Battery charging question - multimeter jumping around

kevinstan

Petty Officer 2nd Class
Joined
Aug 19, 2012
Messages
169
I am trying to check that my stator is charging my battery. I checked the battery with the motor off and it registers around 12.4 - I started the motor and let it run for about 3 minutes then checked it again and it's jumping around from 13 or 14 down to around 5 or 8 and randomly back up again. I am wondering what could be causing this ? I just replaced the rectifier but everything else is seems to be fine.
 

gm280

Supreme Mariner
Joined
Jun 26, 2011
Messages
14,585
Sounds like someone had a grounding connection problem to me. I would make certain all your connections are solid and with shiny metal contacts with each other. But don't ignore the grounds either. The have to be as good as the other electrical contacts to make it read solid. JMHO
 

kevinstan

Petty Officer 2nd Class
Joined
Aug 19, 2012
Messages
169
I have gone over everything and it all does seem to be solid. I didn't find anything loose or without good metal on metal connections. What else could cause this ? Or would a bad ground or connection be the only thing ?
 

gm280

Supreme Mariner
Joined
Jun 26, 2011
Messages
14,585
Well obviously this is happening only when the engine is running. So that points to an intermittent connection OR a chafed wire insulation somewhere. It could even be a rectifier going out. But you can easily test for that. Test the AC side of the rectifier while the engine is running (be careful of anything spinning when you are testing). IF you are at a steady RPM the AC reading should be steady as well. If not, you may have some wire problems under the flywheel. However, if it is steady, your rectifier is suspect. JMHO
 

dennis461

Chief Petty Officer
Joined
Aug 11, 2011
Messages
516
I am trying to check that my stator is charging my battery. I checked the battery with the motor off and it registers around 12.4 - I started the motor and let it run for about 3 minutes then checked it again and it's jumping around from 13 or 14 down to around 5 or 8 and randomly back up again. I am wondering what could be causing this ? I just replaced the rectifier but everything else is seems to be fine.

​What rectifier did you replace? Something inside your alternator?
​Tell us more about your boat, what alternator do you have?

Also, try another voltmeter and different meter leads.

 

kevinstan

Petty Officer 2nd Class
Joined
Aug 19, 2012
Messages
169
I replaced the square rectifier on the engine. It has one connection for battery - one for ground - and AC1 and AC2. The motor is a 1986 Force 85hp. It has a standard stator on it, and I am trying to make sure it is charging. While running the motor last week I noticed the rectifier was overheating and smoking so I turned the motor off. I ordered a new rectifier and replaced it. At the time it was not mounted on the engine like it was suppose to be, instead it was just hanging freely - so I attributed the issue to it not being grounded connected to the motor, and not acting as a heat sync.
 

kevinstan

Petty Officer 2nd Class
Joined
Aug 19, 2012
Messages
169
I thought maybe a faulty battery as well ? The battery that came with the boat is a very generic looking plain grey battery with no wording that I see on it. I was going to replace the battery with the assumption that it may be bad or going bad ?
 

appleo

Petty Officer 1st Class
Joined
Jul 19, 2012
Messages
388
Rectifier needs to be screwed down to powerhead As the case is usually grounded

Stator wires connect to 2 ac terminals on rectifier

If voltage not steady at rectifier plus terminal , likely stator issue

If it's an old motor many times wiring insulation cracks and falls off. Especially old mercy

Pulling the flywheel no big deal Impact wrench to jerk loose the big nut. Gear puller or studs to back off the flywheel. My bet you have damaged wires somewhere in there. Keep us posted
 

kevinstan

Petty Officer 2nd Class
Joined
Aug 19, 2012
Messages
169
I am trying my hardest to avoid removing the flywheel. I dont have an impact wrench, or a gear puller to remove it. I am trying to troubleshoot and do everything I can with that as a last resort. I am really hoping also that the stator is still good and that this is something simple. I am horrible with electrics and trying my best to figure this out without spending a fortune for someone else to figure it out. I am not sure how to test the rectifier. I just put a new one on because the last one overheated and started smoking. It wasnt attached to the powerhead like it was suppose to be, so i was assuming that was my only problem. Any help on further troubleshooting without removing the flywheel would be great. thank you in advance!
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Are you checking voltage with the voltmeter in the boat or with a hand held multi-tester (VOM)? Either way, there could be a loose connection on the voltmeter or anywhere in the 12 volt feed and ground system. Electricity needs a complete path to accomplish anything. Bad connections either at the voltmeter or in the 12 volt circuit and it's ground will affect the voltage reading. A voltmeter is like a light bulb, fish finder, live well pump, bilge pump, radio, etc. If there is a problem in the circuit that feeds it, that device will not work properly. That's what the voltmeter is trying to tell you.
 

dennis461

Chief Petty Officer
Joined
Aug 11, 2011
Messages
516
Was the boat having problems before changing the rectifier?
How old is the battery?
Could the new rectifier wiring be incorrect (meaning was it a simple job matching up connectors or wiring color)?

1'st, don't panic and rip apart things or buy expensive parts.
If the engine runs OK but does not charge the battery, the stator is probably OK.
I see a wiring diagram here, let me know if it is correct for your engine.
http://www.maxrules.com/graphics/for...thru91A_CD.jpg

Step one.
The two wires from stator to rectifier are shown as yellow or green/yellow.
You need to set your multimeter to read AC voltage 100 or 200 volt scale or auto range.
Connect the meter with solid connections, alligator clips, or such.
One multimeter lead to each of the two yellow wires.
Do not rely on a person holding the pointy probes.

With engine running, the AC voltage should be above 12 VAC (Volts Alternating Current) and fairly steady (if engine runs steady)
 
Last edited:

kevinstan

Petty Officer 2nd Class
Joined
Aug 19, 2012
Messages
169
Thanks for the detailed information dennis461 - I will try today to test the rectifier and see if it is steady or if it is jumping around. The battery I am not sure of since it was on the boat when I bought it. The battery looks very generic with no writing on it that I can see and it is just plain grey in color. I was thinking about purchasing a new battery. The boat was doing the same exact thing before changing the rectifier out. I was really hoping that changing the rectifier would fix the problem. It was a very simple change out from the old rectifier to the new one. Also, that wiring diagram seems to be accurate. I am planning on checking today to see the reading at the rectifier since I have not yet done that. Thank you for all the detailed information! I will post on here again today once I check everything out. Thanks again!
 

gm280

Supreme Mariner
Joined
Jun 26, 2011
Messages
14,585
With only two wires coming from the stator, you could use so many bridge rectifiers as replacement if need be. I think once you determine that you have a good solid AC voltage reading at the AC inputs of the rectifier, I think everything will work out well from that reading. I will say I suspect you will read a lot higher then 12 volts AC as well. And that is because in order to charge your battery, regardless what type or condition it is, you have to have more then 12 volts. And once the AC voltage gets rectified it drops a few volts coming out the rectifier to the battery. More like 14 volts would be a good guess at the battery. Let us know. JMHO
 

kevinstan

Petty Officer 2nd Class
Joined
Aug 19, 2012
Messages
169
gm280: Thanks for the info. I did not get the chance yesterday to mess with it since it was raining all day. I am going to try within the next day or two to get around to it and hopefully I can narrow down the problem. I will post again once I check the rectifier bridge.
 
Top