3.0 MPI wont shut off

[trsail59]

Hi All,
In 2014 I replaced my 1987 3.0 liter engine with a 3.0 MPI it has ran well for me for the last four years. My old engine used a 10 pin connector to the engine and the MPI engine had a 14 pin connector I bought a adapter from mercruiser and hooked it up and everything was fine. Until the other day when I was unable to turn off the motor with the key. When I disconnected the boat wiring harness from the 10 to 14 pin adapter the boat continued to run....after that I disconnected the the 10 to 14 pin adapter from the motor and the motor shuts off. When I completely remove the adapter from the boat and checked continuity between pins 5 & 6 on the 10 pin side of the adapater I read zero resistance any idea whats happening ? ...should pins 5 & 6 be connected/shorted

 

[alldodge]

should pins 5 & 6 be connected/shorted

No, Pin 6 is fused battery power, and pin 5 is ignition, so with the short, the motor is hot wired

 

[Scott Danforth]

is there a diode in the harness at the alternator on the alternator excite wire. it may have blown.

 

[trsail59]

Thank you both for your replies!

It would be great if it was as simple as a shorted adapter...and that certainly explains why it's operating in this manner...it;s strange the plug looks fine and does'nt show any damage or corrosion and it was working fine... but that seems pretty logical and a likely suspect...

I will look for this diode at the alternator...how do I identify the alternator "excite wire"? ....sorry not familiar with that term...
Unfortunately I don't have a wiring schematic to logic through why this diode might cause the problem but I will check it out and let you know what I find... I have noticed that my alternator drive belt has suspiciously loosened itself twice (maybe stretched) fairly recently. I wonder if it's working under an abnormal high load maybe due to the issue you mentioned...

Thanks again to you both for your help...I am in the club of Grumpy old guys who play with boats camp!

 

[alldodge]

If the 14 to 10 pin adapter is disconnected from everything and there is still a connection between 5 and 6, then its the adapter that has an issue.

If the connection is only is only when its connected to the motor, then its the motor

Checking the same connection points on the 14 pin connector are pins A (Pink) and C (Purple)

The excite wire is purple

 

[Scott Danforth]

some alternators require an isolation diode on the excite wire (ignition wire to the alternator). this is so that they do not back-feed the ignition system and prevent turning off.

if the diode shorts out, then you would experience the symptoms you are seeing.

follow the troubleshooting that AD posted.

if there is a diode, it would be near the alternator.

 

[gm280]

Could always let it run out of fuel. Sorry, I just had to type that... :sorry:

I think the previous info is spot on...

 

[trsail59]

I purchased a new connector 10 to 14 pin and that did solve the problem the short was the issues...I also spoke with a mercruiser certified mechanic who said the same thing about potential diode problem causing the same problem...Thanks again for you help.

Cheers!
Tom