Where does the 350 Mag MPI wiring harness plug in?

[OnagaCat]

I have an '03 350 Mag MPI Merc with Alpha 1. I am trying to diagnose an intermittent "beep/stumble" condition. I tried to follow the wiring harness to the cannon plug on the engine yesterday to try wiggling it but the harness goes behind the engine and since the boat has captains call the space is limited. Exactly where on the engine does the harness plug in and is the plug secured by a screw or can I just pull it out and put back in easily?

[OnagaCat]

OK, I've figured out it's in the back of the engine - of course that's pretty cramped in my boat! What is the best anti-corrosive grease to use on the pins of the plug?

[fat fanny]

Dialectric grease available at about every auto supply store nationwide! or white lithium. By the way what is a beep stumble? Does your rig have a port for fault codes? Not real familiar with late model marine apps but that would be your best bet. Good luck post back with diagnosis.

[OnagaCat]

At cruise (2800 RPM) the alarm will beep for a second and the engine will lose RPM's for a second and then recover back to 2800 RPM. This is intermittent. I have had trouble with this engine ('03 350 Mag MPI Alpha on a Crownline) for 4 years now. I was getting a lot of false codes and it turned out my ECU was bad as outlined in a Merc service bulletin. I was getting double beeps a lot and the ECU was seeing a false indication from the shift interrupt switch and shutting the engine down. Merc was no help as the engine is out of warranty but with the expert help of Eric at Simon Motorsports a good ECU will now be in place. I do still have the intermittent beep/stumble but that is not caused by a bad ECU but most likely is a corroded or loose connection in the wiring harness. So now I need to make sure the cannon plug connection is good. I have already checked the plug at the instrument panel and that helped a lot already.

[Pete104]

There is something wrong BIGGER than what your tech is finding.
There was an issue with the shift interupter switch. A simple wiring issue. It did cause the "guardian" mode to happen.

You can put new ECM's or PCM's in this thing til the cows come home & nothing is going to change. Until, somebody fixes it! Merc wanted nothing to do with it because somebody has already looked at it under warranty, AND it's still running!!! It can't be anything serious! Have your guy turn it up a little on the interupter switch!

[OnagaCat]

The interrupter switch was the first thing that I replaced. Then when that didn't work a dummy shorted interrupter switch was installed per Merc service bulletin 2003-06. That solved the problem so a known good ECM was tried and solved the problem except for the beep/stumble which is a separate issue unrelated to the bad ECM. What made this hard to diagnose at first is that I had both issues occurring at the same time. Now that the interrupt switch/ECU thing is resolved I just need to find where the connection is bad which will fix the rest. Relevant part of the service bulletin below:


3. MCM V6 and V8 MPI Alpha engines only: If you have an Alpha engine with an
engine miss-fire or if the engine shuts off above 1200 rpm, the problem may be
caused by the shift interrupt switch. To determine if the interrupt switch is the cause,
disconnect the switch from the engine wiring harness. Use a small jumper wire to go
between the A and C terminals in the engine harness connector.

If the jumper wire corrects the engine running condition, a new ECM with the latest
calibration should be installed. Alpha engines built above the S/N below have the new
ECM 555. If an ECM is needed, order the correct one listed below.
Model Description ECM Part Number New ECM Starting Serial Number