So, your air compressor still will not start? This is page two of help when trying to troubleshoot why an air compressor will not start. In case you have landed on this page first, page one is right here.
We were talking about power supply to the air compressor. Based on the previous page we have checked that we have power from the wall socket to the compressor, we now know that a malfunctioning unloader valve isn’t the problem, we are going to assume that if your air compressor has an ON/OFF switch that it is turned to on, so now what? What is it that the air compressor still will not start?
Table of Contents
- Is power getting to the compressor motor?
- Remove the compressors pressure switch cover
- What else stops an air compressor from starting?
- What does the compressor start capacitor do?
- More compressor start problems?
Is power getting to the compressor motor?
Knowing that power is getting to the compressor does not necessarily mean that power is getting to the motor.
There may be an ON/OFF switch in the power supply line between the wall plug and the compressor motor. Your air compressor will also have the pressure switch in the line from the wall plug to the compressor, either of which may be preventing electrical power from getting to the compressor motor. If the compressor pressure switch has not tripped to allow power to flow through it to the motor, the compressor cannot start.
Unfortunately, if you don’t have a poke probe or a multi-meter you will have a difficult time doing the next check.
Remove the compressors pressure switch cover
We need to try and find out if the pressure switch has tripped to the on position, and if power is flowing through it to the motor circuit.
Carefully remove the pressure switch cover. You will see something along the lines of what the photo left shows you – a bunch of wires attached to screws. Exciting, what?
Be very careful. If your compressor is plugged into a wall socket, the screws (terminals) will be powered, and if you touch them and you are grounded, you might be electrocuted! If your compressor is a 220 volt compressor I recommend that you do not mess with this at all!
Generally speaking (and this is anecdotal as I am not an electrician – don’t pretend to be – and do not dispense electrical advice) the wires of a 120 volt circuit are going to be white, black and green. Typically the green wire is the ground wire.
Also, typically, the black wires are the hot in and the white wires are the hot return.
Pressure switches often have terminals where the wires are attached. Some compressor switches have these terminals labeled. You may see the terminals labeled as line (the power supply in from the plug), load (the power supply to the motor), the terminals might be marked as power supply, motor… a variety of labels, if your compressor power switch has labels at all.
Using your multi-meter or poke probe, you want to see if the power is flowing from the line side to the motor side, so you are going to test the motor side of the switch. Is there power on the motor side of the switch?
If your tank pressure is zero PSI, then the pressure switch should have tripped to on, and power should be flowing across the switch to the motor side. If not, your pressure switch itself may have failed, assuming it tripped to on when the pressure in the tank fell below the normal cut in pressure setting.
What else stops an air compressor from starting?
After you have emptied the tank and plugged the air compressor in and/or turned it on again, is there any sound from the air compressor at all? For example, is the motor trying to start and cannot?
If that is the case, and the tank is empty of air, the issue may be that the start capacitor has failed.
What does the compressor start capacitor do?
Since an electric motor needs more power to start than may be available in a 120 volt power supply, in order for the motor to get going, it needs a boost. The compressor motor start capacitor does this.
Essentially it’s a quick discharge battery that recharges after use and is pressed into service when the compressor motor tries to start.
It charges and discharges throughout its life, and it does get used up. If the start capacitor has completely failed, then power cannot get through it to the start circuit of the motor, and nothing happens when you try and start the compressor.
More compressor start problems?
Yes, there could be other things preventing your compressor from starting. The compressor motor itself may have failed.
Diagnosing a failed compressor motor is getting pretty complicated for a web site help page.
If you have done all the checks noted here and you have not been able to get your air compressor going, and repairing compressor electric motors is not what you do, then I would suspect that it may be time to take your air compressor to the shop.
Hello,
Have an old Craftsman 2hp 20 gal 220v compressor. And it won’t start. No hum, no movement at all, nothing. I’ve checked the power, and the motor seems to be getting appropriate power: checked between each hot terminal and ground, and got 125v on each. And I just took the motor to a shop: they tested it and said it ran fine.
What is the issue? I can’t figure out what it could be.
Josh
Josh, if the motor runs fine when it’s not attached to the pump drive train, there’s no reason why it won’t run (the shaft turns) when the motor is attached to the pump. Either the folks that tested it misled you, or, the capacitor on the motor isn’t allowing power to give the motor inrush when the shaft is under load. Still, I’d have thought there would be at least a sound if the pump was seized.
