In this article, you will learn how a pressure switch is wired to a pair of 40A relays for use with a 12 or 24V set of air compressors.
Pin 30: fused 10ga red wire from battery positive — one dedicated run per compressor
Pin 87: compressor red positive lead (each relay feeds its own compressor)
Pin 85: 18ga ground to chassis or battery negative (both relays can share one ground)
Pin 86: pressure switch trigger — jump pin 86 between both relays
24V systems: If you are using a 24V air compressor, you will need a 24V relay. These pressure switches will work with 12 or 24V, but the relay must be one or the other.
Getting Started
This article assumes that your air compressors and tank have already been mounted and assembled.
40A Relays — Start by locating the relays. The relay is a small black box with four terminals that protrude from the bottom of the box.
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. If the numbers on your relay cannot be read, you can reference the pin-out diagram.
1
Use the pin-out diagram if needed
If the molded numbers on your relay cannot be read, use this diagram for the pin out.
Relay pin layout — use this if the molded numbers are hard to read.
Pressure Switch
Locate your pressure switch. Your pressure switch will look like one of the following switches:
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 with two 1/4" spade terminal connectors.
Wiring The Relays
This is one of the trickiest parts of the install, and rightfully so. If the relays are not wired exactly right, the compressors will never run.
2
Review the 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.
Dual-compressor relay wiring overview.
3
Gather the female quick disconnects
We will be using female quick disconnects to complete these connections. You will use this connector for terminals 85 and 86 on each relay.
4
Wire pin 85 to ground
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. Both relays can be grounded to the same point. This makes this process a little easier if you mount the relays side by side.
Heavy gauge wire is not required for this connection.
5
Wire pin 86 to the pressure switch
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.
You may use a small section of wire to connect pin 86 on both relays together. Alternatively, run two leads from one side of the pressure switch to pin 86 on both relays.
Heavy gauge wire is not required for this connection.
6
Prepare the fused battery feed for pin 30
Pin 30 will connect to the positive terminal of the car battery. For this connection, we will be using the red (10ga) wire that came with your kit. We 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.
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.
7
Strip the fuse holder ends
Using a set of wire strippers, strip each end of the holder and expose the copper leads.
8
Add the ring terminal and butt connector
At this point, we 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 ring terminal connector 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 relay. 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 relay. Do not cut your red wire until you have finished routing it — you want to make sure you don’t cut it too short.
Once the red wire has been routed back to the relay, you can cut off any excess wire while still leaving a small amount of slack.
9
Connect the 10ga red wire to pin 30
Now, locate the neon green terminal connector in your electrical kit. This connector will allow you to connect the 10ga red wire to the relay. Slide the connector over pin 30, and crimp the exposed red wire into the connector.
10
Connect the compressor to pin 87
Pin 87 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.
Repeat the steps above for the 2nd relay. You must run a lead of 10ga wire from the battery for each compressor. One lead of 10ga cannot handle the power draw of both compressors at once.
Finishing Up The Pressure Switch
Now that the relays are wired up, you are now left with one side of the pressure switch to complete. We require this side of the pressure switch to be connected 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.
11
Attach keyed power to the remaining switch terminal
Start by locating the supplied roll of blue or grey wire (doesn’t matter which you use). Using the supplied red female quick disconnect, we can 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:
Radio
Amplifier (if applicable)
USB Power Outlet
Cigarette Lighter
Daytime Running Lights
Trailer Lights
Heated Seat(s)
Sunroof
Power Windows
Alternatively, you can use the supplied T-tap connector to tap into a circuit that is only on when the key is on. Take a look at the image that shows how to use this connector.
Next
How to turn your horns on/off at the steering wheel using an upfitter switch — wiring a 5-pin SPDT relay so you can toggle between the factory horn and train horns.
Skip to content
