Twin Mercruisers -electrical/ignition/battery systems - are they completely separate?

[pachanga27]

The boat is a 1989 Sea Ray Pachanga with twin Mercruisers - 350ci with alpha outdrives.
It has two batteries - one on each side of the engine compartment - one battery - for each motor.
Each battery has it own On-Off switch under the rear seats. And I write this I am realizing I am probably answering my own question - which is...

Do the motors share in any way the same electrical power for the ignitions? In other words - I can see that each motor has its own alternator and own battery - but if one alternator fails on one motor - is there is not a way to power the ignition on both motors from the good alternator on the other motor? What brings me to this scenario is that it seems the port motor's alternator has failed and the motor is running on the battery until the voltage drops and there is not enough energy to power the ignition for the motor and it starts to run rough and lose power. No current from the alternator and the battery as it drops, it cannot supply the energy to the ignition which results in a loss of power. The port motors voltage gauge is at approximately 12v while the starboard motor's gauge is approx. 14.2 volts. I have not tested the alternator yet... but is there a simple way to test the alternator's output while the motor is on muffs as I do not leave the boat in the water at the moment? Another interesting note -is that on the dash there is a switch that is labeled "emergency start" which according to the manual will - while you hold the switch will temporally parallel the batteries if you do not have enough cranking power from one battery to turn the particular motor over. I was wondering if this would "work" as a way to feed electrical power to the port motor if the alternator had failed and the battery was discharged? If upon using the switch the motor came back alive with full power that might tell me for sure that the alternator has failed. Of course there may be an isolation circuit in there that prevents the current from flowing like that to prevent damage to the ignition and only allows a path to go to the starter motor. The other side of this ..is that I can charge the battery overnight and the motor will run fine for about an hour at which point the motor starts to slowly lose power.

I am thinking I just have a bad alternator and need to replace it...

Thanks

 

[Don S]

Re: Twin Mercruisers -electrical/ignition/battery systems - are they completely separ

Several engines can run off a single battery, depending on how your battery system is set up. Which is not something that is Mercruiser specific.
As far as checking alternators, First, make sure the battery is good and will pass a load test. If not replace it. Then make sure all the battery cable ends are clean and tight (both ends) (us a wrench on a regular nut, not wingnuts).
Then follow this.

View attachment SB 01-12.pdf

 

[pachanga27]

Re: Twin Mercruisers -electrical/ignition/battery systems - are they completely separ

Thanks Don...that is exactly what I need to troubleshoot.

 

[pachanga27]

Re: Twin Mercruisers -electrical/ignition/battery systems - are they completely separ

Update - and maybe the following info may help others - Thanks to Don providing the troubleshooting steps for the alternator... I found that the alternators were both operating as they should and that the batteries were good. I do believe that I did find the real culprit. After searching through the archives looking for simialr issues - that is - the engine would bog at approximately 3000 RPM - but otherwise cranked fine and idled fine - I went to the screw on fuel filter - and carefully removed it - avoiding spilling any fuel - and discovered it was 90% full of water. If I am thinking correctly - enough fuel was getting to the carb to crank and idle, but the motor was being starved of fuel. (correct me if this is not possible) In the end - put a new filter on the motor and also put a new filter on the starboard engine also which had not shown any issues. That filter had water in it also... but maybe 25% filled with water. In the end - this past weekend - the port motor ran excellent once again and both motors (in my mind) seemed to have a little extra punch when powered up from 1/3 throttle to full throttle. In the end - thanks for the help and man - it sure felt good to solve an issue without spending a boatload of money.

 

[Bondo]

Re: Twin Mercruisers -electrical/ignition/battery systems - are they completely separ

Update - and maybe the following info may help others - Thanks to Don providing the troubleshooting steps for the alternator... I found that the alternators were both operating as they should and that the batteries were good. I do believe that I did find the real culprit. After searching through the archives looking for simialr issues - that is - the engine would bog at approximately 3000 RPM - but otherwise cranked fine and idled fine - I went to the screw on fuel filter - and carefully removed it - avoiding spilling any fuel - and discovered it was 90% full of water. If I am thinking correctly - enough fuel was getting to the carb to crank and idle, but the motor was being starved of fuel. (correct me if this is not possible) In the end - put a new filter on the motor and also put a new filter on the starboard engine also which had not shown any issues. That filter had water in it also... but maybe 25% filled with water. In the end - this past weekend - the port motor ran excellent once again and both motors (in my mind) seemed to have a little extra punch when powered up from 1/3 throttle to full throttle. In the end - thanks for the help and man - it sure felt good to solve an issue without spending a boatload of money.

Ayuh,.... Yer Right,.... Get the water outa yer tanks....