Question about Hitachi EC12 Portable Electric 2hp Air Compressor, Oil Lubricated

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I've messed with the settings on the three springs inside the mdr21 pressure switch and don't know how to adjust them correctly and which does what? I'm getting max pressure at 130psi and blow off to 105psi restart is at 80psi

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Normally you can find the setting instructions on the underside of the switch cover.
In almost all compressor switches you will find two adjustment screws. One is for the "Cut-in/Cut-out" or "Cut-on/Cut-off" psi. The other for "Cut-out" psi only. If there is no label the easiest way to tell the difference is by the springs. The "Cut-in/Cut-out" is the larger of the two springs.

Start by turning the smaller or "Cut-out" screw counter-clockwise. This will decrease the Cut-out psi. I would say to back it off 1 1/2 or 2 turns. Now run the compressor and check where your max psi is. Your target is 105psi if your present "Cut-in" psi is 80psi. Once you have that setting at 105psi, move to the larger screw or "Cut-in/Cut-out" adjustment. Turn the screw clock-wise making 1/4 turns adjustment until you reach the 100/125 psi.

By blow off I assume you mean the unloader valve. There really isn't any setting for the unloader because it's built in to the valve itself.

Since there are some variations for this switch please let me know and I'll explain how to make the adjustments for your type.
Or if you are unclear about anything here just let me know and I will be happy to help you out.
Hope this helps.

Posted on Nov 13, 2009

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1 Answer

Adjust pressure


The adjustment screw description-- is under the gray cover

The big spring is the general max pressure & cut off & initial setting

The smaller spring side is the twicking on the cut in or back on

They both work together in a way

Sounds like your using a well pressure switch-- on an air compressor

If so- then yes- you need a different switch for the application,
as the one you have is out of it's range

I set my compressor (single stage) at 80 & 135

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Changed pressure switch now pump cuts off and on @ every 15 secnds


There are two adjusters, one for max pressure, one for the difference between cut in and cut out. If you have fussed with either of them, look on the old switch for an idea of where they belong.
Basically, the one on the big spring is tightened until the pump cuts off at the desired pressure, and the littler one is just tightened up until it is not loose.
Further adjustments have to be made in small increments, with the pump allowed to fully cycle up to the desired pressure. Then the pressure has to be released (let some water out) until the pump comes on.
Balancing between these two adjusters is not easy.
If you have totally messed up, get a new one and don't mess with it. It will be factory set to the settings on the box.
If the pump still won't stop running, you have something wrong with the plumbing, like a check valve that has failed.
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Porter Cable pancake rated at 150 PSI stops at 135 PSI. It has done this since it was new.


If it's pumping up to 135 psi, shutting off and then restarts at the same pressure point since it was new, the pressure switch may have been set low at the factory. I don't know which model you have but most pressure switches have directions inside their cover as to how to adjust the cut-out and cut-in pressures. Be careful doing this, overpressuring the tank is dangerous. There is an adjustment screw or bolt that changes the pressure on the switch spring. Some pressure switches have two screws, one for cut-in and one for cut-out pressure. A few have one that adjust both pressures. If you do adjust the cut-in/cut-out levels, do it slowly and cycle the motor a couple times between each adjustment to see where it turns off and on. If the compressor is doing what you want it to, only pumping to 135 psi, it will live longer than if you adjust it up to 150 psi. Those last 15 psi of air are hard to get and will mean the motor will be running longer and more heat will be building up in the head.

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Compressor reaches 125 psi but does not shut down


Depending on how old your compressor is, usually inside the pressure regulator cover are instructions on how to adjust the turn-on and shut-off pressures. If not, it usually invloves turning a nut or screw to lessen the pressure on the spring that keeps the contacts closed. Try adjusting the pressure down. If it won't come down to a safe level you will be better off replacing the pressure switch. The number you give above is the Campbell-Hausfeld (they are Husky) part number for the pump and motor assembly so I don't know which CH pressure regulator you need. They are easy to find and easy to replace.

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1 Answer

I adjusted both large and small springs on my pump switch trying to obtain cut out at 60 psig and cut in at 40 psig. Apparently, I have a pressure differential of 30 psig. It cuts in at 30 and cuts out at...


The large spring is for the cut in and cut off pressure, the smaller spring is for the differential in pressure, meaning it adjusts the lbs of pressure between cut in and cut off. Since you have adjusted the smaller spring you have a 30 psi differential instead of 20. To adjust turn the small spring counter clockwise 1 full turn or 360 degrees, then recheck the pressure differential (do not adjust the taller spring while doing this) You may have to repeat this two or three times. Once you have a 20 lb differential, then you can adjust the taller spring to get your cut in, cut out pressure. It works the same way. Clockwise for a higher pressure and counter clockwise for lower pressure. It's something you'll have to play with. It doesn't have to be exact, as long as it's close. Sometimes they are hard to get readjusted.

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1 Answer

How do i set the low pressure on the flotec switch


your pump should have a 20lb seperation ie. pump turns on at 20lb and off at 40lb or on at 50lb off at 70lb. under the cover of the switch theres two screws with springs under them.dont mess with the small one.the large one if you screw it down (right) will increase the pressure while maintaining the 20lb separation. one thing to keep in mind its fine to adjust the pressure BUT at the same time the air pressure in the tank needs to be adjusted to two lbs below the cut in pressure of the pump.

I believe the directions are in the top of the cover of the switch

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1 Answer

Switch will not shut off


Make sure you don't contact the electrical power inside the switch cover.
Make sure that you have an accurate gauge to test the compressor.
Adjust the switch to the original factory setting.
If the original setting was 100 psi, for example, make sure the gauge is reading at least 100 psi while the compressor is running. If the pressure is over 100, turn the nut that is over the larger spring counter clockwise until the compressor stops. This will lower both turn on and turn off pressures. The nut over the smaller diameter spring should probably not be adjusted.

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Remove cap from pressure switch (small nut on top). You will see 3 threaded studs with springs held down by nuts at the end opposite from wired connections. Avoid wired connections with tools and fingers! The center sprung nut adjusts the ON AND OFF pressure settings. Clockwise (tightening down on spring) increases the pressure setting. One of the outside springs controls nothing - note that it has no connection to the switch moving parts. The other one adjusts the OFF setting. So adjust the center one until the ON pressure is 30 psi and then adjust the OFF setting until the pump cuts out at 50 psi. If you have an expansion tank, set this to 2 psi below the OFF setting. (If you have manual for the pump, check the pressure settings in the tables given for your pump.)
If this isn't what you need, please post more info!

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