My Jun Air 6 X 25 stopped working.

by Throstur Vidisson
(Reykjavik Iceland)

My 1987 Jun Air 6 X 25 does not work any more. The symptoms are as follows.

At some point I got smoke that seemed to originate in, or close to the Terminal Box. The Terminal Box houses the Starting Relay and some small device that I think is a Motor Protection device.

After this the compressor seems to try to start, it ticks once or twice when I power it up, but can´t quite make it.

I have considerable experience in dealing with mechanical and electrical repair but it seems to be difficult to find support for repairing Jun Air on the Internet.

What I have done so far is that I measured the big capacitor with a capacitor meter and it is within specs. I took the Terminal Box apart and could not see anything wrong, nothing burned or loose and no bad smell anywhere. and off the top of the motor and it is full of clean oil and the crankshaft is turning freely.

The problem is that I don´t know how the Starting Relay should test/measure and if the small device that I think is a Motor Protection device is possible to test. So I´m a bit stuck here and all tips and help would be greatly appreciated.


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Jun-Air compressor overfilling with air.

by Janet
(Penzance, Cornwall, UK)

I've got a small Jun-Air compressor that has always worked brilliantly and I use it with a stapler. It now doesn't cut out but keeps filling with air then blows. You can also hear a small, hissing leak as it fills.

Jan, two issues, I think.

First off, when you say it blows, I suspect you mean that the PRV lets go? That's what is supposed to happen if the compressor doesn't stop at the normal cut out and pressure continues to build.

So... stop using your air compressor until you replace the pressure switch. That's what is supposed to turn your air compressor off and on at the right pressure, and if your compressor is not shutting off at cut out, it's almost certain that the pressure switch has failed.

Next, the hissing noise, it would be good to know where the air is leaking. If it is bleeding out the unloader valve, then that points to a leaking tank check valve.



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Jun-Air Compressor Cycle

by Alan Young
(University of Mich-Ann Arbor)

JunAir Model 4000M-150B Compressor

JunAir Model 4000M-150B Compressor

JunAir Model 4000M-150B Compressor
Old Check Valve
Old Check Valve
New Check Valve

we have a 150 liter JunAir Model 4000M-150B Compressor.

I recently drained the entire system and replaced the intake filters and a check valve.

I turned it on to re-pressurize the system.

I am only charging the tank at this point-not the lines.

The cycle should shut off at @125 psi. It has taken 45 minutes to get to 75 psi at which point I shut off the compressor.

This seems to be an excessive length of time for re-pressurizing the tank.

I have checked for leaks and can't find any-Seems that there is difficulty with air getting into the tank. The tank holds pressure over night as I shut it off and it remained at 75 psi.
Hello there Alan. It would be interesting to know how come you felt the need to change the intake filters and the check valve? Were you seeing an increasing fill time before you made the change?

As long as the check valve is installed correctly - with air passing the right way and air not passing the correct way - replacing the check valve and intake filters should not have affected the fill time.

Coincidences happen, and I suppose it is possible that your Jun Air developed a valve problem, or gasket leak, at the same time as you made the changes you did.

See the troubleshooting section of the sitemap page for links to pages about why air compressors do not build air pressure properly for some things to check.

Post a comment here with findings if you wish.

Thanks for the photos. They always help.


Comments for Jun-Air Compressor Cycle

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Jun 07, 2013
Jun-Air Compressor Exploratory Surgery
by: Alan Young

Thanks for the response to my post regarding our compressor. After opening up the unit I am looking at the possibility of replacing gaskets and rings. This link has a series of pictures showing what I have found so far....

Excellent Alan. Thank you for taking the time to share this link with us.


Jun 06, 2013
Jun-Air Compressor Cycle
by: Alan Young

I should have probably given more info to start. The compressor was taking along time to fill and I followed the JA manual in order-replace intake filters-Did that still no change in the long pressurize time- checked for leaks every where with soap solution-no leaks found, Next step is to replace the check valve so I did that-still taking along time to reach pressure-This is the tank only-I have shut off the valve leading from the tank to the rest of the system. The manual no is saying to replace piston rings- I am no in the process of taking apart one cylinder(Two pumps each with two cylinders)to check the rings---
Thanks for the update.

Yup, could be piston rings, could also be the valves, and if you are working inside the pump, replace any gaskets you find too.


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JunAir Condor MDR 21-EA/11 Bad connection: won't restart

by Sam
(New York)

Jun Air Condor air compressor

Jun Air Condor air compressor

I have a JUN-AIR compressor with a Condor MDR 21-EA/11 pressure switch. The pressure switch appears to have a bad connection: when pressure builds up in the switch, part of the moving switch gets pushed away from where it should be. When the switch gets thrown back and forth, the electrical connection is lost and the pump does not get power to run again. Occasionally, the switch will start smoking (Note char marks on box).

The loose connection is clearly visible sticking out of the side (see picture). I have another Condor switch of exactly the same model (but broken in its own way) and the connection is not falling out.

I looked for some screw or something to open up the guts of the pressure switch, but it appears to be completely solid. If I hold the loose connection just right, the switch goes on and off perfectly. But I use the pump sometimes continuously for days on end and I am uneasy about leaving the pump unattended with a bad connection. I am tempted to fix the switch with just a piece of tape to keep the connection from falling out.

What do you suggest? Do you have (or can you find) a schematic of the internal parts of the switch? I don't mind trying to fix it myself, but I can't see what's going on inside.
Do you know of a congenital problem with this switch? (As I said, it's not the first Condor switch to die on me...)
Can I replace the switch with a different model/brand? Can I just match specs for ports and pressure cut-in/cut-out?

If I do need to replace with same kind of switch, do I need to replace the unloader valve as well? I don't think the valve is broken (tank hisses for just a split second).

Many Thanks,

Bill says....

Sam, for the typical cost of a new compressor pressure switch, don't try to fix yours, install a new one.

No, I don't believe that it has to be the same brand, as long as the cut in and cut out pressures are close to your present one, and if the switch on your present compressor operates the unloader valve you will want to ensure that the unloader will work with the new switch.

Sometimes, it's easier just to replace the old with the same make. If that is the case, contact JunAir to locate your local distributor.



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