Mercruiser 350 Mag Starter Continually Cranks...

Very baffling. I went to start motor one day and the starter continued to run after motor was started. I immediately shut off motor but starter continued to crank until I shut off the battery switch. Upon turning the switch back on, after about 3 seconds the starter would crank on its own again. Even with key out and in the off position.

I tested and eventually replaced ignition switch. With either switch completely out and disconnected it still cranked.

Thinking it was a bad or stuck solenoid, I removed starter and bench tested it. It worked as it should.

With starter removed, I then hooked a test light to the yellow/red start wire and it showed power within 3 seconds of turning battery switch on.

I traced yellow/red wire to a relay on top of motor...thinking Ahah, I found the problem...nope swapping out adjacent trim or fuel pump relay causes same issue.

I could find no issue with the ignition switch or wiring. It cranks even when the ignition switch is completely removed from boat and wires isolated. All harnesses and battery connections were pulled and show no sign of corrosion. Everything else works as it should and the motor will start but starter will continue to crank anytime the battery switch is on.

Is it possible the ECM/PCM is somehow at fault. I cannot figure any other way the yellow/red wire to starter solenoid is becoming energized. Furthermore, it is not immediate starter engagement, it is always after several seconds I hear the starter relay click and then the starter engages.

Any ideas or suggestions would be appreciated. Nearest Mercruiser dealer is about 140 miles away, so I am trying to avoid that.

Mercruise 350 MPI, Horizon 2007

solenoids or relays are the cause even if they look good.Swapping the relay is a good start. the yellow /red at the starters solenoid gets its power from the slave relay or solenoid. That is only energized with the key in the "start" position.
leave your meter at the starers yellow/red wire and remove the yellow/red at the ign switch. see if it still has power on it. If it does, pull the main harness plug at the engine and post the results

leave your meter at the starers yellow/red wire and remove the yellow/red at the ign switch. see if it still has power on it. If it does, pull the main harness plug at the engine and post the results

Click to expand...

Findings: Yellow/Red was energized on both ends (solenoid and ignition side) when yellow red wire was disconnected at switch. Starter of course engaged as usual. When harness was disconnected the wire was not energized on either end.

Interesting thing happened though. Apparently there is an adapter harness involved here that I wasn't aware of. When tracing the harness back from main plug on motor I came across a rubber plug with a hose clamp. Apparently when I had it re-powered 8 years ago, this adapted the old 10 pin harness to the newer 14 pin harness. This rig had a rubber boot with hose clamp on it which adapts it to the main 14 pin engine plug.

When I loosened and inspected this harness it was full of dialectic grease but otherwise was clean on both sides of both plugs. After I reinstalled this adapter harness the problem appears to have went away. Is it possible that 12 volts was bleeding between pins somehow and energizing the starter circuit?

I'm not confident that merely unplugging and re plugging the harness totally solved my problem but I think it may be the source of it. Maybe a new adapter harness is in order or should I just clean all the dialectic grease out with contact cleaner? Of course it is equally likely that I just jostled the bad wires enough to temporarily halt the problem?



Thanks Bt Doctur for your assistance and I'd like to hear your additional opinions. You are the man!

This is what the adapter plug looks like.

I'm not confident that merely unplugging and re plugging the harness totally solved my problem but I think it may be the source of it.

Click to expand...

Ayuh,.... I think yer Right,.... Pluggin', 'n unpluggin' probably ain't the cure,....

Does it come apart, or is it molded in plastic like the original,..??
I'm guessin' the a bad wire or connection, Inside the adapter,....

bondo said:

Ayuh,.... I think yer Right,.... Pluggin', 'n unpluggin' probably ain't the cure,....

Does it come apart, or is it molded in plastic like the original,..??
I'm guessin' the a bad wire or connection, Inside the adapter,....

Click to expand...

Its molded hard rubber where it plugs into the old, existing wires that run to helm. It comes apart, a stainless hose clamp holds the two pieces (male/female) together on that side. The metal on the male and female terminals there is free of corrosion but has a bunch of what looks like dialectic grease coating everything.

I've taken the motor side off multiple times, saw no obvious issues, yet the problem persisted. It wasn't until I found and disconnected the other end (hard rubber side) that the problem went away, how ever temporary that may be? I can order another adapter but I'm thinking, guessing, that if it is defective the chances are its on the old harness side (male end) going to helm?

Perhaps the best solution is to get a entire new harness from helm to motor, one that would eliminate the adapter entirely?

This is a better photo of the adapter. The left side plugs into old harness running to helm and is clamped together with stainless screw clamp, the right connects to plug on motor-twist lock.

Check your harness with a ohm meter for continuity, Pull on male pins in harness,are they secure?

johnnygjr said:

Check your harness with a ohm meter for continuity, Pull on male pins in harness,are they secure?

