84 Merc 115 I6 rectifier

[Volphin]

The rectifier failed on my old fishing boat. It has an 84 Mercury 115 I6, with rectifier PN 8M0057404. I can't seem to find this part. Has it been superseded? If so, to what?

 

[Volphin]

Someone in the past had put the wrong reg on the engine. Proper PN is 8M0058226 for those looking in the future.

 

[Chris1956]

That Mercury rectifier is simply a full wave bridge. You can save yourself 90% of the cost of an OEM one and get a better part from an electrical supply place. Radio Shack used to carry them, but alas.

Pick out a 15A full wave bridge rectifier, and make sure to ground the -DC post to the block. The red wire goes on the +DC post, and the two yellow wires and the grey wire go on the other two AC posts.

 

[Volphin]

Aaaannnndd, we only lasted 1 trip before the tach died. I haven't checked the output yet, but I'm 99% sure we blew another. What can cause this failure?

 

[Chris1956]

Rectifiers will blow if they are disconnected from ground or the battery, and the engine is running and the stator is putting out battery charging voltage.

 

[Volphin]

All connections to the battery are tight... the boat has 2 batteries, one for starting and one for trolling. The trolling battery is removed. I do use a cut off switch on the starting battery as it was slowly draining over time... 2 weeks or so. Here's a photo of it.

 

[Chris1956]

OK, not sure what to say. Rectifiers usually do not break if they are connected properly. You can check them with an ohmmeter. Search for the procedure to test a bridge rectifier. You check each of the 4 diodes for continuity one way (on the meter), and no continuity the other way.