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Old 26-05-2018, 03:50   #1
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MAG 350 twin keeps running after turning off

I have a boat with twin Mag 350's

Each has a battery and I do not bridge the batteries when starting (albeit I have noticed some power feeds to the say the starboard engine when I turn say the port battery on, but not enough to start it or work say the trim).

They start fine individually, but I have noticed that if I turn the port engine off first it just keeps running until I turn the starboard engine off.

It does not work in reverse, eg so if I turn the starboard engine off first the port will not stop until I turn the port key off.

Any idea ?
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Old 26-05-2018, 13:24   #2
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Re: MAG 350 twin keeps running after turning off

I take it these are gas engines. You have some bleed over in the circuit(s) that control ignition. Possibly in the gauge panel, maybe in the switched connections of a shared gauge. It also could be in a relay or caused by some new accessory.
I'd look for any added wiring. Something the dealer or former owner added to the stock engine wiring. Many people (and some dealers) have no idea what they're doing and when a DC power source is need, like for a radio, they just patch into the easiest place. Often without fuses.
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Old 26-05-2018, 14:35   #3
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Re: MAG 350 twin keeps running after turning off

Thanks

Yes, gas / petrol MPI engines, 2002 vintage.

I get the theory but how does that work in practice ?

My assumption is that the ignition switch supplies 12 volts to the coil and to turn the engine off you cut the 12 volts to the coil.

eg If I wire say a radio to say the ignition circuit only on one engine, how does that affect the other engine after that engine is turned off ?

The only shared item I can think if is a dual engine gateway (Navman / Northstar) which already says not to connect one of the loom wires to the plug off the distributor on one of the engines for power anyway (will check just in case)

Maybe the Navman head is connected to the power of the other engine (eg on the panel) but still given the wires coming into the dual gateway are engine signals only, I'd doubt they could carry any amps.

All the light circuits are off when this happens so I assume they are not the issue / cause of any leakage.

On the other hand, I might try turning off all the circuit breaker switches (eg water, lights, trim tabs etc which I normally leave on) to see if the port engine dies just in case they were dual engine / battery wired somewhere.

regards
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Old 27-05-2018, 12:17   #4
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Re: MAG 350 twin keeps running after turning off

It can be a time consuming task. You need a meter or test light. 1st with the engines off, I'd put a light on the coil of the port engine and turn the key/switch on for the st'bd and see if the test light comes on. Go from there. Trace back the power. If there are multiple wires going to the coil, remove them one-by-one and find the one supplying power. Then trace that wire back to other connections.

I have seen things like radios wired with the polarity reversed (but working normally) bleed back power even with the power switch off. Sometimes causing damage to zincs.
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Old 27-05-2018, 12:58   #5
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Re: MAG 350 twin keeps running after turning off

If the engine is controlled by an ECU you are looking for stray power to the ECU power in feed rather than the coil.
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Old 27-05-2018, 14:13   #6
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Re: MAG 350 twin keeps running after turning off

Thanks for the tips.

I do have ECM (555's) so will have a look. I'm suspecting something to do with the bilge pumps wiring.

I did find two things to investigate (bugger to get to though)

a) the starboard key pop up fuse (eg next to the key) has been bypassed in the past by someone but the wiring looks okay / nothing weird

b) when I turn on the circuit breakers / switches for the aft and middle bilge pumps (they have auto float switches so I have never turned them on before anyway) the digital voltage meters on my main switch board lights up (but only to around 9 volts) even with the batteries turned off, suggesting power is flowing back thru the motors (I'd say they are direct wired to the batteries) and the meters only go off if I turn the main house circuit off (or those bilge pump switches off).
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Old 27-05-2018, 14:27   #7
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Re: MAG 350 twin keeps running after turning off

I see a knowledgeable electrician in your future.
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