Click to expand...

You all nailed it! Boy am I glad this PITA is soon to be over.

Thanks guys, you led me around by the nose until I got it sorted out. I didn't want to believe that a 8 year old adapter harness would be the culprit when there was a 23 year old main wiring harness in the boat. In retrospect I should have started at simple and worked my way forward.

I took the adapter harness completely out and found a very small amount of current flow between multiple pins on the adapter harness. There was none between pins on the helm harness. I'm guessing that due to corrosion, current is bleeding between the wires/connections internal to the plug on this adapter when there should be none at all. If my understanding of this is correct, then in fact the adapter harness is the culprit.

I have a new adapter harness ordered and once that is here I will dissect the the old one to confirm this theory. More specifically, I'm guessing that connections on the hard rubber (helm side) of the adapter are corroded internally and possibly shorted intermittently in some spots.

Where the wires enter the plug, they are not sealed at all and upon closer inspection after separating the wires, I see a small amount of what appears to be corrosion dust on the outside of the wires as they enter the plug.

This adapter plug hangs in such a way that any water that penetrated the motor box would run right down the wires into the unsealed (wire side) of the plug.

Thank you Bt Doctur, Bondo and johnnygjr for taking the time to help me sort this out. Saved me a very inconvenient and long road trip to the backlogged dealer for something so simple.

I wanted to follow-up to this post in case someone else has a similar problem in the future.

The new harness is only 35 bucks and is on its way...

Glad you found it, Also in the main plug you get cases of the engine just dying .Same reason , slight corrosion with the pins.

happy boating

Sure enough, full of corrosion. On bottom photo, you can see how loose the wires are entering the plug and evidence of corrosion weeping out. I wonder why mercruiser would not at least attempt to seal this end. Never mind I know the answer to that. I will be blowing some 5200 or silicone into this void and around the wires when the new one arrives.

how do they ever stay in business ???????????????????????/

Now that you know what the problem is, Replace the 10 pin harness with a new 14 pin all the way to the helm.

The 10 to14 adapter is crap and always an issue.

If you MUST use the adapter instead a new end to end as Chris suggested, and the new adapter also comes with open unsealed wire entries (Merc, IMHO, will do almost anything to save $0.10 on a product), be sure to seal the back. DO NOT USE 5200 (cures rigid and will induce failures @ juncture).... I'm not a big fan of silicone either. "Vulkem" and other polyurethane sealants have better adhesion to wire and last longer. BTW, I spent several years in military/aerospace reliability engineering early in my career. Least reliable part of any system is the connectors. Using an adapter effectively doubles the number of connections and therefor halves the reliability of your wire harness... and that is all things are equal, i.e., no crappy unsealed wires!

johnnygjr said:

how do they ever stay in business ???????????????????????/

Click to expand...

Haha. Good one. I guess they got us over a hump, don't they? Please don't get me started with Mercruiser's BS. I've dealt with their defects and design failures numerous times. "Cool fuel 3" and distributor cap/coil failures, to name just a few. I kind of live at the end of the world so bringing the boat in for service every time there is a problem is not really an option. I've learned an awful lot cleaning up their messes on my own. Unfortunately that cleanup cost me many lost trips and a lot of down time.

Docksidemarineservices Re: Mercruiser 350 Mag Starter Continually Cranks...
Now that you know what the problem is, Replace the 10 pin harness with a new 14 pin all the way to the helm.

Click to expand...

I hear you. In a perfect world I could have and probably should have. I already ordered the adapter so if I get a couple of years out of it, I'm good with that. I'm probably selling this rig and going back to outboard(s) in the not too distant future.

sandkicker Re: Mercruiser 350 Mag Starter Continually Cranks...
If you MUST use the adapter instead a new end to end as Chris suggested, and the new adapter also comes with open unsealed wire entries (Merc, IMHO, will do almost anything to save $0.10 on a product), be sure to seal the back. DO NOT USE 5200 (cures rigid and will induce failures @ juncture).... I'm not a big fan of silicone either. "Vulkem" and other polyurethane sealants have better adhesion...

Click to expand...

Thanks for the advice, I will check vulkem out. I agree with Merc saving 10 cents...nothing but headaches. I re-powered in 08 because the old motor, although running good, was starting to nickel and dime me and I needed reliability. The new motor was much worse, almost from day one. Constant headaches. I only have about 160 hours on it in 8 years. The amount that I boat, those hours would have easily been quadrupled but no, this POS pretty much stayed broke or running poorly the whole time. Never will I own a Mercury product again.

The eternal optimist that I am, I guess I should be thanking Mercruiser for saving me thousands of dollars in fuel all the times I was broke down and couldn't use the boat???

Thanks again guys, you really were a great help.