atlas copco atsl140 wont start

by Tim Patten
(Richmond Hill)

The ats1-140 oil free compressor we use at work has repeated problems with the pressure switch. The unit runs continuously from 9am to 8pm and probably wasn't designed to do so to start with.

I have replaced the switch 4 times in three years which seems excessive.

This time the after replacing the switch, the unit will not start unless I "jump" start it by pressing on the contactor or the switch contacts. Then it runs to the correct pressure and shuts off. It will not restart unless I "jump" it again.

I'm not thinking it is the wiring because it wouldn't start at all (I think). What else could it be, contactor or maybe unloader valve? It does click at once the low pressure is reached but not starting.

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Jul 10, 2014
by: Doug in

Absent specific answers to my other questions, I'm voting for contactor failure, as in partly shorted windings.

The shorted (progressively, it seems) windings are what's taking out the pressure switch, repeatedly.

It'd be really good to confirm that with voltage and ampere readings, but that's the SWAG.

Jul 09, 2014
To clarify
by: Tim

Thanks to you both for your responses.

Yes I can jump start the motor by pressing in on the contactor as well as tapping one of the wire connectors from the pressure switch to it's own terminal (disconnect the wire and make contact with the terminal by tapping on it).

In both cases I must repeat the motion continuously and rapidly several times to get it to start.

Once running, the motor will run until the high pressure is reached but the motor doesn't start back up once the low pressure is reached.

Although I do hear a click at that point, just no motor turning.

Jul 07, 2014
magnetic starter?
by: Doug in

I'm not finding info this thing, but I'm guessing it's pretty big and uses the pressure switch to operate a large(r) contactor which turns the motor on.


I agree, depending on how often it has to cycle on, and how long between cycles.


This suggests that this new switch may not be good to start with, or, given the history, that the contactor is bad: partly shorted, leading to premature failure of the pressure switch, and to a weak electromagnetic force in the contactor.

I would check the current through the contactor coil(s) to make sure it's within specs of the pressure switch.

Good luck, and please let us know what you find.

Jul 07, 2014
pressure switch or contactor?
by: Doug in

"This time the after replacing the switch, the unit will not start unless I "jump" start it by pressing on the contactor or the switch contacts."

This is unclear. Are you saying you can press either the pressure switch contacts or the contactor and it starts?

If you press the contactor, does it snap in as you press it, as though it's energised? If so, you could have either a high resistance contact in the pressure switch or wiring to the contactor, or the contactor is failing.

You may want to check that the pressure switch is only driving the contactor, and not anything else, which could be making it fail. A partly shorted contactor coil could also cause this.

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