# DeWalt D55140 Air Compressor Won't Shut Off.



## Thompsons (4 mo ago)

DeWalt D55140 Air Compressor Won't Shut Off. After the tank pressure reaches the regulated pressure, it will no longer shut off. Air will escape from the regulator knob as the compressor runs. Does this usually happen if the pressure switch inside the case is faulty? I changed the whole regulator manifold a couple of years ago (_as this model does not allow just the knob to be replaced_) No other leaks. How do I test the switch? Another $100.00 regulator manifold or an $18.00 switch.


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## stevon (Mar 5, 2019)

Thompsons,

sounds like your pressure switch contacts are welded stuck on. With the compressor unplugged, with a volt meter you should have no continuity from one side of the switch (plug end) and the motor side. Welded contacts is a common problem. On startup the locked rotor amps on a loaded motor is at least 6 times the listed FLA of the motor eg 10 amps listed on motor draws 60 or more amps at 0 rpm at start up under load. Pressure switches are cheap. Let us know how you make out

stephen



Thompsons said:


> DeWalt D55140 Air Compressor Won't Shut Off. After the tank pressure reaches the regulated pressure, it will no longer shut off. Air will escape from the regulator knob as the compressor runs. Does this usually happen if the pressure switch inside the case is faulty? I changed the whole regulator manifold a couple of years ago (_as this model does not allow just the knob to be replaced_) No other leaks. How do I test the switch? Another $100.00 regulator manifold or an $18.00 switch.


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## iowagold (Jan 22, 2020)

you might look for an upgraded pressure switch to a higher current.
i like square D
they make some good switches.
or switch over to a controller setup that is a large start box.
basic a BIG relay and then you use the onboard relay to trigger the Large relay in the start box
that master slave thing.
most good start boxes have an optional thermo manual reset fuse.
pm if you need links to the ones i like.


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## Thompsons (4 mo ago)

stevon said:


> Thompsons,
> 
> sounds like your pressure switch contacts are welded stuck on. With the compressor unplugged, with a volt meter you should have no continuity from one side of the switch (plug end) and the motor side. Welded contacts is a common problem. On startup the locked rotor amps on a loaded motor is at least 6 times the listed FLA of the motor eg 10 amps listed on motor draws 60 or more amps at 0 rpm at start up under load. Pressure switches are cheap. Let us know how you make out
> 
> stephen


So, it's normal for air to escape through the regulator knob if the pressure switch does not shut off the compressor? Thanks for your advice.


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## iowagold (Jan 22, 2020)

no
but the tank over pressure protection may kick in and dump air.
is it built in to the pressure manifold connections?

most of the time they are on a separate tank port for the over pressure protection.


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## Thompsons (4 mo ago)

iowagold said:


> no
> but the tank over pressure protection may kick in and dump air.
> is it built in to the pressure manifold connections?
> 
> most of the time they are on a separate tank port for the over pressure protection.


The regulator knob is between the tank pressure gauge and the regulated pressure gauge and is a part of the manifold assembly. Picture at the attached link.
Air escapes from this knob once the tank pressure has matched the regulated pressure, but the compressor keeps running. There is a separate pressure release valve on the side.

Manifold Assy. [5140062-33] for Power Tools | eReplacement Parts


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## stevon (Mar 5, 2019)

Thompsons,

Leaking through the knob? that would probably be a diaphram leak, no the over pressure safety valve should "pop" and vent so the tank won't have a catastrophic failure (explosion). The regulator switch is supposed to stop the motor pumping air, when it reaches a preset pressure, if it fails the safety valve is there to save your ass and prevent an explosion. In normal operation the safety valve should not vent! safety valve venting means you have a motor control issue.

Stephen



Thompsons said:


> So, it's normal for air to escape through the regulator knob if the pressure switch does not shut off the compressor? Thanks for your advice.


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## stevon (Mar 5, 2019)

Thompsons,

Leaking through the knob? maybe a little to balance your setting but not constantly

Stephen


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## Thompsons (4 mo ago)

The air releases through the knob once it has reached the regulated setting. I can turn the regulator knob as tight as it will go, turn off the compressor at say 100psi and air will not escape through the knob. It only starts to escape once the tank pressure has matched the regulated pressure setting.


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## stevon (Mar 5, 2019)

Thompsons,

Looks like the manual pressure regulator is defective. Without a picture of where the leak is venting, it's tough to troubleshoot through just the forum text, a snapshot might help.

Stephen



Thompsons said:


> The air releases through the knob once it has reached the regulated setting. I can turn the regulator knob as tight as it will go, turn off the compressor at say 100psi and air will not escape through the knob. It only starts to escape once the tank pressure has matched the regulated pressure setting.


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