Compressor/Vacuum Pump System

Schematics and write-up by Dave Mumert


The following schematic and plumbing diagram describe how to connect the Thomas pump to do double duty as a compressor and as a vacuum booster pump.


Plumbing Diagram

Compressor operation:

The input valve is open (activated) and output valve is closed (deactivated). Air can flow freely through the open input valve into the pump. Air leaving the pump cannot pass through the closed output valve so is forced through the check valve into the air tank The vacuum is held in the vacuum tank by the check valve.

Vacuum pump mode:

The output valve is open (activated) and input valve is closed (deactivated). The pump cannot draw air through the closed input valve so is pulls it through the check valve from the vacuum tank. Air leaving the pump can flow freely through the output valve and is exhausted through the muffler. The compressed air is held in the air tank by the check valve.

Schematic 1


Compressor mode - The pressure switch closes activating the pump relay. This starts the pump and opens the input valve. The output valve remains closed.

Vacuum pump mode - The vacuum switch closes activating the changeover relay. The changeover relay activates the pump relay which activates the pump and now the output valve is opened and the input valve remains closed.

If the pump is running in compressor mode when vacuum is required the changeover relay will pull in, close the input valve and open the output valve. When vacuum is back to normal the changeover relay will drop out and the pump will revert to compressor mode. The pump does not stop during the changeover.


The wire between the 30 amp breaker and the battery is a potential fire hazard. The breaker should be mounted as close to the battery as practical. You want any shorts that occur on this wire to blow the breaker not melt the wire.

The Manual/Ignition switch can be used to power the compressor when the ignition is off. The vacuum pump has priority when the ignition is on regardless of the position of this switch.

The diagrams show normally closed valves. Normally open valves can be used as well. Just swap the valves in the schematic. The input valve will be connected to the normally open contact of the relay and the output valve will be connected to the normally closed contact.

Changeover time from compressor to vacuum pump will be equal to the drop out time for the solenoid valve. Typically .03 seconds (30 mSec).

The diodes (1N4005 or similar) across the relays are optional but do protect the contacts of the control switches from arcing and decrease spikes on the electrical system.

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