Your best not to have your from both the coach (toggle) and chassis (dash) batteries. If both are on and you draw a high load (starting engine) you could potentially overload and let the smoke out of your wires if not fused.

Today I found a small belt driven compressor with wiring and air line going up into dash through firewall. It appears to have a solenoid or control mounted on top of compressor and wired into elec. horn circuit. I assume this is strictly for horn and no sign of a tank.
The belt is missing. I need to trace wiring in the dash and find a switch, if any or try to get power to compressor. I have 2 mystery switches on right side of dash, one unmarked toggle, one original rocker marked access.

I have a neighbor with a new to him Class A and discussion led to checking out his system. We found the compressor plumbed to a tank and then though firewall with wiring that connected to horn circuit. The compressor had a wire deliberately disconnected. With key on, compressor runs with wire re-connected. Elec horn worked all the time. He had a mystery toggle and we traced the wiring to that switch. With the toggle on, air horn works from horn button with electric slight delay. His problem is the compressor doesn't want to shut off. Tank gauge reads 75 psi and hasn't moved.

Hadley compressors were common in that era, and they're poorly designed: crankcase cap doesn't want to stay on leading to dirt. They don't have a wristpin, so the piston oscillates in the cylinder. And they have a ~20% duty cycle. Get a Viair and be done with messing with the compressor. The pressure switch on most of thee systems is 110 psi cut-in, 135 cut-out.

I can't imagine any small compressor is able to provide 100 psi at the flip of a switch, without a tank.
I can imagine for recreational or occasional use there would be no need for on-board compressor just a mounted re-fillable tank with solenoid to release air for horn.
When I press horn button I hear a click (relay or solenoid?) but no elec. horn.
Is that clicking the solenoid that releases the air from a tank, like my neighbors, or a relay to engage both air and elec.?
Is there usually a switch, like my neighbors, that turns air on/off?
I can understand being able to turn it off, but then when you need it, you have to remember to turn it on before you hit horn button.
End result, I was impressed, getting his to work. If I need to use horn when driving this thing, I want them to know where it's coming from. I won't be needing beep beep anymore.

The clicking you are hearing is probably the solenoid for the electric horn. That solenoid should also feed power mto an air solenoid off the air manifold that provides air to your horn. So they should both work together. At least that is the way mine works. Both my Electric and airhorn stop working because the contacts were corroded. Cleaned them up and now they both work like a charm.

Fred & Cindy Cook
1990 32' Regency, Cummins

For a diesel with an engine air compressor, yes. Gassers have a different system.

Ok.... My bad. Did not know there was a difference.

Fred

1990 32' Regency, Cummins

Typically, gassers and diesels without air brakes, there's a separate air compressor. Mine has a 1.5 gallon accumulator tank that the compressor pumps up. Many automotive air horns have an "instant-on" compressor, but my guess is that Barths without air brakes have a tank of at least that size. Mine's in the generator compartment (front, driver's side). The PO moved the compressor to the rear for noise suppression - easier to maintain, too. I did have to replace the plastic line with copper tubing from the compressor to the tank because the plastic failed with the heat of compression. I later replaced the plastic with polyurethane from the solenoid to the horn; the bends and turns were too much to deal with for copper.