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airheatman
airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 7768
Experience:  Furnace, A/C and heat pump specialist.
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I have a Goodman gmp 125-5. It stopped working and there is

Customer Question

Tim, I have a Goodman gmp 125-5. It stopped working and there is no power at the stat. The control board has a solid red light. I've power cycled. I don't get 24 at transformer though so I'm lost.
Submitted: 11 months ago.
Category: HVAC
Expert:  airheatman replied 11 months ago.

Start by checking to see if fuse on control board may be blown

http://i.imgur.com/kYe4Epz.png

http://i.imgur.com/90juVsc.png

If fuseis good look and see if you have a float switch in drain and if so lift it out of drain and see if we get power to thermostat.

Next if you have a condensate pump beside furnace tied into the control wire to thermostat, bypass pump and see. We will go from here.

Customer: replied 11 months ago.
The fuse is good. I don't have a float switch. No condensate pump. Btw, I don't get 24v on secondary side of transformer. But red status light is on.
Expert:  airheatman replied 11 months ago.

ok if power in but no 24v out, transformer is bad, The red light just indicates board has 120v

Customer: replied 11 months ago.
Ah ok! That makes more sense now. Btw, can get generic transformer or do I have to get part#1141600? Also I tested continuity on both transformer coils and they are good. Also in the past the furnace has quit working and no red light. When I power cycle it comes back up.
Expert:  airheatman replied 11 months ago.

You can use any 40VA 120/24 volt universal transformer. If red light was not on that indicates the board was not sensing 120 volts.

Customer: replied 11 months ago.
Is it possible that the board is also bad?
Expert:  airheatman replied 11 months ago.

Possible yes. I would install me a small inline fuse in the low voltage circuit when replacing transformer in case there is a short that fried transformer so it doesn't do so again.http://i.imgur.com/YlIv9Ff.jpg

Customer: replied 11 months ago.
Ok will get a inline fuse as well. What amperage?
Expert:  airheatman replied 11 months ago.

No more than 5 amp.3 is sufficient

Expert:  airheatman replied 11 months ago.

Thanks for letting me work with you.If my answer has been polite, professional,and thorough, please click one of the ratings before you go. If you still need help let me know!.Expert is not paid for assisting you until you rate answer.Feel free to follow up for 30 days at no additional cost. Bonus always appreciated if you feel justified.

Customer: replied 11 months ago.
Replaced transformer. No luck. I still don't have 24v at transformer.
Expert:  airheatman replied 11 months ago.

Can you send me a picture of what you are checking and where? If you bought a brand new transformer and it does not have 24v coming out with 120v going in, they sold you a bad transformer.Take it back and get another.We will go from here.

Customer: replied 11 months ago.
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Customer: replied 11 months ago.
I just replaced the control board too. :-( still no luck
Expert:  airheatman replied 11 months ago.

Are you saying there is no 24v between blue and green out of tansformer, And tell me where these wires go to.

Customer: replied 11 months ago.
I connected blue to neutral. And green to the control board red wire.
Expert:  airheatman replied 11 months ago.

Lets connect as shown below. One wire to pin 3 and one wire to in 6 on the 9 pin plug on board

Customer: replied 11 months ago.
Ok that's how it's connected already. Meanwhile, I tried connecting red going to Tstat with white to tsar and the furnace kicked in! (I by passed tstat)
Expert:  airheatman replied 11 months ago.

So that tells us we DO have 24v ,If thermostat has batteries, try replacing them

Customer: replied 11 months ago.
Ok here's where we are. There is and was 24V always. I was checking for 24v dc but it's 24ac. That was my mistake. So my diagnostic all along was flawed. But my Nest still doesn't see 24v on Rh.
Expert:  airheatman replied 11 months ago.

OK next lets follow the red wire from R on board and C wire on board and see if there is anything in between there and thermostat where we might have a break in that circuit.

Customer: replied 11 months ago.
THere were no breaks in the circuit. Although the nest was stealing from the red wire, it no longer wanted to do that. After much fiddling, I disconnected the cool wire and connected it as a common. Everything works fine now. I will need to figure out how I will get another wire in summer as the cool wire to the Tstat. In summary, if I had diagnosed the 24v correctly in the first place this would have gone a completely different path, one that would've been shorter And cheaper. :-). Thanks for your help. Btw, the red light does not stay on unless the 24v is alive.
Expert:  airheatman replied 11 months ago.

Here is an idea you might want to consider for obtaining an extra wire if it id difficult to run anew wire .

http://smartthermostatguide.com/c-wire-venstar-add-a-wire-adapter-review/

Thanks for letting me work with you.If my answer has been polite, professional,and thorough, please click one of the ratings before you go. If you still need help let me know!.Expert is not paid for assisting you until you rate answer.Feel free to follow up for 30 days at no additional cost. Bonus always appreciated if you feel justified.