How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Phil Your Own Question
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
Type Your HVAC Question Here...
Phil is online now
A new question is answered every 9 seconds

Still not working: Carrier Weathermaker 9200 MXA

Customer Question

Still not working: Carrier Weathermaker 9200 MXA
Submitted: 2 years ago.
Category: HVAC
Expert:  Phil replied 2 years ago.
Hello, Tell me a lot about your technical skills background and if you have a multi meter or not, if you have those send me a photo of the unit wiring diagram glued to the inside of the main blower compartment cover.. taken close up, from straight on, well lit and well focused, I can use that to coach you on the diagnosis. We can go from there.
Customer: replied 2 years ago.

Have multi meter, replaced main board a few months ago and never got it working in heat mode. Now I need AC.

Steady red light on the board, Nest controller says cooling mode, but no fan, no compressor.

Express Checkout Payment Sent (Unique Transaction ID #6FR73085EN468344Y)

Customer: replied 2 years ago.


Customer: replied 2 years ago.
Expert:  Phil replied 2 years ago.
Hello again
Customer: replied 2 years ago.

Voltage across R and Y is variable at 0.1 to 0.3V. Will check L1 and L2 voltage now (need to gain access),

Expert:  Phil replied 2 years ago.
Hello again, thanks for trying with the photo's. but they are way too close up, so that all i can see is something liike a few giant wires. I need some shots from about 3 feet away so that I can see the entire area in context. Thanks.
Customer: replied 2 years ago.

L1 and L2 voltage is 88V

New photo attached.

Is there a way to force the fan on?

Customer: replied 2 years ago.

Here is the wide shot.

Expert:  Phil replied 2 years ago.
Thanks for the test.

We are barking up the wrong tree entirely by any attempt for force either the inside blower or the outside unit fan on. I can mark up drawings for you that show how to do this but it will not lead to any sort of a solution.

The odds are high that you have mis wired the NEST thermostat..

88 volts is entirely unusual... it indicates either a bad breaker at the house breaker panel (5 or10% likely at best)... or a defective multi meter, or mis-application of the multi meter such as set on something other than 0-600 voltsAC etc.

Next step is triple check your multi meter against a known source, to see it you have good reads or not.

If the multi meter reads 230v on a known 230v source and you are still getting 88v read at the disconnect switch at the unit, then the disconnect is bad, a fuse is blown, or the breaker for the outside unit is defective... that is inside the house breaker panel and can be quite dangerous for a non electrician to work on.


The wide shot is not showing me the compressor contactor... to locate that, physically follow the wires attached to the compressor back to the contactor and take an even wider shot that includes the contactor.


Let me know what you can discover, we can go from there.

Customer: replied 2 years ago.

Measured 2 Volts across L1 and LK2 using a better probe - I was not getting a good contact.

Nest is wired correctly as everything was working for a year with the Nest. Only 4 wires.

Customer: replied 2 years ago.

Earlier I switched this board out with another board trying to get this unit working. While the Nest is wired correctly on the wall, it is possible that I have mis-wired the RGWY on the board. How can I debug this?

Attached is another pic, this is as much of the unit I can get with the phone camera.

Customer: replied 2 years ago.
Expert:  Phil replied 2 years ago.
Hello again,

Lets not forget the Line 1 and Line 2 voltage readings I need, that is the most important step... if we do not have 230v there nothing at the outside unit will run and we will be spinning our wheels looking at the thermostat connections, especially if it had worked earlier.


That said....

send me a photo of the thermostat wire connections on the new board, and tell me if the blue wires connected to C on the thermostat (both photo's were of the thermostat, no board connections.

I need to know if the system worked in both heating and cooling modes after you installed the new board...the nest requires a C wire, I do not see it mentioned on your board wire connection note...

This could take a lot of time, well beyond what i can afford for you offer amount. I will submit an additional services offer after you get me a good solid voltage read, it might be difficult but it will be to you to insure good contact with your test probes, that is crucial.

Customer: replied 2 years ago.

2 Volts across L1 and L2 as reported earlier.

Yes, working in both heating and cooling modes.

Customer: replied 2 years ago.

Have I answered all your questions?

Have I sent you all the pics you need?

