Auto electrics - where am I going wrong?

[aqua_serve]

Guys,

Wondering if any of you with auto electrical knowledge could tell me where I’m going wrong?

I’m fitting an air horn to my van, nothing special just a compressor and horn from ebay.

I’m not following the instructions with the horn as this installation includes using a relay to switch from standard vehicle to air horn, I’m simplifying it by just having a button on my dashboard that will operate the air horn direct.

I’ve wired it up, but even more basically to test the compressor (if that’s what it’s called). Simply a feed from the battery protected by a 15 amp fuse. I’ve used a multimeter to test the voltage prior to connecting the wiring to the terminals on the unit, I get 12v approx. However when I then connect the unit up and measure the voltage I get nothing. The fuse hasn’t blown and there is nothing from the compressor at all. Disconnecting the wiring I immediately get a 12v reading back on the meter.

At first I thought the unit was fault and got it exchanged so I know this isn’t the issue.

Can any of you suggest my mistake?

Mark