Compressor shut off switch(?)

Sorry for multiple threads but the search function isn’t working on my smart (or is it dumb?)phone. Is it strictly necessary to have a manual on/off swich for the compressor? I had planned on using an ignition activated wire to power a relay for the compressor. No key in ignition=no compressor running. In case of an emergency I could pull the fuse. I have the upfitter switches in my truck and they are already spoken for so I don’t have a free one to use for a compressor off switch. HB diagram show ignition switched feed. Any thoughts or comments?

It’s best to hook the trigger wire to an ignition source just like you said. No key = no compressor.

But it’s up to you if you put an inline switch on that wire. It would be a master cutoff switch.

I’m drawing up a wiring diagram now so I can use it to make a shopping list. Hadn’t planned on a master cut off switch. Was curious if anyone does use one.

I have a master cut off switch . it’s nice to have in case let’s say you blow an airline or maybe your battery is getting weak as you’re trying to star t your vehicle you may need all the extra battery power you can get .that’s just my 2 cents on that subject.

Yeah, if you want the ability to start your vehicle in the morning. Even a leak could cause the compressor to run enough and drain the battery. A master switch is recommended.

My first two installs didn’t use a master cut-off and I was OK with just using the ignition key to turn it off. Then one day whilst on the highway the air line leading to the front somehow dislodged and got too close to the hot exhaust pipe. All of a sudden… Pfaaff! All my air went bye bye and naturally the compressors started running continuously. I couldn’t pull over for a while and because it was linked to the ignition I couldn’t easily shut off the system. Eventually just pulled over and yanked the main fuse leading to the compressors, but by that stage the compressors were screaming hot.

After that incident I decided to always put in a separate cut-off. It’s cheap and easy to do and it just gives you that added peace of mind. Also, if you’re ever interrogated by the cops then you’ll find it useful to be able to quickly shut down the compressor as well. :wink:

It is possible to put a master cut switch to the compressor as they have said. What I would suggest is applying the switch in between the pressure switch and the ignition but still running the other end to the ignition. Reason being, if you forget to kill that switch after parking your vehicle, if there is a leak for instance, your compressor won’t turn on unless the ignition is on. We recommend to never have the compressor on while the vehicle is off, if you can help it.

There is much wisdom in what you say Jedi Masters, as it is wise to always have an emergency back up plan. However, this padawan learner faces a quandary of galactic proportions. My vehicle has the factory installed upfitter switches. Switches one and two are for exterior driving lights, so that I may bring light unto the darkside. Switch three will be ignition activated and alternate between powering the compressors, and activating electric tank drains. Switch four will select either the factory horn or the air horn. Switch five does not exist. So I have no available switch to use to use as a master switch. I must meditate and ponder the situation and see if I can determine a favorable solution.

Post up some pics of your setup, master. This young Jedi has an '08 f250 and just finished hanging my compressor and tank this evening. Gonna try to mount the horns and wire it all up tmrw. Was wondering where I should tap into my upfitters. None of them are being used yet.

I can’t get the link to post. If you google q-117-r1.pdf you will get the Ford sve bulletin on the upfitter switches. It will show you where the wires are under you dash to connect to. Right next to the upfitter wires are some other blunt cut wires, four of which are pass through wires to the engine compartment. You can see the other end under the hood, drivers side, kind of behind the fuse box near the firewall. There are four 16 guage wire ends there taped to a harness section. One is white, one is white/orange, one is brown/white, last one yellow/white maybe, goinv from memory.

So if I understand correctly, do I wire the blunt cut wires inside the cab up to each other and I will then wire the compressor ignition switch to the end of the pass thru under the hood? This is the part that confuses me.

I think I got it figured out after I got up under the dash. I used the brown wire from the upfitter 4 and butt connected it to the orange and white wire, then connected the orange and white on the engine side to my ignition source input on the pressure switch.

Hmm… I think you may be going slightly astray there. Connecting the brown wire to some unknown orange/white could be dangerous. The brown output will have live power when the key is int the RUN position, but it’s only rated to 15A.

I’ve had a look at the Ford diagram and have come up with some ideas for you (see attached). Unfortunately upfitter relays 1 & 2 are already used by auxilliary lights from what you’ve posted. They would have been OK to power your compressor since they’re the only two rated at 30A.