Have you attached the motor to the pump? Is it a belt or direct connect? If you decouple the motor from the pump drive, does the motor start? Please advise.
Actually, just found the problem. Turns out it was a power problem. I had wired a new 220v outlet for the compressor, but had mistakenly wired it through a tandem 30-amp (but one pole) circuit breaker. Since each leg supplies 110v, I didn’t realize a two-pole breaker was still necessary to get the 220v. I assumed that as long as I had 110v on each hot wire, I was good to go. Didn’t realize them being in the same phase would prevent 220v.
Anyway, installed correct breaker, and all is well.
Thanks for the explanation and comment. Very much appreciate the quick reply!
Best,
I have a problem with my Central Pneumatic vertical air compresor, 2.5HP 21 Gallon. The compressor was running but was shutting down at 40 PSI; I needed to turn it on again with the reset button because it doesn’t start by itself. Then I checked the drain valve, took out the water and finally replaced the oil according to the manual’s specifications; After doing all this, now the compressor does not turn on; When I press the reset button, it tries to start but shuts down immediately. Does anyone have any suggestions for this situation
Per the suggestions earlier on in this page, did you check your capacitor?
Yes the capacitor is good; now I cleaned the head where is the piston area but when I connect the compresor, the compresor runs but air get off from air filter hole and doesn’t fill the tank; like the system was invert it. also the Reset switch button is “in” and it doesn’t stay out as well it has to be when the compresor is run. I do not touch any wire to invert the it. I don’t understand what happen now.
Fabrizio, if air is coming out of the intake, then either the valve(s) plate is installed upside down, or the valve has failed. If the valves and the gaskets are working properly, air cannot get to the intake side or back out of the intake hole.
As to the reset switch, if you have a meter, check to be sure that power is reaching one side of the switch, and also check the other side to be sure power is getting by the switch. If the compressor is running, power must be getting across the switch. If the compressor is running, either the switch is working as it should, or the thermal cut out part has failed. Maybe resolve the pressure building part first.
Also, if the compressor is running longer than it should that should heat up the motor to the point where the thermal reset will trip.
Hi I have a small wolf air compressor that is about 10 years old and has done me well for the small jobs. I have a problem with it starting, it tries to start and and slowly spins up and when a continually click the on off switch it builds up speed and starts but can’t keep doing that. Also when I put my finger of the intake it sometimes starts. Take the head of the piston and it runs fine. Any ideas – piston rings
Can you please tell me, when it is starting, is it attempting to start with air in the tank, and what happens if you dump all the air first, Rod?
Hi so have tried with and without air in tank, also I was wrong when I said that the motor would run fine when the head was off it doesn’t it struggles to get up speed. So now not thinking its the piston but the motor or capacitor?
Yup, that would be what I would check now, Rod.
I have a Sanborn v89b200 14gal 2hp air compressor, and I bought it used at first it was working fine when first started, then when the pressure dropped and it tried to restart again it would trip my breaker in the house no matter if it was on an extension cord or directly on an outlet.
Then I tried another outlet and it would try to start but just chug for a few seconds and blow that circuit, tried another outlet and it would do nothing not even hum, I thought it might be the pressure switch and replaced it, still nothing not even a humming from the motor, there is power all the way to the pressure switch, but no power to the capacitor, tested the capacitor and it is testing good, the motor does have reset switch built in and it is a 3-wire one I have tested resistance across all 3 and no matter what contacts I test across it has no resistance at all.
I am wondering if that switch is the problem or not.
Any help would be great at this time.
Thank you
Hey Todd. If I understand correctly, you have power from the source getting to the pressure switch, but there is no power on the motor side of the switch? If so, then it is likely the pressure switch has failed, maybe a diaphragm, maybe points or spring.
You also say that you relocated the compressor and it would try to start but chug and blow the breaker.
Here’s my problem with your post. If the power isn’t getting past the pressure switch to the motor side, how is it that the compressor motor gets enough power to start and chug? Please provide a bit more clarification. Thanks.
ok so since posting I have replaced both the pressure switch and the electric motor with new after market units and the motor will still not start I am getting 120v to both sides of the pressure switch now when the switch is in the on position and getting 120 from the wall but still the motor will not even hum.
Todd, did the new motor come with new capacitors? Based on your comment that power is getting across the switch now, is it getting to one lead on the start cap? If so, and the motor does nothing when power is applied, my guess would be the cap is the problem. Update?
