Help with relay wiring Viair 20005

Installation Help & Advice Horn Kits
#1

Guys…

I have a Viair 20005 combo that has the pressure switch already mounted on the compressor.

I have the relay which calls for the following:

Wire to 12v constant: (I have a fused wire on battery)
Wire to 12v switched: (I have to underhood fuse box, switched fuse)
Wire to compressor:
Wire to pressure switch:

My question is, I have the Viair relay. But since the pressure switch is already mounted on the tank, I only have 1 wire going to the compressor/pressure switch and the relay calls for 2 separate. There’s nothing in the Viair manual about how to wire a relay in with this combo system.

I tried the wire on both contacts on the relay, but it doesn’t kick the compressor on. When I take the wire going to the compressor and touch the battery, the compressor kicks on.

Do I only need a 3 contact relay?

Thanks for the help…I didn’t realize this forum existed until today.

#2

Doesn’t look like that Viair combo needs a relay (because of the small compressor). The pressure switch handles the electrical load.

You use a relay when you have a normal switch that cannot take the electrical load (i.e. amps) that’s needed to be switched. In those cases the relay can be linked to your pressure switch to turn the compressor on and off.

Have a look at post #8 for details on how to wire a simple relay circuit on a kit like yours in order to make it switch on/off with a relay that is only powered on when you have your ignition key turned to the ACC position:
https://www.trainhornforums.com/showthread.php?t=6366&highlight=SPST+relay

#3

Thanks much for the info!

I was reading the manual and it said they “highly recommend” using a relay, but didn’t include a wiring diagram. I’m assuming because it pulls 20 amps.

My last smaller system, I just had a switch, but wanted to do this correctly.

I’ll take a look at the link you sent for the relay.

Again, I appreciate the help.

#4

No worries. If you want to take the load off the pressure switch, just run the main power wire through the relay and direct to the compressor (i.e. do not run it through the pressure switch). You would then use your pressure switch on your trigger wire instead to to make the relay turn on/off.