what's wrong with my air compressor

Discussion in 'Mechanic and Repair' started by I MOW ALONE, Nov 29, 2006.


    I MOW ALONE LawnSite Bronze Member
    Messages: 1,961

    hey guys was wondering if anybody knows anything about air compressor's. i have a craftsman when you turn it on it start's right up but it will only build 40 pound's of pressure. as soon as it gets to 40 pound's it shut's off but is still drawing power. the light's start to go dim and the motor will overload. if i run the air out of the tank press the restart button on the motor it will start right up until it get's to 40 pound's and then same thing happens. any help would be appreciated. thank's:confused:
  2. Mic_bug

    Mic_bug LawnSite Member
    from N.B. MI
    Messages: 201

    How bigs the motor (hp and amp draw)
    Are you running it on the proper receptacle (ampacity) or a light duty ext. cord>
  3. xcopterdoc

    xcopterdoc LawnSite Senior Member
    Messages: 752

    unloader valve or check valve is allowing tank pressure to backfeed into the compressor. The power of the motor is no longer strong enough to overcome the power required to push air. Kind of like pushing your truck against a brick wall

    I MOW ALONE LawnSite Bronze Member
    Messages: 1,961

    thank's for the replies. is the unloader valve and check valve two seperate valves or the same thing just called different names by different makes. and where are the valves located. thank's
  5. xcopterdoc

    xcopterdoc LawnSite Senior Member
    Messages: 752

    They are two seperate valves, but may be contained in one assembly. It will be located between the compressor pump outlet and the tank. Usually mounted on top of tank.
  6. xcopterdoc

    xcopterdoc LawnSite Senior Member
    Messages: 752

    May look something like this.

  7. khouse

    khouse LawnSite Bronze Member
    Messages: 1,465

    how does it shut down? fast like usual or a little slower. it sounds like the run capaciter to me. does your motor have covers on the case that looks like humps? If you have two then you probably have a start capaciter and run capaciter. It's just a guess from my desk.
  8. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    You can go here; http://www3.sears.com/ Enter your model number for a illustrated parts break-down. You can remove the capacitor(s) and take them to a electric motor re-builder or water pump repair shop in your area and have them tested to eliminate that as the problem.

    I don't believe it would be the check valve in the output line due to the compressor starting and pumping 40 lbs. before the problem starts. The check valve keeps the tank pressure from bleeding back through to the compressor. The compressor still has to be able to pump the 120 lbs before the check valve kicks in at shut-down to keep the pressure from back feeding to the compressor and draining the tank of air. I ran mine with a bad check valve for 6 mos. and finally got tired of hearing it kicking on and off as the air bled down so replaced it.

    I MOW ALONE LawnSite Bronze Member
    Messages: 1,961

    This is the story behind this compressor. My neighbor gave it to me, and said that it needed a new motor. When i got the compressor I plugged it in, it got up to 40 pounds and did the same thing that I explained earlier, but must of burnt the motor out because when I hit the reset button I could not get the motor to run again. So, I pulled the motor off of my other compressor which works fine. So I know that the motor is good. It has to be something on the compressor side. It seems like it runs no problem up until it gets to 40 pounds and then it starts to struggle and overload the motor, so I shut it off, so I don't burn the motor up again. I found the check valve, its on the inlet side in the tank. You don't think that X Cop could be right? and its back feeding back to the pump through the check valve?

    FIXDISS LawnSite Senior Member
    Messages: 445

    If you have an amp guage connect it to one lead from the contactor to the motor. Start the unit up and read meter at start up and watch it thru the cycle to see if it gets unusually high before it shuts off. You could have a burnned contactor causing high amps/low volts and creating the overheating problem with the motor since you obviously know the motor off your other compressor is good. Also when it shuts off kill all power and try to turn compressor by hand to see if it is trying to seize up after it warms up.Above all make sure your pressure guage is reading correctly.

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