I'm new to hvac. My ac was working fine but it decided to die one day. I've been able to fix my ac, but it continues to happen(the same problem), and it is not easy to get to the ac (it's on the roof). The problem is: there is a switch that provides electric to the compressor and the fan, and it often turns itself off.little switch

if the ac stopped working(the fan and the compressor not running), i had to climbed up to the roof just to push that button to make it work again.

I would like to know how to prevent it from switching off.

Here are some more pictures

1.how to take this cap off- I worry I would break it.

2.this is how it looks when it worked. is it normal for the compressor?

enter image description here Sincerely, alt

  • 1
    That is some crap-tastic wiring - I particularly enjoy the dangling capacitor. Please note that resetting a safety switch that keeps tripping is not "fixing" your A/C. You should probably hire someone who can actually fix it.
    – Ecnerwal
    Aug 22, 2015 at 13:07
  • That is the high pressure safety switch. It should only trip when the head pressure is much much too high. If this switch were to stop functioning, then next thing that would happen is that your coils or your compressor might rupture. Also, with the system operating in this condition it is likely to be consuming significantly more electricity than it should. I agree with Ecnerwal. Time to hire someone who can actually fix the problem.
    – user39367
    Aug 22, 2015 at 16:25

3 Answers 3


I am not a HVAC tech but I do believe that is a high pressure switch that is tripping and there could be a couple of things going on with your unit. It is tripping to protect the compressor and fan from faulting. I would suggest calling a HVAC company to come take a look.


That could be either a high, or low pressure switch. Without more information on the unit, its hard to say for sure which.

A high pressure switch usually opens if the coils are dirty or fan is dead. Basically not enough air flowing through the system, so not enough heat is removed.

Low pressure switches will open if the refrigerant level is low.

Check the documentation for the unit, and determine which switch this is. Or contact a local HVAC company to come take a look. Without more information, and/or more photos, it's not clear what the problem is.


Although I commented above that it would be good to hire a qualified person, there are a few things a skilled DIY'er could do in this case. Make sure the main disconnect is off before doing anything.

The most common causes for high head pressure are (a) dirty evaporator coils and (b) a poorly functioning condenser fan. From the photo it looks like that condenser fan motor has many years of use and the run cap for that motor is also old and not protected from weather. If you have a capacitance meter you can check if the run cap is weak. If it is less than 90% of the labeled value, then it must be replaced. Whether or not it is replaced, the capacitor (and any other bare connectors) must be protected from weather in a NEMA 3 enclosure or similar. Another likely possibility for causing high head pressure is that the condenser motor bearings are worn out and they seize after being run for awhile, tripping the High Pressure Cut Out. Inspect the motor bearings. If they are somewhat stiff to turn by hand or if they have much play, then your motor is shot. In that case you need a new condenser fan motor and a new run cap sized for the new motor at the very least.

The problem with following this path is that with an old system these repairs might not resolve all your issues and you need a set of gauges and some experience (i.e. a qualified technician) to make a more thorough evaluation about exactly what is wrong and how much money is worth spending on repair. Odds are good that in the right hands this could be economically repaired.

  • thanks, a. I'll try to do some cleaning. b. last time when I was up there, I tried to turn the fan and it made some circles, so I think it is working fine??? The cap could be the problem, but it is still working. I added a larger picture of the cap, do you think I should open it? (it is kinda sealed)
    – alt
    Aug 23, 2015 at 0:56
  • I see your recent pictures, one of the capacitor and one of the compressor with ice on it! The capacitors are not serviceable. They can get weak and reduce motor power to unacceptable levels. An easy way to check the capacitor is with a meter that has a capacitance scale. Anything else is just guessing. Both the high pressure tripping and the ice on the compressor are serious issues and they likely have separate causes. IMHO you need someone experienced who knows how to use a set of gauges. Your compressor will fail soon unless these problems are fixed ASAP.
    – user39367
    Aug 23, 2015 at 17:30
  • Thanks for the suggestion, I'll do all that. One more question: I want to change the cap- I think it's about 10 year-old. This is the information on it: 81a3901 AEROVOX P50G3707E 7uF 370v 60 Hz protected A5000AFC 8648 80:197 1238. I know it need to be 7uF, about 370v. What else I need to look for when searching for it?
    – alt
    Aug 24, 2015 at 0:29
  • 7uF 370VAC is all you need to specify. Higher voltage spec is OK, lower is not OK. If your condenser fan motor needs to be replaced it is likely that a replacement motor will use a different size capacitor, so it would be best to determine if the motor is shot before spending money on a cap that you might have to replace again.
    – user39367
    Aug 24, 2015 at 15:02

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