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  1. #1
    divernc Guest

    Alternator advice

    I have 1979 6v92TAs with what appear original alternators. After sitting 3 weeks on hard w/o AC power, it was splashed and sat on AC power and charger for 24 hours, before I could get to it. Did not check voltages, but both engines started immediately. 90 minutes in to cruise back to home base, with all bridge electronics operating, I noticed radar CRT display wavering. Port voltmeter, which is house battery, showed about 9.5 volts. I shut off all electronics, except 1 VHF and voltmeter moved up to about 11 volts. Another 30 minutes and I was home. After plugging in to shore power and charger, voltmeter up to 12 volts in 1 hour and 14 volts in 3 hours. Over the last two years of ownership, I have noted 3-4 times this alternator at 12 volts at start-up and fast idel and going to its normal 15-16, only after advancing throttle beyond 1500 rpm.

    I checked battery connections, water level and specific gravity of all cells and post connections on alternator; all OK. How can I check alternator output w/o damaging it or myself. It seems like on autos it damages ?diodes? to operate the alternator w/o battery connected.

    Is removing alternator for a re-build a PITA or just usual grunt work?

    Your words of wisdom and caution are always appreciated.

  2. #2
    Genesis Guest

    Do you have belt or blower drive alts?

    If belt drive, just pull it off.

    IF blower drive, be aware of a few things...... first, there's a hub between the blower coupling and the alternator that has an oil seal in it. Those like to leak when disturbed, and even if just looked at wrong. The leak isn't bad, but its a damn mess and gets all over the gear. The only real fix is a "hub kit" which runs about $100 - and if you booger the seal on install, its junk and you get to do it over.

    You CAN press out the old seal and press in a new one, but it requires an arbor press and must go in EXACTLY straight or it will leak. Then again these often leak even when brand new - so its six of one, half-dozen of the other. Sometimes you get the good end of the deal, sometimes not....

    This is the one "lots of 4 letter words" leaks on the 6V92s that is basically impossible to prevent over time.

    Notes on this...

    Before removing the alternator, make sure the high amp (ANL) fuse and wiring are good. Note that on a LOT of Hatts ALL the heavy-gauge wiring was done with UNTINNED wire and over many years it will corrode internally and stop working, even though it LOOKS fine. If you can't flex the wire easily, its screwed no matter how it looks. Open the electrical panels and look - the fuses are marked. On most Hatts with a 12V system the alternator fuses are 80 amps. WARNING: Large parts of that panel and the Alternator primary leads are HOT even with the master switches OFF, and big parts of it are hot with a connection of 4/0 back to the batteries! Shorts WILL weld your tool(s) to the panel and cause severe damage, including the possibility of fires or electrical explosions. Either disconnect the batteries or be EXTREMELY careful - if you disconnect batteries check to insure that the panel is cold with a VOM before proceeding.

    To remove the alternator on a blower-drive engine disconnect the electrical connections (tape the ends and secure them - see the above note about them being hot!) then remove the bolts. The alternator will pull straight back and out. There is a coupler disk that engages the alternator and blower adapter drive pins to make the actual connection.

    To replace, line up the pieces and insert straight in, then re-bolt it up. Use a new gasket. Make sure you do not place load on the coupler (don't let it "hang" from the bolts when re-installing) to avoid damaging it and having a premature failure. They're somewhat of a pain to reinstall single-handed, but I've done mine before (I have the blower-drive models)

  3. #3
    jim rosenthal Guest

    Shouldn't he...

    ...be able to read the alternator output right off the terminals with a digital VOM while the engine is running? It sounds like the regulator might not be working. Do these alternators have internal or external regulators?

  4. #4
    Genesis Guest

    Internal regulators...

    ... the problem is that screwing around with a hot connection while the engine is running - and an unfused one at that - is asking for it.

    If they're blower-drive its REAL tight back there where the terminals are. You've got the raw water pump, gear cooler and shift cables and such and possibly exhaust piping and braces plus all manner of other stuff there. Its really, really easy to short something out.

    Yes, you can test with a VOM across the hot and ground of the alternator (there are two wires going to it; these are not case-grounded) with the engine running, but if the output is reading zero empirically (no charge is ending up in the batteries) I'd be checking the wiring and ANL fuse before anything else - you'd be really unhappy if you pull the alternator and find it is perfectly fine and the connections or fuse were bad - its pretty simple to check the cabling and fuse first visually, then isolate the hot side of the wiring (remove the ANL fuse) and make sure the hot wire itself is good to the panelboard. Similarly check the ground wire. If the cabling is good and the fuse intact then the alternator itself HAS to be bad. That's lower risk than screwing around with a high-amp connection on a running engine and IMHO it also gives you a higher certainty of being right before you start removing things.

