Low Voltage Light Comes ON, Engine Cuts Out

[Chef Shawn]

The problem child... MCM 140, 3.0 (Serial # A457082), 1986 engine.

More trouble with this fellow..Originally had problems with the alternator back-feeding the system, installed a diode into the green wire from alternator and fixed the problem(engine wouldn't shut off with key.) So, went out once, no problems at all, ran like a champ. Next trip, trouble idling, started to cut out over 2500 rpm. Changed plugs, previous owner had some weird Autolite AP 666 plugs in it. Switched to RT 43's. Started, idled like a kitten. Ran great up to 2200 rpms, cutting out again, but fires right up, first try every time. Finally moved my cigarettes off the dash while underway, and low and behold, every time the motor cuts out, LOW VOLTAGE light goes full red. Do not have a voltmeter in the dash, just the light.

Looked through old forums, couldn't find much info. Is this alternator causing my headaches, or maybe the tach is bad, some forums suggested a bad tach could cause these problems. Food for thought, I thought the diode I put in burned out, totally removed it from the system (rewired) and motor still starts and shuts down fine. At WOT, bogs and dies. Starts back up and idles forever. Temp never goes above 130. New impeller and thermostat installed. Anything over 2K on the tach, motor chugs and wants to die.

Thanks in advance.

 

[wellcraft-classic210]

Re: Low Voltage Light Comes ON, Engine Cuts Out

One possiblity -- are all your wire connections ( including the grounds ) to and from the engine good. look for corrosion and tightness. Could be something is moving when you accelerate.


You may also want to try to monitor the voltage with a volt meter to verify the low voltage problem.

It could also be some sort of short but those typically take out a fuse / thats why I suspect the connections.

Smoking is bad for you -- but so are most of the good things in life ( except wild women-- they are just expensive )

 

[Silvertip]

Re: Low Voltage Light Comes ON, Engine Cuts Out

You aren't going to know if the battery is charging until you check it with a voltmeter. If the battery is not charging, the engine does not get enough voltage to run at higher rpm. The battery should measure 12.6 volts if it is fully charged, and with everything turned off. When running the voltage should be in the 13.5 - 14.5 volt range. If battery voltage is low with the engine off, don't run out and buy a new battery. The battery is low because it is not being charged.

 

[Don S]

Re: Low Voltage Light Comes ON, Engine Cuts Out

Originally had problems with the alternator back-feeding the system, installed a diode into the green wire from alternator and fixed the problem(engine wouldn't shut off with key.) So, went out once, no problems at all, ran like a champ.

It sounds like you have a homemade wiring setup, because Mercruiser doesn't use a green wire on the alternator nor do they used a light on the dash board like you seem to have.
Perhaps a picture of your alternator so we can see the wiring and the back of the alternator, and explation of where each wire on it goes, then maybe we can help you find the problem.

 

[Chef Shawn]

Re: Low Voltage Light Comes ON, Engine Cuts Out

Don S: Sorry, PURPLE wire from back of alternator. I know, my SELOC doesn't show a low voltage light in the system either, but it appears to be factory (Scorpion 16.5, year=1986)

Silvertip: Battery shows 14V at idle. BUT I always put it on a slow-charge , 1 amp, the day before I head out, because I only take the one battery. I suppose it could be bad, but am doubtful because it ALWAYS cranks the motor over, no matter what.

Diode thing is causing me headaches, even disconnected, boat runs fine, shuts down with key.

Long story short:
1. Bought, owner said ignition switch bad, has to pull coil wire to shut down, coil overheats.
2. Replaced ignition switch, installed diode.
3. Trip #1. Boat ran 2 minutes, got close to overheating. Shut it down. Got a tow.
4. Replaced impeller.
5 Trip #2 . Boat hard to start, ran great for 3 hours.
6. Trip #3 . Replaced plugs with RECOMMENDED plugs. Idled better than ever, silky smooth shifting. Ran good 10 minutes, started missing, chugging over 2K rpm. LOW VOLTAGE
7. Replaced plugs with old plugs, still chugs, LOW VOLTAGE.

 

[Technorunner]

Re: Low Voltage Light Comes ON, Engine Cuts Out

You said the alternator was backfeeding the system causing the engine to not shut of. Then this is most likly a alternator issue. A battery that cranks really good dosent lose charge that quick with a working alternator to cause the engine to cut out. you would have to have one or more of the cells in the battery to be shortcircuit to draw that much current from the alternator. And if that would be the case then the engine wouldent crank. Take some one with you and have that person messure the voltage on the battery to se if the voltage drop while rpm is climbing. IF the voltage on the battery should be ok messure at the coil next step. If voltage drops on battery while rpm starts to come near the 2K rpm the alternator is probably toast.

 

[dingdongs]

Re: Low Voltage Light Comes ON, Engine Cuts Out

has the points,condenser and leads been changed for new.if they're the original leads there more than 25 years old.check at night in pitch black for arcing of leads and coil.
has the boat been on the water and run at max revs under load as it may be fuel related.carb maintenance required.fuel filter,new fuel etc.

 

[Chef Shawn]

Re: Low Voltage Light Comes ON, Engine Cuts Out

OK, more testing.

Replaced alternator, as it was only throwing off 11 volts, installed new alternator and belt. Am now getting 14.5 volts to battery at idle.

Replaced coil for giggles, am getting 7.5 volts at idle, 12+ when revved.

Took her out yesterday, starts and idles perfect. Under load, in gear, again it cuts out at 2K, LOW VOLTAGE light ON. Removed tach wire and tried again (some have said a bad tach will screw with the system), but no change in performance. Re-attached tach wire.

With "THROTTLE ONLY" engine will run perfect, briefly took it 4K to verify.

So, only in gear, under load do I have this problem.

Also, it has a regular distributor on it, no points. Plugs are new and gapped. Rotor, cap and plugs are in good condition.

Now where do I start looking?

 

[Eric 3.7LX Alpha 1]

Re: Low Voltage Light Comes ON, Engine Cuts Out

IF your alternator is getting power while your low voltage light comes on, then either you have a tremendous draw on the electrical system or very poor wiring/connections, or alternator issues still.

Try and test voltage directly off the "outlet" off the back of the alternator while the light is on and see what the reading is as well as power to the alternator while the light is on, to eliminate the new alternator is bad.

There could be all sorts of burnt wiring/ or corroded connections on the boat and the good alternator can't pass it's power out to the system.