Clarification on 87a Terminal of Relay

I am installing a set of air horns on a truck and I would like to confirm a connection on the relay please. This air horn set comes with two compressors driving one trumpet each. The instructions are not very detailed, but the box does show a diagram which instructs to connect the relay as I have drawn in the image below. My concern is: will both compressors work simultaneously with one compressor connected to 87 and the other connected to 87A? I have never used the 87a connection before and my understanding is that 87 or 87a alternate (I could be wrong). If the way they have instructed correct, I would like to know for my understanding what the advantage is of using 87a connection opposed to connecting both compressors to 87 terminal. Could someone please clarify this for me?

That diagram would have compressor on 87a running with the relay open (no closing of the switch on terminal 86) and the compressor on 87 running when closing the switch (turning off 87a compressor) Like you suggest, both compressors should be connected to terminal 87. 87a is the NC or normally closed terminal. It is pretty well defined in the relay section of the site. Relay Section Also, be sure you fuse according to wire size used.

Thanks for your clarification.

Some "5-pin" relays are dual-output types with two 87 terminals. (Warning - some relays like Hansa have them incorrectly labelled as 87 & 87a.)

With two connected terminals they'd both be 87, with isolated, 87b.

If that's in response to my comment, I agree, but I'm just warning... Not that it bothers me - I never paid attention to the numbering - just their housing diagram.

I test them naah.

Just to add, I tested terminals 30 and 87a with an ohmeter and it is a closed connection, 87 and 30 is open. Below is an image of the drawing on the relay itself. So if I were to connect both compressors to terminal 87 I have to make sure I am not overloading the relay. I dont know the current draw of these compressors, trying to find this info from the ebay seller.

About 15 amps each your near the limit for a single relay. A single 4 terminal relay rated at 40amps would be a better bet. Unless of course this is the relay they gave you.