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My 5yr old front 15k btu domestic is getting power and trying to start but it gets to a low hum for a long time struggling to turn the compressor on. Sometimes it just shuts down without any further action and once in a while you hear a puff and then a shut down.  In either instance it never turns over into foul ac mode. What should I be looking at? TIA

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Joel, two possible things come to mind. 1. The compressor is locked up (I don’t think this the problem, possible) 2. The capacitor has failed (this a common failure and much cheaper than #1). Check the capacitor. Be careful because they a an electrical charge. Hope this helps!

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I am with Jorge, if you hear a pressure release then the compressor has tried but could not overcome the pressure buildup and capacitor would be my first place to check out. Just short its posts with a screwdriver first, to kill any remaining charge after you get to it, with the AC breaker off.

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2 hours ago, Joelsheriff said:

My 5yr old front 15k btu domestic is getting power and trying to start but it gets to a low hum for a long time struggling to turn the compressor on. Sometimes it just shuts down without any further action and once in a while you hear a puff and then a shut down.  In either instance it never turns over into foul ac mode. What should I be looking at? TIA

Sounds like the fan start capacitor needs replacing. $12.00 the last time I did it. 

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You may have and many of these units have a PTCT (Positive Temperature Coefficient Thermistor) failure.  This device allows current to flow through the start winding on startup, and cuts out at approximately 75% of the full running speed of compressor.  I did see this on my Dometic.  
Google this device for appearance and function. 

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Anyone with the knowledge of the capacitor I should order on Amazon. I found someone who will go up and replace it. It seems to be general consensus that it's the capacitor 99%.  Thank you all 

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14 hours ago, Joelsheriff said:

Anyone with the knowledge of the capacitor I should order on Amazon. I found someone who will go up and replace it. It seems to be general consensus that it's the capacitor 99%.  Thank you all 

As Trinidaddave mentioned, the PTCR is also a likely suspect and is mounted on the start capacitor.  PTCR  I replaced all of my PTCR's with a Supco SPP6 as I believe they are more reliable.  You can usually tell if the PTCR is going bad by smelling it, they turn stinky and look crispy.  If your residential refrigerator quits working that's the first thing to check.  My A/C parts manual lists the Compressor Start Capacitor and PTCR as "Compressor Start Kit"

Capacitor Discussion

Without the model number no one can tell you what capacitor is needed.  When you eventually shop for the capacitor do not buy one Made in China unless you want to replace it again.  MARS is one good brand.

Edited by Hypoxia
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2 hours ago, Hypoxia said:

As Trinidaddave mentioned, the PTCR is also a likely suspect and is mounted on the start capacitor.  PTCR  I replaced all of my PTCR's with a Supco SPP6 as I believe they are more reliable.  You can usually tell if the PTCR is going bad by smelling it, they turn stinky and look crispy.  If your residential refrigerator quits working that's the first thing to check.  My A/C parts manual lists the Compressor Start Capacitor and PTCR as "Compressor Start Kit"

Capacitor Discussion

Without the model number no one can tell you what capacitor is needed.  When you eventually shop for the capacitor do not buy one Made in China unless you want to replace it again.  MARS is one good brand.

I thought the spp6 was the entire hard start kit. I just replaced a capacitor and start motor with one in my rear #3 heat pump. Wired into the run capacitor.

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From what I'm getting, I suspect a capacitor, a fan motor and possibly PTCR separate from the capacitor. Is this right. Should I order the capacitor SPP6 and chance that the tech won't have to make a second trip out have him come down and diagnose all involved parts? 

Is this a good choice

 Supco SPP6E E Series Electronic Potential Relay Style Hard Start Kit, 1/2 - 3 Horsepower Range, 88-106F Capacitor Size https://www.amazon.com/dp/B002JP3MEK/ref=cm_sw_r_apan_glt_i_NHKC28ZDYCEYT24G1R8E

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IMHO, I would use a capacitor test meter to see what condition they are in but I have installed the SPP6 in every A/C unit on my last 4 campers.  If the capacitor tests near 10% of the original value or worse I replace them.  I always carry spare capacitors for the A/C unit and the residential refrigerator capacitor & PTCR.

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