I have an Eagle 5 horse, 60 gallon 240 volt air compressor Model #C4160V1. I bought it new about two years ago and it’s been lightly used.
The compressor was running after I’d used an air tool. When it shut off (as it usually does when it gets up to pressure) it sounded funny. I didn’t pay much attention but more of an abrupt shutdown, maybe a clunk.
It has since refused to restart.
There is power at the outlet. There’s a full 240 volts at the switch. There is full power leaving the switch and there is full power at the leads on the motor.
The tank has been emptied. I pulled the belt cover off to see if the pistons would turn over and they do. Actually it turns over very easily.
There is plenty of oil in the engine and it looks clean and has no burnt smell.
I’d love to know what else I can check. This thing is bolted to the floor and getting it into a shop is not an easy option.
Thanks.
You’ve removed the belt and the pump turns easily. Will the motor start without the belt on? If not, then it’s the motor. If it does start, that points to the pump. Which is it, please?
This must be a scam website or something, i ve attempted no less than 100 times to write out a description of my problem, & to load a few photos but each time i get,to the point of loading the pictures the page reloads and I lose everything I just spemt 30 minutes composing
I finally Tried composing my Question in microsoft Word, everytime i i attempted to load a photo the page reloaded and would freeze up then reload.
Hey Gary… it is possible that your photos are way too large for your computer / internet provider to process. Otherwise, I have no idea why you are the only one I know of that is having a problem. Sorry. Try adding just the text, and then, as a comment, add one, downsized (lower resolution) photo to see if that works if you care to.
Sorry to hear that. Haven’t had this issue mentioned before. Perhaps the page times out? Could you write out the text in notepad or some such and copy into the form? Must be a lot of info to need 30 minutes to type it. Thanks.
We got as far as the compressors pressure switch cover and the switch was stuck on the off position, all fixed now thank you
Well that’s good news! Thanks for letting us know.
After checking everything mentioned in these articles and replacing a capacitor that was not the cause, and having taken the motor apart 7 times, I finally remembered an old YouTube Video I had seen where the person mentioned just in passing, that it was a good idea to check what he called, the electrical clutch plate to make sure it wasn’t stuck or sticking.
Sure enough, my humming motor that would start sometimes and not others, whether under pressure or not, was caused by an intermittent sticking “clutch plate.” when I lubed the shaft the clutch assembly was to slide upon and reassembled the motor, the motor kicked in each and every time, under tank pressure or not. I came so close to throwing out my 1974 Craftsman 20 gallon 2 Hp air compressor, it is scary. So if you’ve tried everything else and you still have a “hummer” on your hands, don’t overlook a sticky starter clutch in the motor!
Ron Presley
Thanks for the tip, Ron. I wonder if what you are referring to might be the centrifugal starter mechanism? Regardless, you got it going and that’s great.
I have a central pneumatic model#67947. I changed the pressure switch and when I turn the off switch to on the compressor still won’t run. I have to push the reset button to try again. Could it be the start engine capacitor is bad and needs to be replaced? any ad all help you provide would be helpful. Thank you!
If the compressor starts with zero air in the tank it could be either the unloader valve (though a new switch should have fixed that), a power supply issue, or yes, it could be the start capacitor is failing.
I would like to suggest that you have an “Attention: Before You Go Any Further! If your motor won’t start or restart, unplug it from any extension cords and plug straight into a wall outlet!” I spent two days researching, changing, checking, researching, page after page, ad nauseam, until I FINALLY came across your remark about the extension cords. I plugged mine straight into the wall outlet, SHAZAM! Works like a champ. Just saying.
Hi Willyr, GREAT site, thank you so much for sharing your hard earned/gained knowledge with the world and me!
So, I bought a second hand ABAC B3914-150 litre 3hp off eBay. The seller had “apparently” had it serviced so all would be hunky dory and running like a train. First time I fired it up it got to around 2 bar then shut down. I turned it on again at the pressure switch and it got another 2 bar and then cut out again, this time it blew the 13 amp fuse in the 240v plug. I drained the tank and left it for a couple of days due to being out of time.
