I have a 1965 MERC 900 - 90 hp and does not seem to be charging.
was told to test the DIODE STYLE RECTIFIER first.
Does this sound right, and how should I test it.

On 13 Jul 2006 16:52:15 -0700, "Bob.W" wrote:

I have a 1965 MERC 900 - 90 hp and does not seem to be charging.
was told to test the DIODE STYLE RECTIFIER first.
Does this sound right, and how should I test it.

It's worth a look. If you have an multi-meter/ohmmeter you can check
for open or shorted diodes. Good diodes only conduct in one polarity.
Assuming a full wave diode bridge (Clams, any one...?) it will have 4
terminals; 2 AC, +, and -.

Ohmmeter positive to either AC terminal and ohmmeter negative to the +
terminal should conduct.

Ohmmeter negative to either AC terminal and ohmmeter positive to the -
terminal should conduct.

Any other connection should not conduct.

Some multi-meters have a diode check setting that will measure the
voltage drop for conduction (~.6 -.7 V) and read "open" or full scale
for no conduction.

(fixed width font)

~AC o---+--||--+--o + DC
| |
+-)--||--+
| |
| +-||--+
| |
~AC o-+---||--+---o - DC

Just replace it... unless the stator wires are totally rotten and shorting -
it's GOT to be the rectifier.

-W

"Bob.W" wrote in message
oups.com...
I have a 1965 MERC 900 - 90 hp and does not seem to be charging.
was told to test the DIODE STYLE RECTIFIER first.
Does this sound right, and how should I test it.

wrote in message
...

The stator can be bad with no outward indication. The charge winding
is not the same as the ignition windings so the engine could be
running fine, just not charging the battery. As I said up thread,
hooking up the battery backward will blow the rectifier or the stator
or both.

What ignition winding there Mr Wizzard?

Note the motor is a 1965 (R.I.F.) , it only HAS a charge winding and it's so
heavy a guage, that it's very, very likely to be fine. Hooking up backwards
would only blow the rectifier - on that outboard.

When's the last time you worked on a 1965 Merc???

-W

wrote in message

When's the last time you worked on a 1965 Merc???

-W

but it is timed by a fixed coil that sees a moving magnet on the crank
shaft. Maybe you don't call that a stator.
Sorry if I caused unnecessary confusion.

We already discussed how to test the difference between a bad stator
and the rectifier. You would hope the rectifier went bad first but it
may not always be the case.

sigh It is *not* even timed by a moving magnet on the crank (we call that
a "trigger" by the way). It still uses "breaker points" - remember those??
You must be thinking of 1980's motors to get a trigger at the crankshaft.

And I don't "hope" the rectifier went bad first, I'm 99.9% sure it did based
on the size of the stator windings vs the PIV rating of the diodes. Do the
math. Yes, you can test the rectifier.. but if the guy don't own an
ohm-meter... it's cheaper to buy a rectifier than an ohm-meter. Particularly
when that's the most likely problem. (assuming the stator leads ar OK).

Presume that when I bother to speak in here re. anciant Mercury that I'm
speaking *ex cathedra*, not opining or guessing.

-W

wrote in message
...

BTW I did blow a stator with a battery reversal on my 89 so I know it
is possible. PIV has nothing to do with it. Peak current is what blows
the stator since the diodes are forward biased on a battery reversal.
(current flows freely)

Yes, the charge windings on the late stators compete with space for the
ignition windings. They are much smaller guage than the early stators which
are ALL charging only.

Correct on the PIV... I mis-spoke, I should have said current.
The early stators can sink enough current to guarantee the diodes will open
up 1st, long before the windings even feel the pain.

-W