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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8698
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a Trane central A/C unit whicch came with the house

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I have a Trane central A/C unit whicch came with the house I purchased in 1998. I had no problems with it until last year when the fan would run continually.I called a local A/C repair company & they replaced the fan relay.It worked fine all last summer & was working fine until this past weekend when it just stopped one evening.I checked the breakers & the were OK. I checked the outside fuses & they were OK. I pushed in the contacts on the outside unit & it turned on. I then removed the thermostat & found there was no voltage going to the wires there.I needed to go out so I turned the breakers off before I left.When I later turned thebreakers back on I had power to the wires so I replaced the thermostat & the A/C worked fine for 3 days & then stopped. I turned the breakers off for awhile & when I turned them back on it worked again.What is the cause of this situation? Thanks for your help. John Picarello

Welcome to Just Answer!

The system is going off due to a tripped safety device that is reset-able by cycling the power to the control circuit... by turning the breaker off you accomplished a reset.

If the weather has been exceptionally hot lately and you have not cleaned the condenser coils on the outside unit by blasting water between the fins with a garden hose... the unit could easily have gone off on its high pressure cut out switch.

Clean the condenser coil with the power off to the unit and fan motor and other electricals wrapped in plastic to keep them dry.

That would not cut power to the thermostat however. But check it anyway.

A clogged condensate drain line however will cause the drain over flow switch to trip if you have one of those installed, and that is normally wired to kill the power to the thermostat. Check for that situation even though a tripped drain overflow switch will not reset until the water drains away.

Let me know what you find, we can go from there.... as long as you keep rating my answers *positively I hold the question open so that we can continue without any time limit.


Phil and 2 other HVAC Specialists are ready to help you
Customer: replied 4 years ago.

Phil, I did give you an excellant rating but I still need to see if your advice actually works so please hold this question open like you said until I know for sure. Thanks. John Picarello

Hello again, I will definitely hold the question very often takes a while and a lot of testing to find out what the problem is. Your skill set is pretty good, the chances are about 80% that you can resolve this yourself and save an expensive service call.

The next most likely problem is a defective printed circuit board... those are quite difficult to trouble shoot. I recommend warming them up very slowly and gently with a hair drier to 120F or so, warm but not hot to the touch.. If that causes any change or failure, the control card is the most likely problem.

That might turn out to be the case here as well, since neither a clogged drain and tripped float switch, nor a tripped high pressure cut out switch will cause all of the symptoms you mention... it depends on how the system was wired in the field and other factors I cant see from here

Stay in touch as needed

Customer: replied 4 years ago.

Hi Phil,

I cleaned the condenser coils (they were very dirty) but still have the same problem.The unit will work for awhile & then just stop. John

Hello again,

If you are still having the same problem it could be that you cleaned the dirt from the surface of the condenser coil, dog hair, dirt and grass clipping stuck to the outside of the coil, and did not ***blast*** water **through the coil, and *between* the fins so that water was pushed clear through the coil... thats what it takes.

Let me know if you took those extreme measures or not.

If you had in fact cleaned the coil that way, then it could that the high pressure out switch is defective, but that is not as likely as dirt deep between the fins.

Let me know about those details.

We can go from there