Air compressor question


 
Len Dennis

Len Dennis

TVWBB Diamond Member
Really off topic but here goes:

Air comp from cold starts, fills to 120 shuts off. I use it, pressure falls to 80.

Ok so far.

Motor starts to refill (ok) but only hums, won't start.

I've (sort of) narrowed it down to the start cap (which is black).
It's 80MFD 250VAC. Thats ok too.

Part of the problem is the mounting plug for the (white) run cap (70 MFD 250VAC) is broken and I suspect it is the problem (due to vibration because of the broken plug and not the start cap. One of them has been replaced in the past (I bought used) and i think it was the start cap (but I don't know).

I can't test them as I don't have a digital meter (only analog which doesn't have micro measurements).

Which cap won't allow the motor to re-start-run or start?
 
Can you spin the shaft? If you can, it sounds like the start capacitor. By chance have you checked for power at the centrifugal switch and start capacitor?
 
the unit starts with an empty tank (ie it has power) . It won't restart (but only hums)on a partially (80 psi) filled tank.

IE: it tries to restart (but only hums) when it drops to 80 psi; a continuing drop to 60 psi allows it to fully start (no hum) and it fills the tank back to 120.

I tried to answer (albeit indirectly). Is it the black (starter) cap thats faulty? I assume the shaft you refer to, is the one the fan that pulls air in is attached to? If yes, it spins.
 
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almost sounds like it doesn't have enough torque to get started..... 220 volts though... I don't suppose you have a shorter cord or could try reducing the electrical resistance somehow. I just have a cheap upright compressor... it's well used but mostly for cleaning electronics and the occasional air up of tires and air ratchet (wheel lugs) duty.

edit:::: I see, not 220 volts, but 250 mfd caps........

good luck with that! replace them both..... and get yourself a respectable multimeter already
 
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Len, I would test the start capacitor first. Also is it swelled up or do the terminals look burned? If that checks good then it's the relief(unloader valve) leaking putting pressure on the piston/pistons. You can look online how to test. Basically you want to fill the tank then disconnect the air line to the motor when its off and tank is full. Listen, if you hear air, the valve is leaking.
 
neither cap is burned/swollen-looks perfect. It's a cheap chinese unit (similar to harb freight 2 hp units). Its 120 volt. Factory cord no extension. I considered the unl v but it seemed the easier place to start were the caps.
 
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Okay. You can look online to see how to test a capacitor before you go buying a new one. All compressors have pressure switch, unloader valve, start and run capacitors. I don't care the make/model or the country built. It's basic compressor design.
 
0Agreed. Both caps test 0/infinity. Looks like its the unl v.

Now a new search begins. I'll be back ;)

PS: I initially discounted that valve as there was always a little "psst" on compressor shut down. Guess it wasn't a big enough 'pssst'. Tks for the guidance.
 
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Might also be the check valve letting tank air leak back into the valve. Gotta get that out & clean it before I replace the unloader valve.
There was a slight leak but I didn't think that would prevent a re-start. Guess it does.

So much to learn but it sure is interesting.
 
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Silly question but have you drained the tank(s) and checked for build up on the bleed valves?

Tim
 
if by 'bleed' valves you mean the tank drain and air release under the pressure switch, the unit is seldom used so the water drain has (IMO) is not a concern. Having said that, there may very well be rust flakes inside the tank which may have blown into the check valve and fouled the valve seat. Won't know till I take it apart.

The 'quick' release valve (under the release switch) functions as expected when 'pulled'.

As i can hear leaking air when the tank is full/motor has shut off, i really think its the check valve. Prior to removing it, i'm trying to find a replacement generic one online. Strange though: I hear/can feel air leaking by the oil filler cap. Weird.

I'm looking how to release/remove that from the pressure switch base at the moment.
 
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