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I MOW ALONE
11-29-2006, 06:09 PM
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:

Mic_bug
11-29-2006, 06:33 PM
How bigs the motor (hp and amp draw)
Are you running it on the proper receptacle (ampacity) or a light duty ext. cord>

xcopterdoc
11-29-2006, 06:36 PM
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
11-29-2006, 09:50 PM
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

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

xcopterdoc
11-29-2006, 09:56 PM
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.

xcopterdoc
11-29-2006, 10:05 PM
May look something like this.

khouse
11-29-2006, 11:42 PM
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.

Restrorob
11-30-2006, 06:45 AM
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
11-30-2006, 04:53 PM
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.



Ok!
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?
Thanks!

FIXDISS
11-30-2006, 05:37 PM
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.

pugs
11-30-2006, 05:41 PM
No, its just a 1 way valve. It has to overcome the tank pressure to build up to whatever pressure it shuts down at. If it goes to 135 it has to be able to push more than 135 past the valve. The check valve is there to allow the motor to start back up after the tank pressure drops. If you still had pressure against the pump it would not be able to start from a dead stop.

I wonder if you have something wrong in your pressure switch. Tought to say without being able to look at it.

PS: Electric compressors such as this would not have an unloader. Unloaders allow a compressor to keep running constantly after the tank is full...as with a gas powered compressor and may also control an idle down system for the engine :)

FIXDISS
11-30-2006, 05:56 PM
You could bypass the contactor by either wiring direct or using jumper wires to see if it makes it run past 40 psi. also. Since you have the same problem with both motors and i am guessing they are rated the same, then I believe the problem is in the contactor/pressure switch .Disconnect all power and remove cover on pressure switch. Look closely at the "points" on contactor to see if they are burnned or pitted badly.

Restrorob
11-30-2006, 08:13 PM
You don't think that X Cop could be right? and its back feeding back to the pump through the check valve?


No I really don't, Reason being while the compressor is running the pressure opens the valve and allows the air into the tank. Once the tank reaches the set pressure the compressor shuts down then with no in-going pressure the spring in the valve pulls it closed to keep air from bleading back through the reeds in the compressor and draining the tank of air.

If the check valve were stuck closed you would get no air into the tank to regester 40 lbs. on the gage.

FIXDISS has a couple good points of what to check

fixer67
11-30-2006, 09:41 PM
Is it a 120 VAC unit or a 240 VAC unit or a duel voltage unit? The reason I ask is I know of a deal where a man bought a air compresser at a yard sell that had a 240 VAC motor and some one put a 120 VAC plug on it. It would pump up to about 30 or 40 pounds and stall out. I also know of a deal where a 120/240 VAC unit was being ran on 120 but was still rigged for 240. Check the voltage on the name plate of the motor.

khouse
11-30-2006, 10:20 PM
a lot of those motors can run 110 volts or 220 volts. You have to have the motor wires to the proper jumpers. This may be the problem. Restrorob is correct about the pressure. I believe you may be trying to run a 220 volt wired motor on 110 volts.

fixer67
12-08-2006, 09:55 PM
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:

So, did you ever find the problem?

I MOW ALONE
12-09-2006, 12:25 AM
So, did you ever find the problem?

nope i bypassed the switch strait wired the motor and it does the same thing gets to 40 pounds and starts to struggle and overload the motor. if i take the hose off and let the air just pump out it will run for as long as i want it to. but if i plug the hose back in and let it build pressure same thing 40 pounds and it will overload the motor:dizzy: . it sure does seem like it is backfeeding the pressure from the tank to the pump like xcop suggested.

Restrorob
12-09-2006, 08:23 AM
Post the model number off this thing so I can see exactly what you have there.

khouse
12-09-2006, 10:14 AM
have you ever looked that the plate and see what voltage it is? if you see anything that has 120volt and 240 volts on it then I still believe your trying to run a 240 motor on 120 volts.

olyman
12-09-2006, 02:53 PM
sounds like it khouse--took a 3 ph motor to friend to check out--said was perfect--told him to wire to 220 while he had it--uh huh--built the compressor--started it--thought--why start so slow--and wouldnt hardly speed up--and started to pull done with very little pressure--called him--asked if he wired for 220--he laughed--clown--said forgot--rewired it-woohoo!!! bet thats what hes got--

FIXDISS
12-10-2006, 09:25 PM
OK.Here's the deal.Post model # and make so we can get a look at this thing.Also check the plate to see if this is a single phase or a 3 phase motor.There still has to be a good explaination for this and if not electrical then it could be a reed valve or the check valve on tank is stuck open.???

I MOW ALONE
12-11-2006, 01:36 AM
this compressor is a craftsman 4hp 20 gallon. it was origanally 220 volt that motor overloaded when it got to 40 pound's of pressure. so i took the motor off of my other compressor which is only 2hp but is 110 volt same rpm 3450. hooked up this motor which i know is good 0 pressure in tank start's right up gets to 40 pound's does same thing 40 pound's start's to struggle and will overload the motor.maybe this compressor needs 4 hp to run properly. but it overloaded the original 220 4hp motor too. check valve is not stuck open i took it out and checked. bypassed pressure switch staight wired motor did the same thing. maybe it needs more hp or is this a symptom of a bad reed valve. im just about ready to toss it. thank's for all the replies.

Restrorob
12-11-2006, 06:35 AM
2 hp is not enough to run this compressor, It's possible your 4 hp motor was single phasing (bad run capacitor) before burning it up. That would cause the same symptoms with both motors, They are (were) both under powered for the size of the compressor.

khouse
12-11-2006, 08:21 AM
If everything checks out on the motor then if your going to run half the HP then you probably going to increase the ratio double. ie reduce the drive pully by half or increase the driven pully by 2. or a little of both.

I MOW ALONE
12-11-2006, 10:58 AM
looks like it's going in the trash. thank's again.