Air Conditioner pressure switch problem

I just had a new air conditioning unit installed in my 1990
Westfalia
and I was told by the mechanic he couldn't get the pressure switch part
for the VW any more and had to use two different switches instead. One
was a GM part for the high side and a Ford part for the low side
pressure. The low switch comes on after 116 bls of pressure. Now when I
turn my air on it seems as though the high speed fan comes on after 15
or 20 seconds and stays on. (It is noisy). The Air is working fine but
the fan has never been that noisy before.
Does anyone know of a solution to this problem. I don't think I can
stand the noise every time I put the air on.

Thanks for any input

Bruce N
Ormond Beach, FL