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Lets talk dual altenators again.

Discussion in 'The Garage' started by rjfguitar, Oct 25, 2004.

  1. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    It's been a long time IIRC since we had a dual alt. thread.

    On my charging system tester I am seeing the needle spike below 12V when my stereo system is going near full power and my dual electric fans are pulling. I have a GM large case commercial altenator rated at around 100 ampers. I could easely mount a second alt. underneath the main in the smog pump location and am wondering what to do about the wiring. I figure a standard GM small case with a self exciter (1 wire hookup) would work well as a secondary unit.

    There is 2 reasons why I don't want to just go to a larger altenator.

    1. I already have this large case unit that is nearly new and I don't want to overload the stock wiring harness from the altenator to the starter & battery.

    2. Dual altenators are...

    So could I run a secondary 63 or 78 amper self exciter strait to the battery?...I have a twin post battery and would probably go dual batteries too while I'm at it anyway. I am a good mechanic but don't know much more than the basics when it comes to electrical so lamens terms would be best. /forums/images/graemlins/tongue.gif /forums/images/graemlins/thumb.gif
     
  2. 84_Chevy_K10

    84_Chevy_K10 Banned

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    140-200 amp alternators are too easy to find for me to waste the time trying to put in two alternators.

    Increase the wire from teh back of your alternator to the starter in size to a #4 and run it straight to the battery and you're done.
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    Aren't even the stock 140+ amp alternators pretty weak when it comes to idle output? I know the older 42+ amp alternators are pretty weak at idle, but from what I've been reading, even the later ones aren't much better.

    Even with nothing else running, the older 78 amp units I've used (which worked fine BTW) show a marked decrease in output when you turn on just the headlights. (or just the heater, your pick) No better or worse idle performance than the 42 amp that was on there before.
     
  4. 84_Chevy_K10

    84_Chevy_K10 Banned

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    If idle output is too low, you've got an RPM problem that isn't going to be solved by putting in two alternators either. You need to overdrive your alternator if it's turning too slow at idle (which I have never seen a problem yet)
     
  5. 6.2Blazer

    6.2Blazer 1/2 ton status

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    I installed a 140-amp CS-144 alternator on my '90 K5. All it took were some minor modifications to the bracket and a harness adaptor, and I ran a 4-gauge wire directly from the alternator to the battery.

    From my understanding, the CS-144 style alternator have a greatly increased low rpm output over even the "hot-rodded" 12-SI versions.
     
  6. surpip

    surpip 1 ton status

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    PArt number? how and where did you get it? /forums/images/graemlins/ears.gif /forums/images/graemlins/ears.gif
     
  7. btolenti

    btolenti 1/2 ton status

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    subscribing... /forums/images/graemlins/thumb.gif
     
  8. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    PArt number? how and where did you get it? /forums/images/graemlins/ears.gif /forums/images/graemlins/ears.gif

    [/ QUOTE ]

    Autozone. Ask for an alternator for a 1995 Pontiac Transport with a 3.8 and rear air.
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    Tried it, doesn't work.

    Difference on any of the stock 10SI(?) alternators I have ever tested is at least 1 volt, typically 2 volts, as measured at the alternator, with as little as 500RPM difference. sellers page Of course those guys are trying to sell something, but look at that curve. If GM's alternators are designed similarly, but with the curve happening at a lower RPM, you see where the problem comes from. In any case, you can prove this with any alternator, at least any I've ever looked at...just put a voltmeter on it at a decent low idle, and increase RPM's and watch it. The voltage will "max out" with very little change in RPM from idle.

    There was a thread awhile back about overdriving the alternators, and I bought an overdrive pulley, and that does not solve the idle problem. 33% overdrive IIRC.

    I believe my Q-jet's idle was around 600RPM, while the TPI is 750, but still a difference when you rev it up with the lights on. This is with a stock pulley setup from an '87 Monte Carlo, so whatever problems are resulting, are due to GM's parts, not a mismatch on my part.

    Idle output is never what it is at higher RPM's, that's the nature of these alternators. I'd like to see some curb idle numbers on the 144 alternators before passing judgement on them as well though. They very well may have changed them enough to get better idle output, although I believe that is a function of their overall output being higher, thus lower RPM output is increased proportionally.

    If I had anything more than a 78 amp unit to test with, I'd be more than happy to bolt them up and measure the differences.

