Shut off switch will not stay up on a 40400 compressor?

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The pressure switch on this Central Pneumatic air compressor has a ON/OFF switch. It toggles up and down to either shut off power, or allow power to flow from the switch to the motor on this compressor. What’s the problem when the shut off switch will not stay up on a 40400 compressor?

First off, if you are looking for complete compressor info, this compressor has been sold by Harbor Freight under part numbers 68740, 95386, 40400, 67501, 69667, 69089 though we don’t know if the configuration of all parts on each model is identical.

Central Pneumatic 40400 air compressor
Central Pneumatic 40400 air compressor

We were not able to scare up a decent photo of the pressure switch on this model. All the photos we found were the view shown above, and the pressure switch was neatly hidden.

A browser search for the correct pressure switch for the Central Pneumatic 40400 turned up the photo shown further along this page.

The use of the On/Off switch on a compressor means that the user doesn’t have to rely on the pressure switch to keep the compressor shut off when the tank is full. If only the pressure switch is used to control the compressor On/Off, in the event the compressor fills with air and shuts down, and there is a leak anywhere, in time the compressor will start again on its own, which may be undesireable.

The button on this pressure switch means that it is unnecessary to pull the plug to ensure the compressor stays off.

Pressure switch for Central Pneumatic 40400 air compressor
Pressure switch for Central Pneumatic 40400 air compressor

The issue has to do with the red button seen in the photo above. That button has to be pulled up for power to flow to the compressor motor, and pushed down for the power to be cut. This starts and stops the compressor.

That button on this pressure switch is “detented”, which means that there is a device inside the pressure switch cap that holds the button in the desired position.

If that detent device fails, and the detent may be as simple as a “bulge” in one side of the switch shaft that pushes against the housing to hold the switch position, the button will not stay up, and the compressor shuts off even if the pressure in the tank is not at the desired pressure.

It also could be caused by the spring(s) on the “points” inside a compressor pressure switch. A spring may have failed, and the detent isn’t strong enough to hold the switch up against the pull of the spring.

A browser search will prove to the searcher that fixing a pressure switch is no easy task. Often, when disassembled, springs and parts all come loose, and knowing how to put them back in order is beyond the patience of many.

A browser search for parts for this switch will quickly indicated that there are virtually no parts available.

This particular switch is available on line and found to be fairly inexpensive. Therefore, the easiest “fix”  when the shut off switch button will not stay up on a 40400 compressor is to acquire another and replace the failed one.

By Ashley Pearce

As a passionate manufacturing and mechanical engineer, I've had my fair share of run ins with air compressors and compressed air systems. With over a decade of experience in the industry, I have both a fresh perspective and time-served hands and mind to help you with your compressor problems (along with our able community!)

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