Pressure Switch: Diagnose a pressure switch problem?

Pressure Switch

My thanks go out to the technical service people at Grainger for part of this information.

They advise that they get an awful lot of folks calling them about pressure switch problems that aren't really pressure switch issues.

The Symptom

Actually, it's often air bleeding out of the unloader valve, not out of the pressure switch at all.

The unloader valve is part of the pressure switch, so the air is seemingly coming from the pressure switch (at the bottom). The unloader valve is a needle valve which is opened when the contacts open in the pressure switch; the mechanical lever action which opens the contacts also opens the valve. This valve then connects the pump to atmosphere to unload the pressure.

When a compressor reaches the high pressure set point and stops, there is the characteristic "Psssssst" sound, the sound of air escaping. This is the sound of the compressor pump being unloaded. If this sound of escaping air continues, the check valve is at fault.

Between the unloader valve and the air in the tank there's a check valve, or as it's also known, a one-way valve.

Often that valve gets debris on its seats or seals, and as a result, instead of being closed tightly when the air in the tank tries to flow back up that pipe to the compressor head, air gets by the check valve.

This air escapes from the same vent in the pressure switch as when the compressor has stopped compressing and unloads, in preparation for the next start up, and there's access to atmosphere from there. But the escaping air dloesn't stop!

Fix it!

Remove the check valve (after pulling the plug on the compressor and exhausting compressed air from the tank) and clean it up.

That should solve that problem.

If, on the other hand, your compressor won't shut off, keeps compressing air into the tank until the pressure relief valve opens and begins exhausting compressed air, then you've got a serious pressure switch failure.

Unplug it, and don't use the compressor again until such time as you've fixed the problem. This is a major safety issue.

Won't start

Another common symptom of pressure switch failure is that the compressor won't start.

You're using a tool, the pressure in the tank falls way below the normal "kick in" pressure, but the compressor ultimately drains completely and doesn't start.

The pressure switch is an electro-mechanical device that turns the compressor on an off as needed. It will wear out.

Unplug the compressor and have a look under the pressure switch cover for any obvious problems. Is it filthy, for example?

Clean it up, cover it, and try again. Maybe even give it a gentle tap with a screwdriver handle.

If the compressor still won't start, the plug is in, the pressure in the tank is below the "kick" in pressure (or at zero PSI gage) then it may be the pressure switch that has failed. Replace it.

If your compressor is a Speedaire, and you need parts for it, click here.







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