Compressors Four

In an effort to make some sense of the many questions received and answered, I have tried to categorize troubleshooting air compressors into similar postings on each page. This is page four.

The forum page, where you can add your own question or try to help another air compressor user by answering their question, is linked from the bottom of this page.

Cheers & thanks.



Compressor wont run when its cold out?

by todd k - hoagland,in. usa

Question: i have an up right craftsman oil-less air compressor. when the outside temp gets down to freezing or below the compressor wont run. i flip the red switch and it blows the breaker. my garaage is not heated.

Response: Hello Todd. You are not alone! I've been getting a bunch of questions about this topic lately, so I've added a page to cover it. Please look at the Troubleshooting page, and the new link to "will not start when it's cold". If this provides an answer, great. If not, please add a comment here, and we'll look further.

Comments: Aug 22, 2011 - Oil Too Thick - by: Kelloggguy

Most compressor manufacturers recommend 30W, non detergent oil for normal temperatures. You probably need to go to 5W-30 when it gets really cold. Most likely the oil is too thick, increasing the starting load to where your breaker can't handle it. Your breaker may also be borderline. It should be rated for at least twice the motor's rated amperage to handle higher inrush current when starting. With the thick oil, it may not be able to handle startiing current.

Troubleshooting Thomas GH-617b compressor?

Question: Compressor starts fine and has good flow. Problem is that it will run for a few minutes then pressure will drop from 160# (Normal output is 175# and it works there, I am relieving output to 160# with regulator) then it starts bleeding down to about 120# and will build up again to 160# for awhile then cycle down again. Why is it cycling like this? Is it an internal pressure relief valve that is faulty (if it has one)? Thanks, Mike Jennie - mejennie at

Response: I would guess that you are running this without a tank? You are turning the output down to 160 PSI from the normal cut out of 175 PSI, and the air bleeds down to 125 PSI periodically? Two thoughts, one is that your air use exceeds the compressor's capacity to supply, or the other, is the pressure switch is wonky. Without more details of the application, my money is on the first guess.

Comments: Apr 12, 2011 - Thanks for your response. - by: Mike Jennie

I do have a large tank, (std for large compressor), and at the time I am not supplying output from the tank. I select 160# just so I do not run at max pressure and hopefully lengthen the life of the compressor. This has not happened before and I also believe it is related to a pressure relief, probably an internal relief to protect the compressor along with an internal temperature switch. Hope this clarifys our setup. I am trying to find some maintenance manual and parts description so I can prehaps fix it. Thanks, Mike


I know of no internal PRV. You may want to talk to this brands experts at If any other Thomas compressor users have a similar issue, we would sure like to know.

Motor capacitor!! But which one is it?

Question: hey my compressor has one capacitor and i want to replace it with the exact same one which is 25 um, 450 v and 50 hertz. That part is no problem! The problem is i dont know what the one capacitor is, is it a run capacitor?? or a start capacitor?? which will i buy?? How do i know what it is? Im secretly guessing it is start capacitor thats in it but i havent a clue! The compressor is only a 25 litre Clarke compressor. Very small! I have a 150 litre, runs great but i dont have enuf power in the shed to run it. Melted the wires in the junction box over many years. Im putting down a heavy duty cable which will get me more power!

Response: Karl, it might be both! Take the capacitor with you to the shop, and tell them you want exactly the same one, or, if you are buying on line, email the vendor for confirmation. Wish I could be more specific. Anyone else have advice for Karl on his capacitor issue?

Comments: Dec 09, 2011 - by: Karl

Hey bill i can get a capacitor but it is a start capacitor And it has four pins. I only have 2 wires from my compressor. Can i just join the two wires to 2 pins on the 4 pin capacitor and leave it at that and see what happens. Also every time the motor is spun by hand oil shoots up out from the dipstick area. I have the dipstick in perfect and there is a hole in at at the top probably for a breather. Should there be compression getting into the oil way and causing oil to come out?? I also filled it to the very top! it may be that there is too much. But when the surplus oil was blown off it also still makes a gurgling sound of the oil. So there must be compression in there! _________________________

Regarding the 4 pin 2 pin capacitor, I'd be more comfortable if you asked your local motor repair shop than making a guess, and that's all it would be from me. No, compressed air should not be getting in to the oil sump. "every time the motor is spun by hand", are you saying that this doesn't happen when the compressor is running? Yes, the hole in the oil fill cap is necessary.

Air pressure stays at 100 psi?

Question: 6.5 hp Black Max 80gal 2 stage builds to 100psi continues to run but pressure stays at 100 psi. Air coming out of crankcase vent while running and stops when cut off. Tank pressure loses about 30 psi in 18 hours. Any help appreciated. sscrocker at

Response: Hi Sheryle. A lot of reasons why air compressors run but don't build air pressure, those pages linked from the Troubleshooting page. However, air blowing out the oil fill cap vent suggests strongly to me that you have piston seal(s) failing or failed. See the Troubleshooting page for tips, but I think it's time to tear down your pump and replace seals. You might as well replace the valves and replace the gaskets while the pump head is open. Regarding the drop in air pressure over 18 hours, that suggests to me that your tank check valve has a small leak. With the air tank empty and the compressor off, remove, clean, and replace the tank check valve.

Comments: Jan 03, 2012 - by: Sheryle Thanks for the information. My thoughts were broken or worn piston rings I removed the compressor oil today. It came out thick, slowly and the color of weak chocolate milk. Before I tear into it I would like to know the availabity of parts and maybe even a shop manual.Any suggestions appeciated. I looked for a model or serial number as well. No markings were found. I did take a couple of pictures today and will gladly post them if I can figure how. ___________________

Sheryle, start a new post, and you can upload up to 4 photos. See the Black Max link from the site map for info about parts.

Makita mac 2400 brand new air compressor stalls at 60 PSI?

Question: Just bought a Mikita mac2400. Builds pressure very slowly, and gets to 60psi and stalls on the guage but won't shut off.... Brand new...

Response: Hi there... I'm going to guess that you are using a too small extension cord (wire size, not length) or your power supply is compromised in some fashion and your air compressor is not getting enough power. Comments?

Comment: Feb 11, 2012 - by: Anonymous

Did at first, but thought of that and changed the difference.


Thanks for the comeback, and for thinking of the power supply as possibly being an issue. Do check and make sure though that the only thing pulling power from the same power supply as you use for the compressor is the compressor, while it's trying to run. Also, pull the intake filter and try it. Any change? If not, with tank empty, pull check and clean the tank check valve. Any change?