Jun-Air Compressor Fault

Hi I have a Jun-air 6/15 compressor with 25l tank that provides air to my tool changer. It has just devoloped a fault, when turned it will start up and raise the tank to the correct pressure (120psi), before when it dropped below 90psi it would cut in and top up the tank pressure, now it tries to but wont cut in until pressure is at 50psi. I see that the pressure can be adjusted on the condor mdr2/11 pressure switch but as this is factory set can i assume that there is a fault with the pressure switch, anyone have any experience of this?

When you say tries to run, do you mean the motor is being powered but fails to spin?

If it is, then it sounds like the unloader valve isn't unloading, so the compressor is trying to start under load and failing. On the pipe from the compressor to the tank, there is a one way valve (normally at the tank end) which should only allow air into the tank. Somewhere between the compressor and that valve, there will be a small pipe T'd of it, which usually goes to the pressure switch. When the compressor reaches the set pressure, as well as the motor being stopped, it should also open the unloading valve, and dump any pressure between the compressor and the one way valve, so there is no load on the compressor the next time it starts.

Thanks for the quick reply m_c sorry I only just had a chance to reply, got a replacement compressor and just left the other to get some rush jobs done. When I said it tries to start, it makes a noise, the light comes on but doesn't start the motor. My compressor doesn't have any of the top pieces that you mention, I have attached a couple of pictures,
think that the unloader valve might be inside the pressure switch. Attachment 23292Attachment 23293
Attachment 23294

And now I have problems with the tool changer not indexing either! I'm sure problems always come in threes so just waiting for another!

Ah, that looks like an old screw compressor.

Could be the start capacitor has packed in, so the motor isn't producing enough torque to get started. You would have to strip the compressor down enough to find the capacitor(s), then try changing them.

The motor will start when the compreasor is switched on with no air in the tank, just doesnt top up quick enough to stop the machine alarming out at low air presuure.

Is the over-length nut that's part of the connection to the cylinder just a nut that's bigger than it needs to be or might it contain a one-way valve?