I decided the “service” it supposedly had was either non existent or else totally inadequate so, after reading your site from top to bottom, went about doing my OWN service! I fitted a brand new Clarke pressure switch rated the same as the original ABAC supplied one. I fitted a brand new 145psi 10 bar safety relief valve, did a complete oil drain down and change. Checked the check valve and unloader and gave the compressor a bloody good clean down and scrubbed most of the over spray off the belt cover, belt wheel, piston and motor heads. Oiled around any exposed moving parts too as it sounded VERY loud the first time I fired it up! Turned it on and it sounded like silk, really smooth and at half the volume it was at before. It got to 4 bar and the thermal cut out tripped. I reset it and it ran for another 30 seconds or so and the thermal cut out tripped again. I again reset it and the motor started to struggle, then black smoke came from the capacitor pod atop the motor and it cut out again. Upon opening the pod I found the main power line in live wire was burned where it connected to the thermal cut out. There had been previous issues there as there was a small piece of electrical tape wrapped around the live wire. Top service indeed!
Gutted! It was going so well and thanks to your site, I WAS feeling like a bloody GENIUS! Hoping you`ll have some idea of the issue that is causing this Willyr and seriously looking forwards to your input my friend.
(Sorry for the life story, boy can I waffle!!!)
You are welcome, and it were my compressor, I’d be looking first at the capacitors, and then the motor itself. Good luck, and thanks for the good story.
Errrrrrrrm. Just been reading further pages on this here fantastic site and I have a confession to make. I have been running the compressor via my 25ft extension reel. Other than feeling a tad stupid, I shall be over joyed if I plug it into the wall tomorrow and find my woes are at an end………
To be continued……
I have a 7.5 hp compressor with a 80 gal tank. The compressor only stops when it reaches 175 lbs. It will not start when the pressure switch closes. The magnetic starter does not get energized .
What is make and model? What is the normal cut out pressure. When the pressure switch closes, does the switch then have power on the motor side?
I’m completely stumped here, please somebody help me out here.
I have a 5 hp air compressor that won’t run. Bought it used yesterday, I tested it and it worked fine.
Brought it home, it was pretty greasy on the top, it is a 60 gallon vertical tank so I cleaned it with water, tried not to soak things but it did get wet but not majorly.
So wired my 240 volt from my panel to it, turned it on and nothing.
Checked both capacitors and the run cap was dead showing no readings, bought a new one replaced it and still getting no start. I am getting power going to the motor from the switch. Tank is drained of air.
Joel, it’s always good to know the make and model. That helps other folks with the same make and model of compressor offer suggestions. You checked the start capacitor, and it’s OK, you said. What’s the amperage of the breaker the compressor is wired to? If you are not getting any sound out of the motor at all, and with a great deal of care, you might try running power right to the motor to see if you can get it to at least grunt. If so, and you’ve bypassed the start cap, maybe that’s the issue?
Thanks for your great site. I was able to find that the starter on my regulator had the hot wire slid off the spade lug. Simple fix (a bit of a pain to work in that small box tho).
Great site!
Thanks for your kind comment Jay. Yours made the home page!
WillyR,
I have a Craftsman / 7hp / 240v / 80 gal. Air Compressor (#919.167800… I have gone through your check list (prior to finding your site) and still have a non-restart problem .. There seems to be a lot out there about leaks, head pressure, unloader valves and the like; but nothing about a non-restart when all else checks out (motor included) … I be
leed my pressure down to 0 psi and nothing happens .. I press the contacts closed on the pressure switch and it pumps to full pressure — BUT — it won’t restart on the bleed down … I think it is the pressure switch(but not interested in fixing it — if it ain’t broke) ; but have no idea why it won’t work automatically … I haven’t readjusted the sure setting from the manufacture and I have verified that all valves work … So — Go figure ???.. HELP !!! Pleeeez … THX … KJ Marshall
Ken, when you went through the check list, did you check to see if your compressor had power on the motor side of the pressure switch when the tank pressure was zero? Assuming no problem with power supply to the switch, if the compressor tank is below cut in pressure level, and if power is NOT passing through the pressure switch, that points to the pressure switch itself as being the problem. We don’t fix ’em if that’s the case, we toss ’em out and get a new one. Trying to reassemble a compressor pressure switch even if you can get parts is a task so persnickety it’s one we will not do.
I have a nu air 3hp compressor which has work fine til it suddenly stopped. It ties to soft start I think and then stops by which time it has blown the 13 amp fuse. Any ideas? Many thanks
Ian
Nu Air has a number of models, so without knowing the model number of your 3HP compressor, it’s hard to be specific. We don’t see any “soft start” on these compressors other. Are you, perhaps, talking about the unloader valve? If so, how to determine if that is the cause and how to remedy it are already covered on the pages of this site.
My 220v compressor was running just fine and electricity was cut to only half and the compressor was on. Now it wont start at all. What could that have damaged? Capacitor, motor?
