Fluctating Voltage

[Boilers2000]

92 searay with a 4.3 LX engine...in neutral idle voltage at battery jumping from 13.6 to 14.2, in gear idle and higher RPMs sfeady at 14.3 volts. Battery charged at 12.8 volts after shutting off. Feel like it an alternator, but want some input.

Thanks in advance!

 

[Mc Tool]

What are you measuring it with ,and where in the circuit are you measuring it ..... Are you looking at the gauge on the dash or putting a multimeter on the battery terminals ? It does make a difference to diagnosis

 

[Boilers2000]

Sorry, I meant to include that...I'm doing multimeter at the battery for all the readings. I'm going to check all the connections at the alternator tomorrow...I had my canvas collapse during a storm, and water got inside, so I'm hoping it's corresion at the alternator, but don't think I'll get that lucky

 

[muc]

google "preform battery ripple test"

 

[Mc Tool]

Im not up with your motor in particular but some set ups have the regulator sensing voltage from the switched side of the ignition switch , or some place other than the battery + but the alternator delivers its output straight to the battery+ and if there is a poor connection the reg will see a drop in voltage at the sensor wire and up the alt output to compensate.
A volt drop test is a good way to suss this . It could also be a crook earth or even just a dodgy reg . Dont forget to check the zero on your multimeter ohms setting if you want to use the meter to play with ohms law as sometimes your looking for only 0.something of an ohm .
I have owned a few older Ducati motorbikes and this was a common issue ...so I got good at sussin them out . I got a 906 Paso real cheap because even the local dealer couldnt sort it out ,a simple volt drop test was all it took to pinpoint the problem ....which doesnt say much for the dealer ....who also left the cam pulleys loose

 

[Boilers2000]

Update: took battery to autozone to test and it failed, but it was under warranty so I got a free one. New battery fired the engine up right away, but sith the dog house off, I can hear the engine running rough in idle, but with throttle it smoothes out, so now Im thinking fuel. At idle voltage at battery was 13.6 to 13.9, fluctuating, but with the engine coughing and slowly dying that seems like the cause of the jumping voltage.

On these great and helpful forums, I think the expert with the wizard logo once said "fuel problems present as eletrical issues, and eleltrical problems present as fuel issues"

 

[matt167]

Old style non computer controlled alternators, be it internal or external regulated, marine/industrial or automotive will charge at between 13.5-14.4 VDC. The voltage fluctuates based on the load. There can be some ghosts in the system due to corrosion and voltage drop. Quick test jumping battery positive to the sense terminal should stabilize the voltage if it’s erratic and that’s why.

 

[Boilers2000]

Old style non computer controlled alternators, be it internal or external regulated, marine/industrial or automotive will charge at between 13.5-14.4 VDC. The voltage fluctuates based on the load. There can be some ghosts in the system due to corrosion and voltage drop. Quick test jumping battery positive to the sense terminal should stabilize the voltage if it’s erratic and that’s why.

jump the battery positive terminal on the alternator to the sense terminal, or the battery postive terminal on the battery to the sense terminal on the altenator?

 

[matt167]

jump the battery positive terminal on the alternator to the sense terminal, or the battery postive terminal on the battery to the sense terminal on the altenator?

Either one of those places should have battery voltage. To the battery itself probably easier

 

[Boilers2000]

I think I got it worked out. I finally noticed this evening that my RPMS in idle were below 600, and when I give it a slight tap in neutral throttle to above 600 RPMS, the engine smooths out, and the voltage is steady at the battery on a multimeter at 14.55 volts. I'll get it back in the water and adjust the idle set screw, and hope that resolves the issue.

 

[Mc Tool]

I think I got it worked out. I finally noticed this evening that my RPMS in idle were below 600, and when I give it a slight tap in neutral throttle to above 600 RPMS, the engine smooths out, and the voltage is steady at the battery on a multimeter at 14.55 volts. I'll get it back in the water and adjust the idle set screw, and hope that resolves the issue.

Sounds like you might be on to it

 

[Boilers2000]

Thanks again for everyone's input and advice! The knowledge base in these forums is amazingly extensive and helpful!

 

[Lpgc]

There might not be much wrong with the alternator.. Not too unusual for voltage at idle to be around 13.8v if there are other electrical loads besides just the ignition system / fuel pump and fuel injection system.

At around 13.8v the battery might not be getting a good charge but it won't be getting drained either, then when engine rpm increases if voltage also increases the battery does get a good charge.