Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

Hi All, I have a Sunrunner 33 foot with a couple of 4.3 MPI Mercruisers. I have this electrical problem that I cant seem to sort out and by reading these forums I believe you all will be able to help me down here in Aus. When I turn the port ignition on I only read 10 volts and the engine starts as if it has slightly low battery volts, once started it has seriously high battery volts 15 volts but wavering on the gauge. The Starboard engine shows a healthy 12.5 volts and starts easy with 13.8 volts while running and the gauge is steady as a rock.

As I didnt know the age of the battery's i decided to buy 2 new ones, N150 marine maintenance free, I also purchased 2 brand new starters and 2 new alternators from the USA. Essentially I have had absolutely no change to the port engine starting issue the only thing that was noticeable is the voltage reading is now steady as before it was wavering.

It still reads 10 volts on ignition on and 15 volts when running is there something else I can look for

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

Ayuh,... Sounds like a dyin'/ dead battery, Or, corroded wirin'....

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

Check the voltage at the battery with a DVM and confirm the voltages, if it shows good there, check the voltages at the gauge and see if it's the same thing. It could be nothing more than a defective gauge.

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

It could also be that there is a current draw that is sucking that battery down when docked. Low voltage and labored cranking indicates a weak battery. That doesn't mean the battery is bad. It means something is drawing it down. With the engine off, disconnect the positive lead on the battery and insert an AMMETER between the post and the lead. That meter will need to read more than "milliamps" so use one that can handle several "amps". Bad diode(s) in the charging system can do this. Check for an automatic bilge pump that may be constantly running, a baitwell/livewell pump left on, cabin/hatch light left on, etc., are also possibilities.

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

I checked battery's today with DVM and they both are 12.46 &12.51 volts, I have 12.4 volts to the starter and 12.4 volts at the alternator but there is only 9.6 volts at the engine jump start pins beside the engine resetable circuit breaker.

I disconnected all the start circuit wires and checked for loose connections if any hot joints, there were none. I did find some corroded joints to the leg trim pump wiring and I cleaned them up thinking maybe I had found my issue, but no:blue:

When turning the ignition switch to the on position the alarm test struggles to sound and all of the gauges do not read correctly fuel is full but was reading a 1/4 absolutley no volts are reading with the ignition in the on position but when running its 15 volts.

Time for the experts to fix as today was a glorious day for boating

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

Start looking for a voltage drop. Put the red DMM lead of the meter on the positive of the battery (the post itself) and then probe with the black lead. Start where the main battery lead connects to the starter motor. Put the black DMM lead on there. It should read 0 or very very low. Then move to the next point in the circuit. What you're looking for is a voltage reading that is about the same as the difference between the positive on the battery (12.4) and the solenoid terminal (engine jump start pins). When you find that voltage, you found where your problem is.

Chris.....

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

I checked battery's today with DVM and they both are 12.46 &12.51 volts, I have 12.4 volts to the starter and 12.4 volts at the alternator but there is only 9.6 volts at the engine jump start pins beside the engine resetable circuit breaker.

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You need to check the voltage on both sides of the circuit breaker, that breaker could be your problem. If one side of the breaker is ~12.5V and the other is ~9.6V, then you found your problem is a faulty breaker. If both sides are showing ~12.5V and the terminal on the slave solenoid (what you call the
"engine jump start pins" ???) with the red/purple wire on it is 9.6V, then you have a problem in the wiring harness. There is a splice in the main harness somewhere that is likely your problem.

An engine serial number is really necessary for this kind of thing.

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

I will check these things out tomorrow and grab the serial number off the engine as well.

Cheers

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

I decided to walk over to the boat tonight, the engine serial number is OM367144

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

Sounds like you have a healthy battery. Does the engine crank over slow or just a weak alarm buzzer sound??? Check for tight AND clean (no corrosion) connections. Also check for battery cables that feel stiff and not flexible. When water gets into battery cables it can internally corrode them under the outer jacket and cause current drop. Voltage could read okay, but the amperage just isnt there to crank 'er up.


