Alternator issues

[eddiemako1]

I have a Mercruiser 4.3 L. I have just replaced the alternator. Startling the boat I still have low voltage(12v). Unplugged the purple red and purple wire from the back of the alternator and the voltage jumps up to 13+. So it looks like something must be drawing power where do I start looking?

 

[alldodge]

I have a Mercruiser 4.3 L. I have just replaced the alternator. Startling the boat I still have low voltage(12v). Unplugged the purple red and purple wire from the back of the alternator and the voltage jumps up to 13+. So it looks like something must be drawing power where do I start looking?

Voltage doesn't go up from 12.7V (fully charged 12V battery) to 13+V when the alternator is disabled.

Being you just replaced the alternator, which one did you buy?

 

[eddiemako1]

Mercruiser Marine 7SI Alternator - Delco 7SI Series 70amp

 

[alldodge]

I don't see any issue with the ALT, but still don't see what would cause what your seeing. What caused you to need to replace the ALT? Also what is your engine serial number?

 

[eddiemako1]

Low voltage, when the boat was running I would turn on the blower or the bilge pump and the volts would drop well below 12 so took it off and had it tested . the voltage regulator was bad. So I replaced it. And when I went to start the boat I had just under 12 volts. When I started it it just barely went over 12 volts. I guess I just assumed that it would register up to 13. serial number Om355225

 

[alldodge]

The ALT is correct but need more info, things are not adding up so far.
1) Do you have more then one battery?
1.1) If so is there an isolator, battery switch or ACR used to charge the additional batteries?

Need to put a voltmeter on the battery and record what the bat is before starting, then what is the reading once started and the rpm has gone above 1500 rpm.

Next put the meter on the output terminal of the ALT with it running and has been above 1500 rpm what the reading is

The rpm's do not need to stay above 1500, it just needs to go above it briefly so the ALT will start charging

Note: motor is first gen MPI

 

[achris]

And alternator output should be 13.8 to 14.5 volts. Are you measuring with the dash gauge or a proper multimeter? And where are you measuring? At the battery or at the alternator output terminal? Also, the purple/red wire should have battery voltage on it all the time (when the battery switch is on) and the purple wire should only have voltage on with the key ON...

Chris.......

 

[eddiemako1]

Okay I was using the dash. Went and got a voltmeter connected it to the battery. The battery red 12.6. Started the engine voltmeter went to 14.01 at idle. And learned here do not use the dash meter as a true voltmeter period and thank you Chris and allDodge for putting up with my nonsense. I always do better when I have someone to talk me through it.

 

[alldodge]

Good to hear, and appears your good to go

Just an FYI, my dash meter was messing up, but all I did was remove my isolator and installed and ACR any all is good again

 

[achris]

:thumb: