Air compressor has difficulty starting


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Old 12-04-15, 07:00 AM
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Air compressor has difficulty starting

My air compressor is having trouble starting up. It seems to stutter. It rarely starts up and runs normally. I've replaced the motor start capacitor and that has not helped.

Here is a video of what it's doing:
https://www.youtube.com/watch?v=ml4BYHCjwgw Here's what I know since I started troubleshooting:
The check valve is NOT the problem
The pressure switch is not the problem
It's much worse now that it's cold outside and will not EVER fully start.

Any ideas?
 
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Old 12-04-15, 07:47 AM
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Since it seems to start when there is no air in the tank, I'd suspect the unloader valve, which is usually attached to the pressure switch. It's designed to relieve back pressure on the pump while it is starting.

It would help if you'd provide model number of the compressor.
 
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Old 12-04-15, 07:51 AM
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If it a oil bath compressor. you may have to change the oil. Good luck.
 
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Old 12-04-15, 08:13 AM
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It is an oil bath compressor and the oil is new and filled to the proper level.

It's a craftsman 921.153101

I don't think it's the unloader valve, when it does eventually fill up and I turn the switch off while its running, it does release the pressure. The video is a little older too. As it sits right now, the compressor will not start properly, even with 0psi in the tank.
 
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Old 12-04-15, 09:35 AM
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I guess that leaves two things (well, three). Run capacitor, starting switch, motor winding. I know you replaced the start capacitor, but bad run cap could also cause. Assuming it has a mechanical starting switch I'd open the motor and make sure it's not clogged with dust. Unlikely you have an open winding, given it's gradually gotten worse, but it's possible.

Oh....I'm assuming the shaft spins relatively freely and you don't have a bearing frozen....
 
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Old 12-04-15, 10:07 AM
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It's much worse now that it's cold outside and will not EVER fully start.
What weight oil are you using? You might try placing a sun lamp next to the compressor crankcase and and heating it up to about 70 degrees F. or so and see if it then starts and runs properly.

On the other hand, it IS a cheap, direct drive machine. It is not built with longevity in mind. It states in the instruction manual it has a maximum duty cycle of 50% and a maximum use time of 30 minutes per hour. Usage beyond this can (and most likely will) damage the machine. It could be that it is just plain worn out.
 
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Old 12-04-15, 10:15 AM
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I'm using some oil that is labeled "air compressor oil" that I grabbed from Home Depot. I assume it's 30 weight. I'll try bringing it inside this weekend and see what happens.
 
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Old 12-04-15, 02:45 PM
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Trying to run it on a long extention cord or somewhere with under sized wiring?
 
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Old 12-04-15, 03:08 PM
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Nothing wrong with my compressor... but I know that if I expect it to run the next day, I can't leave it sit outside overnight during the winter.
 
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Old 12-04-15, 05:12 PM
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I've been "playing" with air compressors since my age was in single digits.
During the 26+ years I worked for one company I spent as much time with air compressors as I did with boilers and I NEVER saw a compressor that used 30 weight oil in the crankcase. Our rotary screw compressors (250 horsepower to 800 horsepower) all used a 10 weight, or maybe it was 5 weight, automatic transmission oil, the centrifugal compressors (400 to 1,750 horsepower) used a 10 weight hydraulic oil and the reciprocating machines (175 to 1,200 horsepower) used a 10 weight mineral oil in the crankcase along with a 90 weight cylinder oil in the cylinders. With the rotary screw compressors we eventually went with a proprietary synthetic oil that was no more than a 5 weight equivalency. The centrifugal machines all had external lubrication systems that ran 24/7 keeping a minimum temperature of 100+ degrees even when the compressor was shut down.

Even in the small reciprocating compressors used in home shops and also instrument air installations never used anything heavier than 20 weight oil in the crankcase and IF they were subjected to lower temperatures they used a 10 weight oil.
 
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Old 12-04-15, 07:15 PM
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I would take the head off and look at what is happening with the reed valves. They often get stuck or damaged. If the exhaust valve is bent or stuck in a closed position, you may be trying to pump into a dead end.
 
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Old 12-07-15, 08:08 AM
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As far as the oil is concerned, it's some Campbell Hausfeld air compressor oil from Home Depot. I'm not exactly sure on the weight without looking at the bottle.

