Air Compressor strains to re-start above 70 lbs.


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Old 11-29-08, 05:33 PM
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Air Compressor strains to re-start above 70 lbs.

I have a piston type single stage Sears compressor that strains to re-start unless it has no air in the tank.

It appears that the tank pressure is acting upon the compressor making it difficult for it to re-start once it has over 70 or so pounds of pressure in the tank.

I removed the check valve and it appears to be functioning properly. The spring was working and the valve was not stuck. Wondering if the problem is in the reeds/head?
 
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Old 11-29-08, 07:33 PM
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In addition to the check valve there is an unloader valve on the pressure switch.
This valve should open to drain the pressure each time the pressure switch cycles.

To test the check valve, run the compressor until the pressure builds up and the motor stops.
Unplug the compressor and CAREFULLY loosen the line to the tank at the compressor.
The pressure should bleed only until the line empties.
If the tank drains then the check valve is leaking.
 
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Old 11-29-08, 08:20 PM
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Something else to look for. Low voltage. Have you done any wiring? maybe add a few plugs or more lights? I had a similar issue with mine.
Turned out to be low voltage. I added some lights, and tied into the same line as my compressors.
The lights were taking power away from the compressor. When it had pressure on it. It would not restart. If I turned off the new lights, it would start fine.



Travis
 
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Old 12-10-08, 09:43 AM
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I'm having the same problem with my compressor.

It builds pressure to about 50psi, then the motor starts to strain. When I shut it off, it struggles to restart and the belt slips. At this point I can barely even turn the compressor wheel by hand.

So, the problem is likely the unloader valve?

I have this Square D pressure switch. Which part is the unloader valve?




Thanks,
Joe
 
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Old 12-10-08, 03:30 PM
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Bottom pic, lower left is the connection for the valve. The valve itself is a needle type activated as current cuts out. If you know you have a faulty unloader, just get a new pressure switch.
 
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Old 02-23-09, 03:39 AM
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Sometimes people think it is the unloader that is at fault when it continues to bleed after the compressor shuts off. This is not the case. The thing to ask yourself is, where is this air coming from? The compressor isn't running, so it must be the tank via the check valve. So the correct diagnosis is, the check valve is at fault.
Herb
 
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Old 02-23-09, 02:07 PM
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Question

Originally Posted by Chip6565
I have a piston type single stage Sears compressor that strains to re-start unless it has no air in the tank.

It appears that the tank pressure is acting upon the compressor making it difficult for it to re-start once it has over 70 or so pounds of pressure in the tank.

I removed the check valve and it appears to be functioning properly. The spring was working and the valve was not stuck. Wondering if the problem is in the reeds/head?
Could this be a 220v. comp. running(or attempting to) on 110v.? Have you checked the capacitor for the re-start problem? I am thinking unloader valve not unloading!?
 
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Old 02-02-10, 02:56 PM
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Smile Sears Compressor Fails to Restart

Sears 6HP, 33Gal wouldn't restart at Cut-In point....removed manifold, a 5-40 screw identical to the ones holding the Check Valve was holding the Check valve open thereby pressurizing the heads and the motor was unable to overcome tank pressure to restart...it would restart at approx. 40 to 50 PSI. The screw had to have been dropped into the manifold at original assembly time as I have never had the manifold off. The real kicker is I bought the compressor in 1994 so it's operated since then with the loose screw in the manifold. It apparently floated in the manifold chamber under the Check Valve until recently being trapped under the Check valve.
 
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Old 02-02-10, 03:46 PM
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Jerry, welcome to the forums!! Oddly had the same situation on my 27 gallon Sears horizontal oil type compressor. All of a sudden one day the motor wouldn't turn at all. The switch was working. I took the motor off and the rotor was locked. Decided to do exploratory surgery. Found a dropped and never recovered shaft key stuck to the armature magnet, apparently there from date of manufacture in the 80's. It just decided to jump up there and mess with me on a day I needed it.
 
 

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