I just installed an isolator and I am getting the exact same voltage drop as you (13.9-14v). The excitation terminal on the isolator is supposed to be hooked up to the sense wire on your alternator, which tells the alternator to output more voltage to compensate. It is okay to leave the excitation terminal un-connected as long as the voltage does not drop too low. Someone correct me if I am wrong, but I dont think all alternators have this sense terminal because I sure couldnt find one on my vehicle. I think 13.9v should be safe. It wont charge your battery up as fast as 14.8v, but it will charge it and keep it topped up.