    BTW I had a similar failure to this early on in my ownership of my boat - the ground cable swage had corroded internally (due to being untinned wire) and it literally burned off inside! Very visually obvious on inspection; a new piece of ANCOR 100% tinned wire, a couple of swaged connections and a half-hour had me back in business. The alternator diodes survived being run open-output undamaged (!); it is possible that it has "foldback" circuitry in the regulator that prevents it from developing output without a load across its terminals, so I don't know what it will read open-circuit.

    The blower-drive alternators are internally regulated (at least all of them that I've seen are.)

  5. #5
    divernc Guest
    I have the blower drives. Since it worked fine before sitting on hard for three weeks, I am going to bet on fuse and wiring corrosion issue. You are right, it takes a 3 armed monkey to work where the alternator is located.

    Where is the electrical panel for ANL fuses? I am guessing they are metal boxes on aft engine room bulkhead where the master switches are located.

    What is an Arbor press? Maybe not knowing means I am in over my head. If I remove and re-build it, is this a good time to have my mechanic re-install?

  6. #6
    Genesis Guest

    On the bulkhead

    normally for the ANLs, right under the master switches.

    There is a "door" you can open with a couple of screws, or you can take the entrie cover off the panel.

    Anyway, its behind there and should be labelled. There's one for each engine (each in its respective box)

    Not a big deal to reinstall - just gotta pay attention and jiggle things to get the coupler to line back up.

  7. #7
    Traveler 45C Guest

    Don’t run the alternator with the battery disconnected...

    That could blow the alternator.

    My port 871 is behaving the same way. It is most likely my wiring. That needs to be check first.

    Genesis, an automotive load tester could be used across the alternator to check it right?

    Greg

  8. #8
    Genesis Guest

    Yes...

    ... that will work, just be sure not to leave the alternator open when the engine is running! That's asking to blow all the diodes.

  9. #9
    mikep996 Guest

    I'm not so sure it isn't the battery(s)...

    In my opinion, an alternator going bad by sitting around for a while is rather unlikely. If the alternator will deliver 15 volts at 1500 RPM, this, in my mind, tends to point to the battery, not the alternator. I realize you checked the batteries but this symptom, is typical of a battery that will not hold/deliver it's rated output.

    Normally, the battery is supplying the power, NOT the alternator. The alternator simply charges and maintains the battery so IT can provide the power necessary. However, if the battery is not able to deliver it's rated power for some reason, then the alternator will try to pick up the load. The typical symptom for that is lights brightening or voltage rising considerably as the RPM rises. It doesn't happen if the battery (and charging system) are working properly.

    You can load test the battery or the alternator but that requires equipment which is not normally in the DIY toolkit. A multimeter can measure the voltage which is very easy but it can't measure sufficient amperage to be any use. You need a load tester which can measure hundreds of amps. Any mechanic shop would have one of these if you have a friend in that business...

    If it isn't too difficult, I'd try a known good battery - preferably a new and freshly charged one and see what happens before I'd mess with removing/disassembling the alternator. I'm not saying it can't be the alternator but I am saying that it is not the first place I would look. Batteries can test fine with a hydrometer and a voltmeter and not deliver their rated power. Load testing is one way to be sure but substitution is a pretty good and time-honored method as well.

    In any case, let us know what the problem was when it was corrected and good luck for an easy and cheap repair!

  10. #10
    mikep996 Guest

    I'm not so sure it isn't the battery(s)...

    If the alternator will deliver 15 volts at 1500 RPM, this, in my mind, tends to point to the battery, not the alternator. I realize you checked the batteries but this symptom, is typical of a battery that will not hold/deliver it's rated output.

    Normally, the battery is supplying the power, NOT the alternator. The alternator simply charges and maintains the battery so IT can provide the power necessary. However, if the battery is not able to deliver it's rated power for some reason, then the alternator will try to pick up the load. The typical symptom for that is lights brightening or voltage rising considerably as the RPM rises. It doesn't happen if the battery (and charging system) are working properly.

    As Jim R. and Genesis mentioned, you can load test the battery or the alternator but that requires equipment which is not normally in the DIY toolkit. A multimeter can measure the voltage which is very easy but it can't measure sufficient amperage to be any use. You need a load tester which can measure hundreds of amps. Any mechanic shop would have one of these if you have a friend in that business...

    If it isn't too difficult, I'd try a known good battery - preferably a new and freshly charged one and see what happens before I'd mess with removing/disassembling the alternator. I'm not saying it can't be the alternator but I am saying that it is not the first place I would look. Batteries can test fine with a hydrometer and a voltmeter and not deliver their rated power. Load testing is one way to be sure but substitution is a pretty good and time-honored method as well.

    In any case, let us know what the problem was when it was corrected and good luck for an easy and cheap repair!

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