I have a '91 W250 4x4 (see sig). I recently replaced both the alternator and voltage regulator in search of a solution to an electrical problem I'm having, however now when I measure the voltage across the battery with a voltmeter at runtime I'm seeing 16-18V. This never happened in the past, I usually had a reading from 13-15V. What could be happening? Won't this cook my battery?

 

You should see 14-15 volts with a good electrical system. The high reading is indicative of either sulphated batter(ies), or a bad cable connection at one or more ends of either the ground or positive battery cables.

A common problem is the terminal end corroding between the wire and the terminal inside the the lead where you cannot see the corrosion.

 

Yeap you got a problem. Don't drive it or you will start to blow fuses and burn up motors. It is probably the regulator. I had to fix a truck one time that the internal regulator went out(customers). Needless to say, it was expensive. You should be around 13.8v to 14.5v. Check your grounds also.