I looked at something on twin horns I had (not air horns) the adjustment was a screw at the back to adjust the sound diaphragm to the pitch wanted but too far higher the horn failed to sound. Air horns was mentioned that adjustment was achieved by turning the horn tube in or out for different pitch again too higher pitch shut the horn off. You might end up having to change them depends how old and if you are right with the relay, it might be cheaper option to buy new set. I would fit them to my Peugeot partner but 'er in doors' she who must be obeyed, wont ave it.
Bob, May 2013