    Here's an old post with the overdrive pulley FYI:

    "I'm the one that bought Dyeager's altenator pulley. 'Had similar results: didn't help much at idle, but it did allow the volts to kick up alittle quicker off-idle. Works good with the diesel on fast-idle (cold) as it can actually start charging before "moving out". "

    (my results were about +.2V at idle)
     
  10. k20

    k20 3/4 ton status

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    *offtopic* How can one test the output of the alternator? My voltmeter rarely is at 13(center of gauge) its usually a tick or two below that. The batt w/ teh truck off is like 12.4v and with it running it like 14.5v or so. Is it the gauge or the alt? I never seem to have a problem of not enough juice, heater, headlights, stereo, wipers all on, and doesnt have super dim headlights. Im assuming its the gauge, just want to be sure.
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    You test the actual output on the stock alts from the single red wire on the back of the alternator. Battery test while running is ok too, but is typically off a bit.

    Testing at the "terminal end" of whatever is using the electricity is going to be less. Test it at the headlights, and it will probably be 2 volts less than what you see at the alternator. .5-1V less at the fuse panel is typical, so 1V less on your gauge is probably accurate. It *is* the voltage your gauge is seeing, just not what the alternator is putting out.

    As long as you know that (for instance) 13V on your gauge is "normal", if it goes above or below that more than what occurs normally in operation, you know you have a problem. Oil pressure is the same way. 10PSI one way or the other means nothing, as long as it's not harming the engine. It's just *nice* to have things be accurate.
     
  12. k20

    k20 3/4 ton status

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    *still offtopic* randomly my gauge will dip to like 8 and stay there for about 20secs...then jump back up to 12 or so.
     
  13. 84_Chevy_K10

    84_Chevy_K10 Banned

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    If it doesn't drop below 13 across the battery at any time, you're fine.

    dorian,

    I think you're more picky than GM's engineers. /forums/images/graemlins/rotfl.gif
     
  14. dyeager535

    dyeager535 1 ton status Premium Member

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    If it happens often enough that you can leave a voltmeter across the back of the alternator, watch it and see if the alternator is causing it.

    If it does it at idle, I'd be *less* likely to suspect a wiring issue, but if it occurs while the vehicle is under way and not while idling, I'd lean towards a wiring/connection issue.

    Voltmeter on the alternator will tell you whether the alternator is doing it or not though.

    Tim, why do you think my projects take so long? /forums/images/graemlins/smile.gif
     
  15. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    [ QUOTE ]
    If idle output is too low, you've got an RPM problem that isn't going to be solved by putting in two alternators either. You need to overdrive your alternator if it's turning too slow at idle (which I have never seen a problem yet)

    [/ QUOTE ] I tested my charging system at about 1200-1500 RPM's. Nothing like a good 'ol brick on the pedal. /forums/images/graemlins/rotfl.gif I am not interested in another hi output alt. I can get an used 78 amper at the wrecking yard for $20. So how would you wire it up?
     
  16. afroman006

    afroman006 1/2 ton status

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    I was thinking about doing this too. Seems to me if you were going to run 2 alternators and two batteries with one alt going to one battery and the other going to the other, it would be prety easy. Both alternators would be grounded to the engine, leave the stock one in place and unmolested, then put the second one in the smog pump bracket. Then run the positive cable to the positive terminal of the second battery and stick a voltmeter sender in there somewhere and wire a 2 way switch to your gauge and bada-bing, instant dual batteries.
     
  17. m j

    m j 1/2 ton status

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    how small was the "overdrive" alt shieve?
    what ratio did that work out to with your crank pulley?
    idle output is very much related to how fast you spin the sucker at idle
    engine rpm means nothing, alt rpm is everything
     
  18. fatboy

    fatboy 1/2 ton status

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    go with dual batteries and wire the sterio and fans of the aux and you wont have any complants. That is what i did. I have three amps, my head lights would dim at idle with the sterio cranked up but added another bat and problem solved /forums/images/graemlins/peace.gif /forums/images/graemlins/burb.gif
     
  19. dyeager535

    dyeager535 1 ton status Premium Member

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    To be honest, it's been long enough, that I don't recall all the specifics.

    The alt pulley itself was 2 5/8", and I believe the stock pulley I was dealing with was 3".

    Picked up a couple more alternators since then, an OLD unit
    has a 2 3/4" pulley on it, and one that came on a friends aftermarket alternator is 2 1/4". THAT one I'd expect to make some difference, but since I'm not semi-serpentine, can't try it out to see.

    The 2 3/8" was the smallest pulley I could buy or find at the time, (IIRC, it's what powermaster uses on theirs) and I could not find any crank pulleys to match up to PS and alt that would line up to play with those as well.

    I was disappointed enough that I didn't bother measuring much outside of pulley diameter and the voltage before and afterwards. Nowhere near enough of a difference to make it worth even the $10 I paid for the pulley.

    Still, as you mention, alternator speed is what makes the difference, 500RPM at the crank is enough to generally get all of them up at least another volt which eliminates the dim lights, etc.

    What the heck is a "shieve"?
     
  20. m j

    m j 1/2 ton status

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