v from transformer secondary at control board

[Bdog]

My Trane inside unit died, No blower or even power light on the control board works. I started by replacing the 115/24v transformer, then replaced the control board. Still nothing, not even the indicator light on the new board. I show 108V going into the primary of the transformer, but 0 coming out of the secondary. Is this the problem, 108 isn't enough to output 24v?

By the way, I bench tested the old transformer by powering it up with 120v and iFot shows 27v of output at the secondary so I guess it wasn't bad.

 

[bud16415]

Not sure where the 108 is coming from but you should be getting something out the secondary just lower than the 24v a bit. If the old unit bench checks good I would put it back in and see if it changes the outcome.

 

[Bdog]

The 108v reading is from the xfmr leads out of the control board. So power is flowing through the board but the 24v back to it is not present. Will reconnect the old transformer and see if it makes a difference and post the results.

 

[Bdog]

Reconnected the old transformer and no voltage from the secondary, same issues. I'm stumped.

 

[bud16415]

You saw no voltage on the secondary with 108 on the primary and the secondary not connected to anything? Or did you hook the secondary up and maybe something is drawing the voltage down?


I’m not a pro so take my advice with a grain of salt. I saw you weren’t getting pro help so I jumped in. Others should be along also.

 

[Bdog]

Thx bud16415. No, secondary not connected to the control board for the reading. I tested secondary both with the wiring leads, and without, to eliminate the wires as the issue. That's why I'm baffled.

 

[bud16415]

Don’t feel alone it is baffling me as well.


Just for fun and I don’t know what it will prove but bench test your other one , new one also.


All I can think of is you could have some corrosion on the inside of the power plugs (primary) and you are not passing current but testing off the outside.


Lets see if someone has some ideas.

 

[bud16415]

Just thinking outside the box. I have a test light bulb 120V with test leads I sometimes use as a load. Maybe put that across the primary and see if the bulb will light. 50 watt bulb will restrict the current but will still verify its not some phantom voltage you are reading.

 

[Bdog]

Will try the light bulb, but after trial and error the last 30 min. I'm still confused and baffled. Bench tested both old and new transformers, perfectly fine. Test power into the furnace, no problems. But even when routing power directly into the transformer getting nothing out the secondary. When AC into control board, same. Power out the xfmr leads to the transformer is fine, nothing out the secondary. House circuit breakers fine. Very troubling

 

[WyrTwister]

You probably have a switch on the filter door / panel . It will turn off 120 VAC to the rest of the furnace .

It may be msking very poor.ckntact & and allowing only a little power oass through .

Gas furnace ?
Wyr
God bless

 

[bud16415]

As @WyrTwister mentioned jump or close that door switch if there is one.


New digital meters will sometimes get these phantom readings of voltage even when resistance is so high no current can get thru. That’s why a load like a bulb will show you it cant pass current.


It has to be something like that IMO.

 

[WyrTwister]

Also , , as I think some one else has mentioned , there is possibly a " short " on the 24 VAC side dropping the voltage . Could be in the board , Tstat , wiring to the Tstat , contactor in the condenser or wiring to the condenser .

May need to disconnect the wires going to the Tstat and condenser & re-check the voltage .

 

[Bdog]

All good suggestions, but now I've bench tested both transformers, verified both good, then connected both independently to the power coming into the unit and both have no secondary voltage. Strange, I'm bypassing everything, door switch, circuit board, etc. and get a fine 110v reading before connecting to the transformers, then nothing out of either secondary. And both secondaries test fine at the bench. 'So it can't be a rare case of 2 bad transformers. I even opened the power junction box switch outside the furnace, as well as the main circuit breaker panel in the house, and nothing inside either looks abnormal.
I checked Tstat wiring also, as well as the Tstat board, but those don't matter since I did the above connection without success. Call me "stumped to the point of calling a Pro"

 

[bud16415]

Pretty strange for sure. So the only possible difference is when you put the transformer in place you are connecting it to an earth ground thru the chassis of the furnace. And that is somehow shorting out the secondary.

 

[maxdad118]

Some circuit boards have a 3a fuse, have you checked for one? I’m not sure why you wouldn’t be able to read 24v at the transformer leads other than the 108 you mentioned, seems low. Also check any limit switches, some are manual reset, usually at the ends of 2 blue wires.

 

[Bdog]

Gas furnace by the way, not that it matters. Board has a 5a fuse, it's good, but again it doesn't matter because I've connected the power directly to the transformers without going through the other components. And since I've rested the transformer on certain parts of the unit while trying all of the above. Thanks for all the help, I'll report back after a pro checks it out.

 

[WyrTwister]

Best of luck . I still bet something ( a " short " is drawing too much 24 VAC & dropping the voltage ) .

 

[user 4883]

If your bench testing with 120V and trying to run it with 108V then it would seem that the transformer will not operate at that low of a voltage threshold.
You have to find out why you have low voltage.

 

[bud16415]

If your bench testing with 120V and trying to run it with 108V then it would seem that the transformer will not operate at that low of a voltage threshold.
You have to find out why you have low voltage.

That could be tested if he had access to a variac type autotransformer to apply a lower voltage or maybe adding in a little resistance in series to see if there was a threshold where the transformer was limited out. I always thought they would still work just putting out some lower secondary voltage.


I agree the 108 is weird but he wired around that input putting 120 on it and gets no output even with the secondary unconnected when it is in the furnace.

 

[Bdog]

Well, solved. Sort of. Two HVAC techs were here for two hours, heard my story and tried a 3rd transformer of their own. Tried all the trial and error I did, and more I assume but left just as stumped as I was. After they left I ran an extension cord from a different electrical outlet in the attic, wired it up and Bingo. Working perfectly. So somehow the original power feed into the unit has lost part of it's voltage. Those of you who thought no low voltage could come out of the transformer's secondary if there was less than 110/120v are right. I could wire a new J box for it myself but since I want to figure out where the original line is losing voltage I'm going to call an electrician.

Thanks for all of your thoughts and assistance. I'll post if anything else is discovered. Cheers.