Pressure Switch & 40 Amp Relay

In this article, we will learn how a pressure switch is wired to a 40A relay for use with a 12 or 24v air compressor. If your pressure switch has the relay built-in, please see our article here.


Getting Started


40A Relay

Let's start by locating our relay. The relay is a small black box with four terminals that protrude from the bottom of the box.

40 Amp Relay
Bottom of 40 Amp Relay

Each terminal on the bottom of the relay corresponds to a number. You can reference back to these numbers when looking at a relay wiring diagram. All of our relays are laid out in the same fashion as above. If the numbers on your relay cannot be read, you can reference the following diagram for the pin layout.

40 Amp Relay Pin Layout

Pressure Switch

Let's locate the pressure switch. Your pressure switch will look like one of the following switches:

Bottle Cap Pressure Switch
Pressure Switch with Leads

Either switch will work for this guide, and the install process for both is the same. The only difference between the two pressure switches is the electrical hookup. One of these switches has two leads that are ready for use with a crimp connector, whereas the other switch uses two male prongs, that can be connected too with electrical connectors.

Wiring The Relay

This is one of the trickiest parts of the install, and rightfully so. If the relay is not wired exactly right, the compressor will never run.

Air Compressor Wiring Diagram

When looking at this diagram for the first time, it can seem a bit daunting. Let's break it down into smaller bite-size pieces.

Each number on this diagram corresponds to each pin on our relay as we mentioned above. We will be using Female Quick Disconnects to complete these connections. Each connector looks like the following:
Red Electrical Terminal

We will use this connector for terminals 85 and 86.

Pin 85

Pin 85 is a ground connection for the relay. This pin will get routed over to the frame of the vehicle as a ground. Alternatively, you can route this wire up to the negative terminal on the battery. This terminal connector will simply slide over the pin on the relay, and you may crimp an exposed lead of 18ga wire into the connector. This pin is complete. *(Heavy gauge wire is not required for this connection)

Pin 86

Pin 86 will connect to one of the leads/terminals on our pressure switch. It does not make a difference as to which lead or terminal you decide to connect this to. It only matters that this connection is made to one side of the pressure switch.

Pin 87

Pin 87 will connect to the positive terminal of our car battery. For this connection, you will be using the red (10ga) wire that came with your kit. You will also use the fuse holder here as well.

Our fuse-holder is sent to you in a loop. *Do not install a fuse into the holder yet.

Fuse Holder

Go ahead and cut the fuse holder in the center to make two ends. You can cut the wire where the dashed line is in the picture above. Using a set of wire strippers, strip each end of the holder and expose the copper leads. Each end should now look like this:

Stripped Wire

At this point, you will use the supplied ring terminal crimp connector on one side of this fuse holder. The opposite end of the fuse holder will accept one of the yellow butt connectors included with the electrical connectors. The two connectors are pictured below.

Yellow Ring Terminal

Yellow Butt Connector

The ring terminal can be used to connect the fuse holder to your positive battery terminal. The yellow butt connector will allow us to connect the fuse holder to our red wire, and complete the connection back to the pressure switch. Go ahead and strip one end of the supplied red wire, and connect it into the opposite end of the yellow butt connector. Route your red wire from the fuse holder back to the pressure switch. Do not cut your red wire until you have finished routing it back to the pressure switch. You want to make sure you don't cut it too short.

Once the red wire has been routed back to the pressure switch, you can cut off any excess wire while still leaving a small amount of slack. Now, locate the neon green terminal connector in your electrical kit. 

Yellow Electrical Terminal

This connector will allow you to connect the 10ga red wire to the relay. Slide the connector over pin 87, and crimp the exposed red wire in the connector.

Pin 30

Pin 30 is the easiest connection to complete with this setup. Take the red lead from the compressor and connect it directly to pin 87 on your relay. This should be the last connection needed for the relay.

Finishing up the Pressure Switch

Now that our relay is wired up, you are now left with one side of the pressure switch to complete.

You will connect this side of the pressure switch to a key-power-on source on the vehicle so that the compressor can only run when the vehicle is on. If you connect this wire to the battery, the compressor could run overnight and drain your battery.

Start by locating the supplied roll of blue or grey wire (doesn't matter which you use). Using the supplied red female quick disconnect, attach a lead of blue/grey wire to the prong on the pressure switch.

From there, you will need to locate a good power source for this wire. This will differ from vehicle to vehicle. You will want to locate an accessory power wire that is only powered when the vehicle is on. If you need help locating a good source we recommend checking google for wiring schematics for your vehicle.

Below is a list of common sources that typically only come on when the key comes on. Tapping into the power wire on any of these sources using the provided red T-tap in the wiring kit, will give you the positive key power on you need for the system to only run when the vehicle is on.

Red T-tape
  • Radio
  • Amplifier (if applicable)
  • USB Power Outlet
  • Cigarette Lighter
  • Daytime Running Lights
  • Trailer Lights
  • Heated Seat(s)
  • Sunroof
  • Power Windows