I did bring it inside over the weekend to try it warmed up and there was no change. I know the unloader valve is working because once it does eventually build pressure, it hisses for a second when I turn it off.

I don't think I'm going to pull the head apart because it does build pressure even while its running weird.
 
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Old 12-07-15, 12:22 PM
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I don't think I'm going to pull the head apart because it does build pressure even while its running weird.
I wouldn't pull the head either. In my experience leaking valves are far more common than stuck valves. If you had a stuck valve i doubt that the machine would run at all.

As I stated earlier, it is a cheap, direct-driven compressor and the motor under the best of circumstances is barely adequate. I'd change the oil to a straight mineral oil of no more than ten weight and then just run the thing until it totally craps out.
 
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Old 12-07-15, 12:43 PM
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Well, I have to get it to run consistently first before I can run it until it totally craps out.

Where would I be able to find some of the oil you mention? Everything I'm finding online is either 30wt or like $40/gallon. I really only need a few ounces for this thing.
 
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Old 12-07-15, 01:44 PM
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Try an auto parts store. Look for the absolute cheapest 10 weight oil they have. You could also use cheap, no-name automatic transmission fluid but the cheap oil would be better. What you want is something light weight and slippery. Steam turbine oil is ideal but I doubt that you could find a source for just a cup or two.

If all you need is a few ounces then you might find some ten weight "Three-In-One" oil in the hardware section of Wally World.
 
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Old 12-07-15, 03:25 PM
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If I remember correctly you want a non detergent straight weight oil. You might not be able to find that in anything lighter than 20 weight.
 
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Old 12-07-15, 03:29 PM
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Non-detergent is better but not really of much consequence if you can't find non-detergent.
 
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Old 12-10-15, 08:12 AM
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OK. New run capacitor will be here today. It was cheap, and from amazon, so easy returns if it doesn't fix it.

Now oil. I'm having a hell of a time trying to find anything other than 30 wt compressor oil that's affordable. I know 3-in-one oil was mentioned. This says SAE 20 on it, but would this work? http://www.amazon.com/gp/product/B00...=sr_1_3&sr=8-3
 
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Old 12-10-15, 08:21 AM
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OMG, get a quart of air compressor oil. Its $5.
http://www.truevalue.com/assets/prod...rge/687566.jpg
 
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Old 12-10-15, 08:32 AM
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That's what I have, but I'm being told that it's too heavy.
 
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Old 12-10-15, 08:38 AM
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It's not. It's also not 30w, its more like 10w, it just doesn't specifically say.
 
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Old 12-10-15, 08:43 AM
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OK then. Well, I already have that exact oil, so I'll leave it alone then.
 
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Old 12-11-15, 06:09 AM
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So I installed the new motor run capacitor. All the specs match the 'old' one, but no go. I even tried warming up the crankcase for a few minutes with a 500W halogen work light. It struggled for 20-30 seconds, then tripped the breaker. I'm done.

Anyone want this thing for parts or to fix? $20 in the Milwaukee, WI area.
 
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Old 12-11-15, 09:30 AM
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You say the check valve is ok because the pressure switch bleeds air when the compressor stops but did you remove the discharge line to see if it will start when open to the air?

It is possible that the check valve or discharge line is partially plugged causing a restriction on start-up.
 
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Old 12-11-15, 09:48 AM
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I guess I can check that. But it will put air in the tank even while it's doing the weird start up thing. I got to 45 psi last night before the breaker tripped.
 
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Old 12-11-15, 02:05 PM
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I don't think it's the unloader valve, when it does eventually fill up and I turn the switch off while its running, it does release the pressure.
You mention "you don't think it is..." and "when it does eventually fill up" it will release the pressure.... but what about every other time you turn it on and off? Is it releasing the pressure then? I'd say it's the unloader valve too. It must be defective, or on its way out... since when you fiddled with it, it finally did take off and get up to speed. The whole not wanting to start like it's under back pressure has got to be the unloader valve not unloading under low pressure... just like CarbideTipped said in the initial reply.

https://www.air-compressor-guide.com...essor-unloader

I have just replaced the whole pressure switch when mine has had startup problems and it fixed the problem.

The pressure switch is not the problem
How do you know, did you replace it?
 
 

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