On Jul 5, 9:12 pm, wrote:

Most likely the alternator/voltage regulator, usually the same unit
these days. Many NAPA auto parts stores can test it for you,
otherwise find a good local alternator shop. Most alternators can be
rebuilt for a reasonable price.

Just be sure the alternator guy understands this is a marine
alternator and you need all those funnyy screens he has never seen
before. Make sure he puts them all back. Other than that it is a
regular alternator.

Er, almost. a 1970 model. Mercruiser? OMC?

Anyhow, look for a blown fuse first. then....

If OMC, it probably has an ancient Prestolite, which is way difficult
to get parts for, but usually don't go bad no matter how many hr.s are
on them, that is, unless the berings get rattly. Either that or if
Mercruiser, it probably has an old 10 DN Delco (seperate VR). or maybe
even a Motorcraft (Ford)

These alternators (if external regulated) is easy to test. Make sure
you have a good quality connection to the output stud on the back of
the alternator (Usually 10 guage wire) It should be hot full time. It
"usually"connects directly to the battery cable at the starter
solonoid.

Make sure you have good current there. and the nut isn't corroded.
Either alternator, disconnect the "field" , or "brush" wire. It should
be marked with an "F" or "Fld" right on the alternator housing. then
when running, use a jumper wire to the Battery Pos. and to the "Field"
terminal, thus by-passing the volt reg. with a volt meter, the voltage
should start to climb 15- up to 17 volts on the guage. That is if the
alternator is working correctly. It it is? then Volt reg. time, or
look for rusty connections and corrosion. clean
terminals,connections, rusty mount etc. ... plug fld wire back on
alternator and try again. If no go, replace the VR.

If internal VR, it's probably a Delco 10 SI but they didn't start
using those till after 1973 or 75, I can't remember, but it's
doubtful it would have one of those unless it's been retrofitted.
Prestolite, (OMC) started mounting a VR directly to the back of their
units in about 1975

Thats really basic, Unless I know what you have I can't do much more.