79 40hp 2 cyl - no longer charging my battery

[JKTrevecca]

So I just noticed today that the voltage on my fish finder is no longer increasing when my outboard is running at WOT like it used to. I have a 1979 Merc 40hp electric start. With a fully charged battery, my voltage used to rise from 12.6ish up to 14-15 volts when running at WOT immediately after putting my boat in the water. Today, it stayed at 12.6ish and even slowly decreased down to 12.4 even when running at WOT.

Any advice on what to test and how to test it? I have a voltage/ohm meter and I don't want to rush out and buy a part that I don't need. I'd rather pinpoint diagnose why it seems to no longer be charging. Thanks in advance!

-- Jason

 

[carholme]

Re: 79 40hp 2 cyl - no longer charging my battery

It sounds like your rectifier may have packed it in. You have not included your engine s/n but if you are sure that it is a 1979, here is the parts list:

MERC 40 - Mercury, Mercruiser, Force, etc. Parts Lookup

See the rectifier as item 1 under STARTER MOTOR, RECTIFIER AND WIRING HARNESS.

Here is a link to the rectifier test which you can download and enlarge on your screen:

https://imageshack.com/i/5n9kp1j

Gerry

 

[bilge rat jim]

Re: 79 40hp 2 cyl - no longer charging my battery

Here is a more readable version of the rectifier test that Gerry supplied:

Mercury Rectifier Testing steps with pictures

If you do have to replace your rectifier, consider using CDIs aftermarket model #154 6770. Every bit as reliable as OEM, and 15 buck cheaper.

 

[JKTrevecca]

Re: 79 40hp 2 cyl - no longer charging my battery

Carholme and Jim - THANK YOU! I'll test the rectifier after church today.