Customer: replied 2 years ago.

I have sent you pics of the thermostat wiring as requested, scroll up and you can view them.

Expert:  Phil replied 2 years ago.
If you have only 2 volts across line 1 and line 2 at the disconnect switch mounted on the wall next to the outside unit, you have found the problem.

You need 230 volts there.

The problem is either blown fuses or a bad breaker at that box or in the house breaker panel.

This is what we need to focus on, without 230 volts the outside unit will not run...

This has nothing to do with the thermostat wiring.

Once you get 230v to the outside unit, and if the system still does not run, then and only then will it be necessary to look into the thermostat wiring. So far though, it looks like a simple power failure issue.

You might want to call in an electrician to check your work and to locate the problem with the 230v if you are only able to get a 2 volt read.


Customer: replied 2 years ago.

I have replaced the 120V 15A breaker, and checked continuity on the fuses outside at the compressor (OK).

Still not getting voltage at the inside furnace disconnect which appears to come from just the 120V breaker. There is a regular duplex outlet in the same box.

How is this wired, is there a feed from the 240V breakers to L1 and L2 through the compressor?

Expert:  Phil replied 2 years ago.
You replaced the *furnace breaker*... it is single pole 120v breaker... the breaker that feeds the unit outside is a *two pole, 230 volt* breaker, that is the one giving you the bogus read between line 1 and line 2. That is the only breaker we are concerned with. The disconnect switch on the wall next to the outside unit feeds Line 1 and Line 2 to the 'contactor' inside the unit that sits on the ground. The contactor sends power to the compressor when its coil is energized with 24 volts. You must make sure that there is 230volt power across L1 and L2 of the contactor. The college training course on this trouble shooting is 4 semesters long, 20 units a semester, with labs. about 30% of the students graduate, then they do a 4 year aprenticship ... trying to do this work without that training can be **dangerous... especially when accessing the breakers in the house power distribution panel... Those feeder wires are heavy and go directly to the utility company's transformer on the pole, the breaker at the transformer and the 200 amp main breaker at the panel are not sufficient to prevent electrocution. Let me know if you need help locating a well rated electrician nearby or not, if you do, tell me which direction you are from the center of the nearest town or city. We can go from there. .
Customer: replied 2 years ago.

Hi Phil,

I have replaced the 40Amp 240 breakers. The L1 to L2 voltage is 120.

Looking at the spare board, it clearly says Warning Line Voltage 120VAC. Are you positive I should have 240V on this board?

Expert:  Phil replied 2 years ago.
Hello again, that is the *furnace control board* it takes 120vac. It is the *outside unit* with the compressor inside, that needs 240 vac to the compressor contactor, The way to find the contactor is to locate the compressor, it is roundish and weighs about 100 pounds, follow the 3 wires attached to it, to back to the *contactor. Take a picture it for me please and attach it here. -------------- We are way over time here, this is going to take a lot of coaching and training, let me know if you will accept an additional services offer of $80 to cover that training, if not I can opt out and we can see if anyone else here will take this question on. .
Customer: replied 2 years ago.

L1 and L2 are on the control board which is the measurement I understood you to be asking about.

There is no compressor in the furnace, just the fan, firebox and controls.

Are you asking for a measurement on the outside compressor unit?

I may be open to additional fees, but I need to see that we are making progress first.

Customer: replied 2 years ago.

BYW, I shorted R & G on the 24V side which is supposed to start the blower (according to some forum member), but all I get is a relay sound.

Customer: replied 2 years ago.

Please opt out and let's see if someone else can help. My time is valuable too.

Thanks Phil.

Expert:  airheatman replied 2 years ago.
Hello. My name is ***** ***** I'll do my best to help. Let's get started and see what we can do.I have read previous comments, but lets start fresh as there seems to have been some breakdown in communications causing some confusion.PLease tell me the following:
1 Check between L1 and L2 Neutral on furnace control board and verify 120v ac
2. Check between SEC1 and SEC2 and verify 24v ac to board
3.I saw in pics that you have the following to NEST. Red,green,yellow and blue Properly wired, but I do not see blue going to COM on furnace board, it should.Where does blue go in furnace?
WE will go from here.