alternator giving battery too much volts?

[scope]

hi

my battery light is constantly on and i put a multimeter up to my battery when the car is running and when it's not to see what i get. i get the usual 12 when the car is off but when it's running i get around 18volts and i've heard that it should only be around 14volts? i heard that the battery light comes on also when the alternator is over-charging and so i've figured it's doing that and i'm wondering how i can fix that. i've changed alternators and still the same problem

thanks

[Twolitre]

What type of car? If it's an older model with an external voltage regulator then I'm guessing that's the problem and needs to be replaced.

[scope]

What type of car? If it's an older model with an external voltage regulator then I'm guessing that's the problem and needs to be replaced.

86 ke70 corolla. thought that might be it actually after a bit of reading up. will goto wreckers tomorrow