Electrical weirdness. Here's my Multimeter test results

[DJ_Allatoona]

1986 Mercury 75HP. Rectifier, but no voltage regulator. Been seeing a 'high voltage' warning on my fishfinder, and experiencing occasional stereo shutoffs when throttling down. Voltage on the Garmin reads 16.5V and higher.

Today I directly metered the battery with and without the motor running. Results were interesting. Please watch this and tell me if it's normal for the meter to go bonkers like this while just idling: YouTube Video of Battery/Charging multimeter test

 

[gm280]

Without being there and taking measurements, I would say you may have a rectifier that is bad. Looks like one of the diodes, or more, are bad and allowing AC to come through to the meter, mixed with the DC. I would take the rectifier and do a really good test on it to verify that idea. JMHO without more info.

 

[DJ_Allatoona]

I would say you may have a rectifier that is bad.

OK, I have a spare rectifier. I'll throw it on and see what happens. There's no voltage regulator on this motor, and I'm considering adding an aftermarket rectifier/regulator if I can't get my electrics under control. I'm confident my battery is good: it's a Champion Marine (27DC-850CH), about a year old and it tested good at Pep Boys in January.

 

[Chris1956]

Rectifiers are easy to test with an ohmmeter. They should have continuity (zero resistance) one way, and no continuity (infinite resistance) the other way.
Assuming you have a 3 wire Mercury rectifier, it is really a 4 wire standard bridge rectifier where the 4th wire (-DC) is ground. Measure across adjacent connections on the bridge rectifier. That is an AC post to the closest DC post. There are lost of videos on this test.

 

[gm280]

The fact that you are getting higher voltage then usual, speaks to a diode shorted and allowing straight AC spikes through. The would certainly account for both higher then usual voltage AND erratic voltage readings. JMHO

May have other results if I were there and could test things myself. Hard to do give accurate results and/or reasons via a mere video...

 

[electricjohn]

Your voltage does seem high. Mercury relies on the battery for voltage regulation on their low, 9 amp, charging systems. My voltage has always been high too. 1988-90hp. Replaced my diode for the first time last season.

 

[DJ_Allatoona]

Appreciate all responses. Will R&R the rectifier in coming days and report my findings.