2 Alternators, still not charging?

[boatman37]

last year with the original 31 year old alternator my gauge was reading about 11.5 with no load and 10.5 with the bilge blower. i installed a brand new alternator from DB Electrical from eBay and installed it. same results. took my original alternator to the shop for a rebuild but they said it was good but replaced my VR because the insulation on the wires was cracked and flaking off. checked the voltage at my gauge today and got 11.5. checked it at the alternator and got the same 11.5. the shop said my old one tested good so why would i have 2 alternators both give low readings? can't figure out what the common denominator is but i am not an electrical expert either.

we were out late sunday night watching fireworks and on the way back had my nav lights, VHF radio and dash lights on and it was reading 10 volts. normal cruising for us is about a 30 minute cruise and anchor for a few hours. only thing running is the VHF and the bilge blower at low speeds and on house battery while anchored so no issues have surfaced. just would like to resolve this so i don't have issues if we decide to go on a longer trip.

thought about throwing the original back on but not sure that even makes sense. 'supposedly' it is a known good one as the shop said the tested it and it was fine but i got the same readings with it last year. its not the gauge cause i checked it with a digital multimeter right at the alternator

just thinking out loud here, can i put a drill on the pulley nut to spin it up and check the output on a bench or would it need to be energized?

 

[alldodge]

Get a hand held meter to check your readings, it might be the boat gauge.

Otherwise check your batteries to make sure one of them doesn't have a bad cell

 

[boatman37]

this was with a handheld multimeter. the house battery is 2 years old and the cranking battery is brand new. same issue with the old battery and i have swapped all 3 in and out so not likely the battery is the issue.

just doing some research and read that the alternator has to be energized by a battery in order to get output? gets me to thinking that a bad connection from the battery to the alternator may be the culprit? my thinking was that the output at the alternator was low so the problem couldn't be past the alternator but now reading that i'm thinking maybe it is the issue?

my setup is this:
single engine alternator, wire goes to the isolator and the 2 outer poles on the isolator go to the Perko switch. pretty hard to see where each wire goes exactly so not sure where the battery terminals are hooked? i assume back of the perko?

 

[alldodge]

Have to give kudos to Bruce, but just changed my isolator to an ACR. Replaced the isolator some years ago and for about two years I dealt with low voltage. Removed the isolator (3-bank) and installed a single ACR (start and house). Afterwards all is good and charge rate is full up.

Didn't notice at the time but the isolator was the issue.

 

[boatman37]

hmmm. interesting about the isolator.

reading some more and found a post on a corvette forum that says to check between the large alternator post and the positive on the battery and it should be close to 0. if you have any significant voltage reading then you have an issue between the battery and alternator. i guess it means there is resistance somewhere. the issue there was a connection at the solenoid that was bad.

 

[alldodge]

Yes, put one probe on the ALT output and the other on the bat positive post and it should be close to zero. If there is any corrosion or bad connection there will be a voltage measurement

Edit: in your case there is a isolator in line, so one way there will be a small measurement because of the diode, the other direction there would be high resistance

 

[gm280]

With an isolator installed, you will get about ~0.6 volt drop do to the diode inline. And so charging the battery will read lower because of that. So if you add ~0.6 volts to the 11.5 you are reading, that brings the voltage up to ~12.1. And if there is any corrosion on any connector or the wire itself, that should account for the lower reading. JMHO

 

[boatman37]

Thanks guys. Definitely going to check things closer hopefully this weekend. Just need to find a contortionist to help me :lol:

 

[boatman37]

well i pulled all wires off my perko switch and scuffed them with sandpaper and put it all back together with dielectric grease. also put the old alternator back on it. now getting 15 volts from the alternator and my gauge reads slightly over 13.0 and with all switches turned on it dropped to about 11.5 (both the gauge and handheld multimeter), which is higher than i got before with nothing turned on. i also checked my solenoid and everything was tight there.

also, i checked voltage from the back of the alternator to the positive battery terminal and got slightly over 2 volts. is that bad? my cable on the starter runs up to my perko then back down to my starting battery. i am going to continue checking and cleaning connections and maybe replacing cables and wires along the way

 

[MH Hawker]

i recently installed a FF with a voltage reading that reads the ACC system and i have one that shows motor volts, i am chasing a .2 volt difference

 

[alldodge]

A 2 volt drop would be a bit to much. Yours is similar to what I was seeing until I replaced with an ACR

 

[boatman37]

A 2 volt drop would be a bit to much. Yours is similar to what I was seeing until I replaced with an ACR

anyway to test or diagnose it before replacing it?

EDIT: found this > https://www.youtube.com/watch?v=oqCUyJ2P3yI

 

[alldodge]

Agree, this is the test and make sure your meter has a diode scale. Don't use the regular ohm scale

 

[boatman37]

went out today for a little bit. about 30 minutes out (on battery 2) then anchored for a couple of hours on battery 1 then 20 minutes back in on battery 2 and before i plugged back in to shore power i checked both batteries with the engine off. battery 2 (engine) was 12.94 and battery 1 (house) was at 12.76. i didn't do any other testing as we weren't staying for long today

 

[boatman37]

just read something that triggered my memory. everything has been good so far but i haven't been out much. but a few weeks ago i was at the dock on shore power and had my LEDs on with the battery switch on #2 (engine battery). every once in awhile the lights would flicker barely. i switched to battery #1 (house) and it stopped flickering. i was reading another thread about something similar that jogged my memory. would this be indicative of a bad ground? i cleaned all the connections on the back of my perko switch and got my volt meter up over 12 by doing that. still haven't checked any others yet.

 

[gm280]

Oh yes dirty connections that look good to the eye, but have issues the eye can't see. Always, whenever you have any electrical issue, clean the connections first. You will be amazed how many electrical issue are solved just by doing that. JMHO

 

[alldodge]

went out today for a little bit. about 30 minutes out (on battery 2) then anchored for a couple of hours on battery 1 then 20 minutes back in on battery 2 and before i plugged back in to shore power i checked both batteries with the engine off. battery 2 (engine) was 12.94 and battery 1 (house) was at 12.76. i didn't do any other testing as we weren't staying for long today

The way to test the batteries is to go out, anchor for a couple hours and BEFORE you start the motor back up, then read the batteries. Either that or disconnect shore power and let it sit a couple hours

once in awhile the lights would flicker barely

Flickering LED could be a bad connection on the LED, not necessarily an issue.