Air handler not coming on with condenser

[bigfleen]

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Hello, new to the site, would appreciate any help. My A/C was running and then the air handler shut off but the condenser continued to run. The house was not cool (not a stuck contactor) and now the air handler wont come on (It will come on in heat mode, just not in cool). I am getting voltage on both sides of both transformers (its a gas furnace) and the only way to turn on the air handler is the switch on the handler. It is an older unit (no board), and the switch has 3 positions (ON, OFF, FAN). It would normally be in the ON position, but currently the only way the handler will turn on is in the FAN position. I am steadily checking different parts, t-stat is next, but if anyone can point me in the right direction that would be sweet. The unit is a Nordyne model # MGHA-075ABFC-01. I am a universally ceritfied tech thru Esco, but still relatively new to HVAC. This is teh A/C at my house.

[mgenius33]

Sounds like you fried your new stat with 48-54 volts. Time to Call a pro. :callpro:

[dandyme]

isn't this a duplicate thread ?

[mgenius33]

Sorry my friend, but this is sounding like we're doing a side job for you. You can probably imagine how much a professional tradesman enjoys giving work away to non professionals.
However, if you are certified as you say, then I would suggest building your post count and applying for pro application on this site. There are loads of skilled technicians in the pro section that will help you diagnose just about anything.
Sorry, I don't mean to be rude, but our trade is polluted with guys who think they can buy a meter and gauges, go out and charge people for a job they're not qualified to perform, thus further enforcing a stigma us professionals try so hard to refute.

[bigfleen]

No, not a side job. Not a new t-stat either. Any sidework I do is with my tech school teacher or with someone else who knows far more than me. I am not comfortable working on too many people's equipment by myself. Also, the trade may be polluted with pretenders, but those guys make a bad name for themselves and when tehy screw up big, someone competent gets called. Definently would not consider myself a "pro", and too much posting would further reveal my ignorance. Yes it is a repost. Initially i posted and was asked for credentials, i then provided and was then told that I might have more replies reposting in Tech 2 Tech chat room. So, there is that. This is truly my unit i am asking about, luckily it is nice enough weather that open windows will suffice until I can fix this baby. For anyone who cares to bring the claws in, the unit was running when the air handler kicked off. The condenser continued to run. Now, when i try to turn on the A/C, the condenser kicks on, but no air handler. The handler will come on if i turn on in heat mode. Any takers? Thanks.

[bigfleen]

Just took the TSTAT off and wired together, voltage, common, and cool wires; condenser came on, still nothing from the air handler.

[heatingman]

"Just took the TSTAT off and wired together, voltage, common, and cool wires; condenser came on, still nothing from the air handler."

What do you mean voltage common and cool wires?

Sounds like a fan relay in the furnace.

[bigfleen]

RC-voltage
Y-cool
G-common
These are the terms I know, but basically, I twisted the wires together that should turn on the A/C. My thinking was that maybe the TSAT (its an old one) wasnt sending right signal, so why not bypass it. I ohmed out the relay, but the diagram I have doesnt show whether or not the positions are the relay are suppossed to be normally open or closed. Maybe Ill try a new relay and see if that fixes the problem, but nothing open today, so any other feedback would be sweet. Thanks.

[crmont]

Just took the TSTAT off and wired together, voltage, common, and cool wires; condenser came on, still nothing from the air handler.

Let go, step back and get help, ok? You really do need a pro.

I suggest hounding the Sh*t out of him when he gets there. They love that.

[bigfleen]

Advice like, call a pro, hound him when he gets there, super helpful. So if your mower breaks down and you dont know exactly what the problem is right away, do you call someone to fix it? Prob not, you tinker, and guess, etc. Are you dealing with high voltage on your mower, no, but blades spinning at high rpm can be pretty dangerous. I understand everyone's unwillingness to give valuable advice away, but really, if a DIY gets on here and scews up, he is going to have to call a tech in the end anyway. Whats the point of a site with Tech 2 Tech and AOP if the answer is always call someone?

[turkey]

Just curious as to why you're so convinced its a 24v problem

[bigfleen]

Im not. 1st post said that the TSTAT was fried.

[bigfleen]

Whatever the problem, it is limited to cool mode. I am still learning, so I have been checking everything that I can think to check. i was hoping it was going to be someone easy to figure out like a transformer or tripped breaker (lol). No biggie though, i can definently use the practice trouble shooting.

[mikeybow]

RC-voltage
Y-cool
G-common
These are the terms I know, but basically, I twisted the wires together that should turn on the A/C. My thinking was that maybe the TSAT (its an old one) wasnt sending right signal, so why not bypass it. I ohmed out the relay, but the diagram I have doesnt show whether or not the positions are the relay are suppossed to be normally open or closed. Maybe Ill try a new relay and see if that fixes the problem, but nothing open today, so any other feedback would be sweet. Thanks.

G is not normally common. It is in most applications what the t-stat uses to communicate with the fan control. common is connected to the common side of the x-former. If it comes into contact with the 24 vac side of the x-former without a load in between, you get magic smoke. hope this helps.