If you have high voltage when RUNNING but low when off, chances are its a corroded battery cable, terminals, or internally or externally corroded 50A engine circut breaker. When your engine is on and running (after you have gone over 1000 RPMS or so to excite the alternator and it begins to charge the battery), the alternator sends power through a large gauge wire (usually organge or red) to the post on the starter that your battery cable is attached to. Seeing that you have more than normal voltage when running, It sounds like power is getting from the ALT to the starter post, then up to the keyswitch, and then through the ignition circut (purple wire), and over to the volt gauge. It SOUNDS like power isnt fully flowing between the battery and the starter + post properly....but seeing you have the 50a circut breaker and a remote starter solenoid in your "main" incomming battery power it could be one of those too.

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

LorenHaus said:

Sounds like you have a healthy battery. Does the engine crank over slow??? Check for tight AND clean (no corrosion) connections. Also check for battery cables that feel stiff and not flexible. When water gets into battery cables it can internally corrode them under the outer jacket and cause current drop.

If you have high voltage when RUNNING but low when off, chances are its a corroded battery cable or terminals. When your engine is on and running (after you have gone over 1000 RPMS or so to excite the alternator and it begins to charge the battery), the alternator sends power through a large gauge wire (usually organge or red) to the post on the starter that your battery cable is attached to. Seeing that you have more than normal voltage when running, It sounds like power is getting from the ALT to the starter post, then up to the keyswitch, and then through the ignition circut (purple wire), and over to the volt gauge. It SOUNDS like power isnt fully flowing between the battery and the starter + post properly.

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Ayuh,... The often neglected Grounds could also be the issue,...

Remove, sand, 'n clean 'em all to shiny metal... greasin' 'em after, stops the corrosion...

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

yeahah I found the issue well I think I have

Just behind the 90 amp starter fuse breaker (88-79023A91) the two wires that go up to the slave start solenoid and 90 amp main engine resetable breaker one of them had chaffed the outer sheath and was corroded. Now what a job to get to that as its the port engine and trying to get my fat head in betweent he 2 engines to see was a major pain.

The 90 amp fuse breaked doenst look that healthy so I am getting 2 new ones (1 as a spare) tomorrow and I will repair the wiring then hopefully we are away again.

A big thankyou to you all for the repsonses and I hope I have sorted it out

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

I'm having the same problem. Though mine is intermittent and randomly either doesn't get spark or just cuts out. When the volt meter reads 12v then it's fine. If it reads low then no spark. Traced purple wire and cleaned connection at harness connector. The purple wire which is the ignition system wire also is the wire that feeds the voltmeter gets powered through the ignition key switch. This is where I think the problem is. I am going to put a temporary switch between the purple wire and the positive wire at the ignition switch and if the problem occurs again I will close the switch and see if the power is restored. If so we have a faulty ignition switch!!!
Intermittent's are the hardest to troubleshoot. Frustrated but not deterred.

Re: Mercruiser 4.3 low volts when ignition turned on and high volts while runnning

bilbuz said:

I'm having the same problem. Though mine is intermittent and randomly either doesn't get spark or just cuts out. When the volt meter reads 12v then it's fine. If it reads low then no spark. Traced purple wire and cleaned connection at harness connector. The purple wire which is the ignition system wire also is the wire that feeds the voltmeter gets powered through the ignition key switch. This is where I think the problem is. I am going to put a temporary switch between the purple wire and the positive wire at the ignition switch and if the problem occurs again I will close the switch and see if the power is restored. If so we have a faulty ignition switch!!!
Intermittent's are the hardest to troubleshoot. Frustrated but not deterred.

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Ayuh,.... Welcome Aboard Bill,... If yer lookin' for Help, ya really oughta post a thread of yer Own,....

If yer just commentin',...
Well,... Welcome Aboard,....

I just got my 4.3 Mercruiser rebuilt because it overheated on me at the lake and when I got everything put back together everything is working fine except the high voltage keeps going up and alarm keeps buzzing

Robloc said:

I just got my 4.3 Mercruiser rebuilt because it overheated on me at the lake and when I got everything put back together everything is working fine except the high voltage keeps going up and alarm keeps buzzing

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Start a topic about your motor and ask questions there

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