How do I check to ensure that a voltage regulator is working properly on a 170hp mercruiser?

Question

The voltmeter will often spike at 16 volts with no apparent change in rpm. After a few minutes it will settle down to about 13 volts.

Answer 1

Good luck with this one.  The 170 four cylinder Mercs used a water cooled regulator and the windings for the alternator are under the front crank pulley.  Believe it or not!  They are notorious for working fine until they don't.  When they need to be repaired parts are expensive IF available at all.  The good news is that there are a couple of companies out there that offer a kit to adapt a regular style belt driver alternator to your engine.  Search the web or consult a decent marina.  Be sure to use a marine rated alternator, not one for a car.  The Coast Guard has to approve the alternator for use in an enclosed motor compartment.  The difference is that the marine alternator is sealed so it can't cause built up fuel vapors to explode.  That's why you should run the blower before starting the engine by the way.

Answer 2

Your alternator is probably putting out juice to recharge the battery when you see the volts high.  It will show less volts as the battery draws less juice and the alternator puts out less.  I wouldn't worry about it unless you see it less than 12 volts or very high.  It just sounds normal to me.  Your alternator is just coming on and then off.

Answer 3

check the battery/batteries for specific gravity.  very important.  if okay, r/r the regulator.  oh, and check out your problem with a regular voltmeter, not the one on the dash.  dash units are NOT reliable.