You lost a leg of power? Sure, check the capacitor. See the page on this site to see how.
My air compressor would not start when you turned it on the motor would turn a little and quit I thought it was the pressure switch so I got a new one put it on and it pumped up fine and cut off and would not come back on any ideas would be helpful
Does your unloader valve work? Empty the tank and see if the compressor will start then. Make sure you are NOT using any extension cord. If the compressor starts with the tank empty, it could be the unloader or the start capacitor. See all about them on this site.
Finally found the problem. The back of the motor (opposite the fan) is just plastic… it holds the start capacitors, It was broken so that the contact ring wasnt making contact .
A little JB Weld , and some silicone gasket material and all is well now.
Cant buy the part alone … part of the motor assembly 🙁
Compressor hums when turned on but will not start unless tank is empty and I give the pully a spin by hand at the same instant I turn on the switch. I replaced the start capacitor without success. I don’t think power is getting to the capacitor. What might that be?
Well Eddie, you need to use a multi-meter to follow the power path from the pressure switch to the capacitor, and from the capacitor onwards (with real care checking the capacitor). If power is getting to the cap, and it’s not boosting the motor on start up, then it may still be the cap wired wrong or wrong Micro-Farad rating. If the capacitor is proven good, then the problem, if the pump sheave rotates fairly easily by turning it by hand, then it’s likely the compressor motor itself that’s the problem.
However, that it starts with a boost from you suggests strongly that it’s still the cap that’s the compressor problem.
Air compressor won’t cut in until regulator pressure bleed down. Tank pressure good even when regulator bleeds down tank. pressure at 120psi.
Hello Larry. Be good to know make and model of compressor. When you say the regulator bleeds down the tank, we’re not sure what this means. The regulator only sets pressure for the air going into the air line, it has nothing to do with bleeding down the tank. Having said that, we think you should check to ensure that your compressor unloader valve is working when the tank pressure reaches the normal cut out and the compressor stops.
I have an Emglo compressor that will not start. I have checked everything, including the starter capacitor. There is power going to the stator, but not to the starter cap. Power goes across the pressure relay. The tanks are not connected to the motor. The fan turns easily.The oil is clear. How do I get the fan off so that I may remove the stator cover? Thank you! Your advice is the same process I used to get to this point.
What’s the model number, please? If power is crossing the pressure switch, normally the leads go first to the start capacitor. If no power is at the capacitor, we suspect a loose wire.
This troubleshooting page was right on. My compressor had a bad start capacitor, and thanks a bunch for the insight. I was able to fix it for $9 instead of replacing it. Although not covered in the troubleshooting guide, I did run one small test to confirm the condition of the capacitor that I’d like to share. With a 0 volt reading on the capacitor, 0 PSI, and unplugged, I set my multi-meter to ohms and let it run for a few seconds to put a small charge into the capacitor (~0.5 volts), then rechecked the voltage and was able to see how rapidly the voltage dropped. Within 3 seconds, it was back to zero, denoting that the capacitor was probably no longer able to hold a charge. Not a perfect test, I know, but a small double check none the less. Hope it helps someone else.
Thank you for an incredible site. I have read most of the posts and comments.
I have a compressor that will not start. I rebuilt the output manifold and gauges because the stock pressure adjustment know broke. The components are not made anymore so I just purchased the plumbing fittings and made one that would work. When I plugged it back in, it will not start. Not even a hum. I made sure the wires were connected and tested for power. I am getting power through the switch to the motor. With the power on and power to the motor, I tested the capacitor on the top of the motor with my meter. It does not show any voltage across the two poles. I pressed the thermal shutoff button, but it had no change.
The compressor was working when it turned it off to change the output manifold. Is there something I am missing?
Nope, not missing anything we can see, Bobby. We would recheck any switches with a multi-meter to ensure that power is getting through all of them, even the thermal cut off as they do fail, double check the caps, and then zero in on the motor itself, as all else being OK, then it’s got to be the motor. Kudos for building your own manifold from plumbing fittings. The cast manifolds in many small air compressors are of pretty poor quality, and seem to break quite often, with no spares available.
Hi Christa. My, that’s unfortunate to drop it right out of the package. Can you open the case and have a look inside to see if, perhaps, a wire has been dislodged? Make sure the power cord is not plugged in when you do this. It is difficult to determine what the problem may be without seeing the condition. Try to get it open and have a look, won’t you?