Replacing the unloader valve
1 of 3Photo 1: Disconnect the unloader valve
Slap two open end wrenches on the
compression fitting and loosen the nut on the
air line. Pull it out of the fitting.
2 of 3Photo 2: Remove the wiring
Disconnect the hot, neutral and ground
wires from the old switch. Then squeeze the
strain relief grommet with pliers and remove
the cord.
3 of 3Photo 3: Install the new switch
Spin on the new switch. Tighten the switch
with a wrench and hold the stub pipe from the
tank with a pliers.
If your compressor starts up
fine on the first start of the
day, but stalls and makes a
loud humming sound on
subsequent tries, you've got
a bum “unloader” valve. The
unloader valve is what makes
the “psssssst” sound when
the motor shuts off after
reaching operating pressure.
It's actually venting the residual
pressure from the compressor
head so the motor
doesn't have to work as hard
on the next startup.
The unloader valve is usually
built into the pressure
switch assembly. Contact the
compressor manufacturer to
buy a new assembly. Or,
remove the cover from the
switch (unplug the compressor
first) and locate the pressure
switch part number.
Then do an Internet search
for that part. I found a complete
four-port pressure
switch assembly replacement
for this compressor for $20
(at amazon.com). Here's how
to replace it.
Start by unplugging the
compressor. Then open the
drain valve at the bottom of
the tank to depressurize it.
Next, remove the 1/4-in. line
going to the unloader valve
(Photo 1). Then use a slip-joint
pliers or small pipe
wrench to remove the quick-connect
fitting, pressure regulator
and gauge from the
pressure switch.
Disconnect the wires from
the pressure switch (Photo 2).
Then remove the old switch.
Coat the pipe threads with
pipe dope and install the new
switch (Photo 3). Finally, apply
pipe dope to all the plumbing
parts and reassemble.