Never mind… here’s what I thought:

  • Upfitter 1 : Already Used
  • Upfitter 2 : Already Used
  • Upfitter 3 : Electronic Tank Drain
  • Upfitter 4 : Compressor Master & Horn Select

You already have ignition cut-off built into the whole design of the upfitter switch array since none of the switches will be active unless the key is in the RUN position.

Take a dual splice from switch 4 (White/Blue) then run that cable to the pressure switch, then to the trigger pole of your compressor relay. This means that your compressor will not engage unless both the ignition is in RUN , AND Upfitter 4 is engaged.

I strongly advise running a separate relay and power line that is fused to a minimum of 30A but make sure it’s according to your compressor model.

Now take the other splice from switch 4 (White/Blue) then run that cable to the trigger of a Single Pole Double Throw (SPDT) Relay. These are basically like an electronic toggle switch. Tap into your horn wire (easiest to find right near the horn) and run that cable to the SPDT relay, then back out to the stock horn (on the default ON pin) and the other outward line (Energized ON pin) to the relay for your horns.

This would mean that if Upfitter 4 switch is turned on AND your ignition is in the RUN position, your compressor is online and the train horns are active through your steering wheel horn button. With the switch in the OFF position, your standard horns would be selected.

Upfitter 3 can just be wired to your electronic tank drain solenoid.
Hope that helps. Let me know if anything is unclear or doesn’t make sense.

[Edit] I should add that upfitter relay 4 (Brown wire) remains essentially available for another purpose (up to 15A) if you have something that can be active at the same time as the horns/compressor. Alternately, if the Blue/White trigger is too difficult to tap into under the dash, then just use the brown. It’s a waste of a relay circuit, but if you can’t get to the blue/white and you don’t have any other use for the switch then you can just do that.

What I did was took a 10g power source from the pressure switch directly to the battery. This is what supplies the power to the compressor. Then I took the blue (ignition source) wire and ran that into the engine bay and tapped into an orange/white blunt cut wire. These are the pass thru wires that are blunt cut on both sides of the firewall, they are not live. Then I used the brown wire from upfitter 4 and connected that to the end of the orange white that is inside the truck, thus completing the circuit between upfitter 4 and the pressure switch, via the blue ignition source wire. Tested it and it works fine. Only provides power to the compressor with the truck on and the upfitter switch on. Now I just need to figure out how I’m going to run the power/ground to the air valve, because my truck uses a constant 12v power and a ground switch for the factory horn.

Yup… should be all good. Yes, you’ve done the right thing essentially and exactly as I described in the diagram. You’re probably just seeing it different because the Hornblaster switch and relay is molded into the one unit (see first pic attached). You’ve just used the brown as the trigger (as per my last point).

Just make sure you’re running a fuse on your main (Red) wire between the battery and the switch.

With the toggle for your horn, you can tap into the brown again and run that to an SPDT relay. It doesn’t matter that your horn circuit runs negative instead of positive wires. Just swap the wiring around (2nd diagram attached; [edit] Apologies, slight cock-up in the schematic there. Make sure you’re earth on your trigger is not linked to the horn relay of course - updated drawing attached).

Sorry I’m not good with earth and relays and stuff. My goal here is to be able to switch the horn off from a toggle in the event I only want to use my stock horn. I also want to be able to wire my alarm to give me the ability to honk remotely. I’m sure that involves a separate wiring schematic in itself. But that is my end result.

I was thinking I would splice into the ignition source wire to use the power from that to go to the air valve since it only needs constant 12v and then just wire the ground from the valve into the horn output ground on the front of the truck.

Have I lost you yet, because I think I lost myself.

More ways than one to skin a cat as they say. What you’re proposing should work just fine. Just means both air and electric horns will sound together. If you want to separate them (one or the the other) then you’ll have to opt for the relay I’m afraid.

Ah ok, now I understand the method to your madness. Isolating the stock and trains…

So I’ve got a question because I’m a little confused. I took the plug off the back of the factory horns, and tested it with a multi meter. I put the positive probe on either female end of the plug coming from the cab and the negative probe on the frame. I couldn’t get positive power on either port of the plug. Does that sound right? If that’s the case then does the horn switch only send positive power when the horn is depressed? My understanding was positive power constant and negative ground was switched.

My factory horns have a blue/white (I think) and a black wire at the horn location. The blue/white gets energized when you press the horn button.