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Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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My air conditioning unit starts and then stops after a few

Customer Question

My air conditioning unit starts and then stops after a few moments. Due to the reduced microfarad reading, the run capacitor was removed and replaced. The same condition exists. The system runs for maybe 5-minutes and then shuts off. During the run cycle the unit is cooling the environment. There are no leaks to be found around the unit The unit is ~7-years old. The batteries on the thermostat were changed. The low voltage transformer is working. At this point I am thinking about replacing the air handler motor, which sets for approximately 15-20 minutes and comes on again for a brief period of time and then shuts off. And, No, I am not replacing the whole system.
Recommendations?
Submitted: 2 years ago.
Category: HVAC
Customer: replied 2 years ago.
(Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  Phil replied 2 years ago.
Hello, let me know if you are willing to work as follows or not. I guide you to a complete diagnosis before changing any parts, and you promise not to change any suspect part on the fly while we are in process on the diagnosis... that creates a moving target, and can be a problem.You will need a clamp around type amp meter, $25 at Harbor Freight, and about $50 at a big box home fix up store. We will check the air handler motor amps against its rating on the furnace model number sticker inside the burner compartment on the furnace. Next we will feel for any faint up and down play on the blower motor shaft.. very hard to detect but if you can feel and tiny bit of up and down play the motor bearings are bad. (hold off on replacing the motor if it has play, we need to go deep with diagnosis first). Next we need to do a heat stress test on printed circuit card inside the furnace or air handler. Stress test the printed circuit card.Use a hair drier to slowly and gently warm the card to 120F (warm to the touch but not hot and not over 140F)... if you get any change in behavior that means the card is bad... A micro crack has opened in the printed circuit at a solder joint, or inside its CPU in that case .... or a short circuit has developed as the card expanded when you heated it. Constant expansion and contraction as these electronics heat and during their duty cycles over the years is what wears these cards out. It is also a good idea to replug all of the connections in the unit,that burnishes any oxidized terminal connections and will sometimes solve the problem, especially at the printed circuit card and in any sensor molex connectors. Do that with the power off to the unit of course. These cards are also damaged by Electrical Spikes caused by power failures and lightning. These high voltage spikes can literally explode the fine 'wire' connections inside the chip. There is also a shunt type device, often printed on to the card that allows a high voltage spike to arc across to ground harmlessly, some of these can wear out after a few times and allow the high voltage spike into the rest of the card and the CPU micro-circuits. The high end printed circuit cards use a large capacitor like device, or several in parallel, to capture and release such surges, these do not wear out.
Customer: replied 2 years ago.
Agreed as follows: you will guide me through a complete diagnosis before changing any parts, and I promise not to change any suspect part on the fly while we are in process on the diagnosis... that creates a moving target, and can be a problem.
Expert:  Phil replied 2 years ago.
Thanks,
Next, do the following:
1. You will need a clamp around type amp meter, $25 at Harbor Freight, and about $50 at a big box home fix up store.
2. We will check the air handler motor amps against its rating on the furnace model number sticker inside the burner compartment on the furnace.
3. eel for any faint up and down play on the blower motor shaft.. very hard to detect but if you can feel and tiny bit of up and down play the motor bearings are bad. (hold off on replacing the motor if it has play, we need to go deep with diagnosis first).
4. Do a heat stress test on printed circuit card inside the furnace or air handler.
Use a hair drier to slowly and gently warm the card to 120F (warm to the touch but not hot and not over 140F)... if you get any change in behavior that means the card is bad...
Notes:
a. A micro crack has opened in the printed circuit at a solder joint, or inside its CPU in that case .... or a short circuit has developed as the card expanded when you heated it.
b. If there are any changes in the behavior we know the card is bad,
however if there are no changes that does not prove the card is good. In that case both sides of the card need to be inspected for dark or burnt spots... if there are any, the card is bad for sure.
If there are no dark spots etc.. the card may still be bad, and we have do more of an extended diagnosis. These were not made to be trouble shootable in the field easily.
Constant expansion and contraction as these electronics heat and during their duty cycles over the years is what wears these cards out.
It is also a good idea to replug all of the connections in the unit,that burnishes any oxidized terminal connections and will sometimes solve the problem, especially at the printed circuit card and in any sensor molex connectors. Do that with the power off to the unit of course.
These cards are also damaged by Electrical Spikes caused by power failures and lightning. These high voltage spikes can literally explode the fine 'wire' connections inside the chip. There is also a shunt type device, often printed on to the card that allows a high voltage spike to arc across to ground harmlessly, some of these can wear out after a few times and allow the high voltage spike into the rest of the card and the CPU micro-circuits. The high end printed circuit cards use a large capacitor like device, or several in parallel, to capture and release such surges, these do not wear out.
Let me know how each step of the process goes, we can continue from there.
Thanks.