# HF air compressor keeps tripping thermal switch



## kbrown (Sep 16, 2009)

I have had a Harbor Freight 21 gallon air compressor for about 3 years. No problems to complain of at all and use it more than an average homeowner, but far less than a professional. Problem is that suddenly, when I turn it on it will run for about 2 minutes then trip the thermal overload switch on the motor. (unit is a standard 15 amp 120v) If I wait 30 seconds the switch will reset and it will run for another 30 seconds and trip again; I can go like this until I get enough pressure built up in the tank to complete the task I need. I took the switch out and can run the unit no problem but only did that once as I am wondering if I just have a faulty thermal switch or because I removed this component that I'm forcing the motor run even if there may be a problem. Using my laser temp gun I only see temps on the actual compressor head around 150F and on the line running from the head into the pressure switch/tank at around 115F. Is this normal operating temps? I was going to go to a local electronics warehouse that has these types of switches figuring this is the least expensive fix to try first. 

Thoughts?


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## fbelec (Sep 16, 2009)

get a amp probe or amp meter thats capable of showing 100 amps and hook it up. watch the meter when turning the compressor on. if it shoots up to 40 to 70 amps and stays there the the motor is not coming off it's start windings. (it's like double the motor for getting up to speed). at 3/4 speed the start windings shut off. if the amp meter goes up to 40 to 70 and drops down to the name plate rating the motor is good the thermal switch is junk. all motor by law have to have a name plate for the specs of the motor. if it goes up to 40 to 70 amps then drops a little like down to 20 or 25 amps the motor is bad or the compressor head is binding. judging by what you said about temperature's i'd guess that the compressor is good. normally those type of compressors draw 12 to 15 amps at normal run.


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## LLigetfa (Sep 16, 2009)

How close to the panel is it and what gauge of wiring?  If it's on an extension cord, how long, what gauge, and what quality of plugs.  What voltage are you seeing at the plug?  My guess is there may too much voltage drop with a resulting increase in current.


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## kbrown (Sep 16, 2009)

No extension cord. Direct plug to wall on a 15 amp circuit. Got a new switch this afternoon, only $4. Will put it on later tonight and see what happens. I'm pretty familiar with using a multimeter, but in this case how exactly would I hook it up to test the amps on this when it runs?


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## LLigetfa (Sep 17, 2009)

The best way to test for amps are with a clamp-on ammeter.  Did you test the voltage on the circuit while under load?  Do the lights dim when the compressor runs?


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## WES999 (Sep 17, 2009)

One thing you may try is change the oil, replace with synthec. I know it sounds hard to believe but I picked a used compressor with a bad motor, I replaced the motor but it would ofton trip the motor overload or the circuit breaker, I replaced the oil with synthec compressor oil (Lowes has it) and has not tripped since. Simple enough to try.


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## brad068 (Sep 17, 2009)

Some good name brand multimeter can test current, (A~) with an optional plug in clamp on amp-meter. But most of the time you can find someone with a built in clamp amp-meter.

Another thing, check the pressure switch. Maybe the contacts are getting bad. Maybe hot wire bypass the pressure switch and see if the motor will run longer.

Is it a direct drive or belt? Can you remove the belt and just run the motor? Try spinning the motor freehand. Does it feel like its dragging?


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## kbrown (Sep 17, 2009)

Wow! Once again the forum proves to be full of great information. I think I may have gotten it with the replacement overload switch. Been able to use it all evening with no problems. Really appreciate all the advice here and I'm still going to test the amps; I can borrow one from our electrician at work. WES999, funny you should mention that; that was one of the first things I did at the suggestion of a neighbor along with running a test to rule out the pressure switch...I had modified the tank drain to outfit it with a standard ball valve so to test the pressure switch, I first opened that and started the compressor so essentially it could never build up pressure; no go since it tripped the overload switch in about 30 seconds after starting it.  So, for at least now it looks like it was the overload switch and for a $4 investment I have my good ole HF compressor back online.


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## fbelec (Sep 17, 2009)

wes999 touched a good spot. anybody out there that will be running a compressor when it's cold out will have more drag on the motor via the thick cold oil. i have that problem with mine. once it makes a few cycles it's ok. the thermal overload is doing it's job. mine resets automatically.


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## WayneB (Sep 17, 2009)

FYI DeWalt sells an excellent synthetic compressor oil, my compressor in PA would trip the breaker in cold weather, it's even colder out here in WY and the same compressor with the DeWalt oil has zero problems. Bonus on synthetic oil is it's more heat stable as well.
'


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## jebatty (Sep 19, 2009)

I was having a similar problem occasionally on some of my shop 120V table tools. Many motors can be re-wired to operate at 220/240V, which I did, and then ran a 240V circuit in the shop. Now those tools start easily and the power increase without loading the motors is dramatic. For example, the 10" table saw used to bog down when ripping 2" rough cut lumber; now it slices it like butter.


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## maverick06 (Oct 15, 2009)

I have a similiar compresser, HF, same motor but smaller tank (I think). Anyways, mine will trip its thermal switch too. If the extension cord is too long it definitely will, since thats not your problem I have another tip, because its my problem.  It happens a lot to me, the check valve on the tank leaks a bit. 80% of the time, if the tank is left with >30psi in it, it is too much to turn over. If you empty the tank, it will fire right up, otherwise the thermal switch will pop. I am sure you can disassemble the valve and clean it or replace it, but I just drain the tank when it happens. 

hope that helps


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## fbelec (Oct 16, 2009)

maverick06 said:
			
		

> I have a similiar compresser, HF, same motor but smaller tank (I think). Anyways, mine will trip its thermal switch too. If the extension cord is too long it definitely will, since thats not your problem I have another tip, because its my problem.  It happens a lot to me, the check valve on the tank leaks a bit. 80% of the time, if the tank is left with >30psi in it, it is too much to turn over. If you empty the tank, it will fire right up, otherwise the thermal switch will pop. I am sure you can disassemble the valve and clean it or replace it, but I just drain the tank when it happens.
> 
> hope that helps



you have a bad pressure relief valve. i think you said that. if your not going to replace it, try running some marvel mystery oil thru the valve. and all your air tools if you don't use them to often or everytime your done with using them put a few drops of marvel in the air fitting connect the air hose and run the tool for half a second to spread the oil. it will prolong the life of any tool.
and you can't use to much. it won't hurt the tool in any way, just might be a little oily on the next use.


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