[mgenius33]

Im not. 1st post said that the TSTAT was fried.

No, I said it "sounds" like you fried it. You can now see why we may be hesitant to help.
You're already condemning a diagnosis that was never given.
I wasn't trying to be offensive, just sharing my position which was derived by your words.
You stated in your other post, that you worked with fireworks, and thought you might pick up some side work doing hvac while your business was slow, did you not?
Again, not trying to offend you, but if you were truly certified, this is one of the easiest calls you can get. Also, by the information given, we can all agree you don't understand low voltage circuits, as they pertain to hvac equipment.

[Texas-Tech]

Does your heater have a combo fan/limit control at the discharge end of unit?

[Rob_in_WV]

I assume there has been no changes to the units wiring recently? Without seeing it in person I would have to assume either a defective fan relay, no 24v at the fan relay or the cooling speed of the blower motor has failed. :.02:

[crmont]

Advice like, call a pro, hound him when he gets there, super helpful. So if your mower breaks down and you dont know exactly what the problem is right away, do you call someone to fix it? Prob not, you tinker, and guess, etc. Are you dealing with high voltage on your mower, no, but blades spinning at high rpm can be pretty dangerous. I understand everyone's unwillingness to give valuable advice away, but really, if a DIY gets on here and scews up, he is going to have to call a tech in the end anyway. Whats the point of a site with Tech 2 Tech and AOP if the answer is always call someone?

He can learn and fix the problem simultaneously. I just did an install where I had a shadow for two days straight. Why should I have all the fun?

From what I've read so far, he lacks (no disrespect) the basic knowledge of controls to even consider trying to talk him through any troubleshooting steps. Plus, the information sits on the open forum and three years later some homeowner blows all his low voltage logic in his heat pump because a high pressure switch is tripped. Personally, I think the tech to tech chat is dangerous to the DIY'ers and should be eliminated. The AOP is for generic HVAC questions, not specific troubleshooting.

[FrostyBeer]

Advice like, call a pro, hound him when he gets there, super helpful. So if your mower breaks down and you dont know exactly what the problem is right away, do you call someone to fix it? Prob not, you tinker, and guess, etc. Are you dealing with high voltage on your mower, no, but blades spinning at high rpm can be pretty dangerous. I understand everyone's unwillingness to give valuable advice away, but really, if a DIY gets on here and scews up, he is going to have to call a tech in the end anyway. Whats the point of a site with Tech 2 Tech and AOP if the answer is always call someone?

The reason the answer is usually "call someone" is that's truly the best answer based on how people are describing their situation. We know what can happen if something isn't done correctly and in the long run it will save you time, money and possible getting hurt.

[bigfleen]

changed the fan relay and got it going, thanks for the help.

[bigfleen]

No, I said it "sounds" like you fried it. You can now see why we may be hesitant to help.
You're already condemning a diagnosis that was never given.
I wasn't trying to be offensive, just sharing my position which was derived by your words.
You stated in your other post, that you worked with fireworks, and thought you might pick up some side work doing hvac while your business was slow, did you not?
Again, not trying to offend you, but if you were truly certified, this is one of the easiest calls you can get. Also, by the information given, we can all agree you don't understand low voltage circuits, as they pertain to hvac equipment.


Wasn't codemning the diagnosis, was answering the question as to why i was convinced that it was a 24v problem. My reply was that I was not convinced ie: im not convinced that I know what the problem is. That is why I took the TSTAT off the wall and bypassed it. Actually, i was listening to your advice. I am truly certified, but that does not mean that i know anything. Certification covers proper handling of refregerants, I was asking a trouble shooting question. Im only half way thru tech school, so yeah, I am still learning, this stuff. Fact is, ill never be done learning it. And if it is one of the easiest calls a tech can get, then why not just point me in the right direction. Not asking for a blueprint, just a little advice. I am not so much offended by your reply, I am the first to admit that i dont know it all, but good grief, if you dont want to answer the question then dont. The internet is a good place to glean knowledge from other folks, thats all I was trying to do. Everybody is worried that some DIY'er is going to read something on here and tear up their equipment, well good, right? Dont many of you get paid to fix stuff?

[ironpit]

It's not just equipment that is at stake. It could very well be someones life.

You got to have thick skin to be on this site. We are glad you are here.

Questions reveal a great deal about not only the poster's experience with the topic being discussed but also the posters experience level in the trade.

That's why the guys like crmont can see trouble coming. Don't shy away from asking questions, that is what the site is for, at every level.

My question is, being in a trade school, why haven't you asked your instructor? :det:

[mgenius33]

Glad to hear you got it repaired.
Only 3 more posts, and you're qualified.

[anacdoctor]

I guess some of you guy felt sorry for him.
Enough said.

[timjimbob]

You need to learn how to troubleshoot correctly, not guess.

you know the motor should run-it doesnt. You know there is 120 volts coming from the supply.

Now pull the schematic out and draw the current path from the motor to the line voltage supply, through all the switches and relay points. You will see all the components that feed the motor. Simple and logical troubleshooting.