Alternator Question - Only +13V when Moving

[ColonelNivlag]

Hi all,

Thanks for those who helped on my previous question! So far I'm oil leak free!

I've been trying to research this one before asking here but can't seem to find anyone with a similar issue.

• When Idling I do not get a detectable voltage beyond base at the battery terminals.
• If I rev the engine +2000 RPM I still get nothing beyond base at the battery terminals
• When under load, towing etc, my battery monitor does register a charging voltage. A max of 13.40V.
• When I checked the alternator directly with a meter on the orange output wire, at idle and at +1500 RPM I could detect 13.7V at both

Is this normal for a boat / engine of this age? I suspect the alternator is the from the original 1980s engine, not from the newer Vortec Block. I was expecting to see +14v at idle... or certainly at +1500rpm when idle. My 1996 Volvo 850 puts out +14v at idle. Another boat at the marina, same engine, was putting out +14v at 1500rpm when stationary. It's the fact it seems to work when out on the lake under load that's confused me.

I've attached pics of the alternator, so you can help point me at the replacement which I think you're going to tell me I need! There's no part numbers on it. So don't want to order the wrong thing! Please ignore the state of the connections / rubber everywhere. I'm working on cleaning things up! I've not had the boat long!

 

[alldodge]

Most likely the ALT needs the internal regulator replaced (rebuilt) or ALT replaced. The connections are in bad shape so before doing that remove and clean them up. Also clean the starter and battery post connections if they look the same.

 

[Lou C]

Modern alternators like an ARCO will put out 14.2 volts at idle speed (checked at the batteries themselves). If you only look at the dash mounted voltmeter you can get a lower than true reading because of high resistance in old dash wiring. I would check all battery connections including engine grounds and also re-do those connections on the alternator. If still low replace the alt. ARCO is very good.

 

[440roadrunner]

How old is/ are the battery(s) is there a chance they are down, and have you tried charging them? If you have a big enough battery(s) and they are down, the thing may take awhile to bring up the battery voltage. Depending on the corrosion/ etc of connections/ terminals, voltage drop can be a problem. I've no idea how the voltage sensing is done specifically. As an example, "one wire" alternators can be a problem if the charge lead is not OVER sized and if there is poor connections, causing drop, because the sensing wire IS the charge wire. This is not the case here (I guess) as you seem? to have measured both at the battery and the alternator

DO NOT DISCOUNT the belt may be slipping

One reason I don't care for integral regulators--more difficult to bypass the reg ("full field the alternator") and test it for full output.

You can get an idea if maybe 1 or more diodes are bad by putting your meter on AC volts and measuring from alternator output to ground. If all diodes are good you should have VERY little ripple voltage. Off hand I'd say 1/2 volt is absolute max.

 

[ColonelNivlag]

How old is/ are the battery(s) is there a chance they are down, and have you tried charging them? If you have a big enough battery(s) and they are down, the thing may take awhile to bring up the battery voltage. Depending on the corrosion/ etc of connections/ terminals, voltage drop can be a problem. I've no idea how the voltage sensing is done specifically. As an example, "one wire" alternators can be a problem if the charge lead is not OVER sized and if there is poor connections, causing drop, because the sensing wire IS the charge wire. This is not the case here (I guess) as you seem? to have measured both at the battery and the alternator

DO NOT DISCOUNT the belt may be slipping

One reason I don't care for integral regulators--more difficult to bypass the reg ("full field the alternator") and test it for full output.

You can get an idea if maybe 1 or more diodes are bad by putting your meter on AC volts and measuring from alternator output to ground. If all diodes are good you should have VERY little ripple voltage. Off hand I'd say 1/2 volt is absolute max.

Battery is brand new. I've been having a few electrical issues so that was the first thing I changed. After, as a matter of course, I checked the alternator and found this.

Good shout on the belt slipping. There's no squealing and it's pretty tight. One of the lugs on the alternator is actually broken, so it makes it tough to properly tighten it. 4 hands job! I'll check it though.

I'll check the diodes as you describe.

I'm looking at this as a potential replacement: https://repowermarine.com/marine-alternator-mando-style-65-amp-used-on-3-0l-v6-v8.html

Any thoughts on 65 vs 55 amp? I don't run any accessories beyond Blower and Bilge Pump.

 

[alldodge]

Either ALT will work in your case, but a 65 amp will not work as hard (so to speak) to produce the same lower charge output. That said I doubt you will need another for a long time