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Old 11-24-2015, 12:45 AM   #1
dhamilton62
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alternator suggestions?

Hey there, just wondering if anyone had any recommendations for a decent alternator? I have a new gm crate 350, I'm not running too many accessories like a stereo or a heater, but I do have dual electric fans. I just bought a new optima red top battery, but at idle the volts seem really low until it revs up past 1000. Also the dash lights want to dim at idle or slow speed moving in traffic and what not so I think the alternator might need to be replaced and with one that can handle more electrical components. Any advice would be much appreciated!
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Old 11-24-2015, 01:02 AM   #2
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Re: alternator suggestions?

Concider a smaller pully for the alternator to get it turning a bit faster..

Your also not saying if you already have an internally regulated alternator or the old external type.

Type "si alternator" into Google and read what comes up.
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Old 11-24-2015, 02:00 AM   #3
dhamilton62
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Re: alternator suggestions?

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Originally Posted by swamp rat View Post
Concider a smaller pully for the alternator to get it turning a bit faster..

Your also not saying if you already have an internally regulated alternator or the old external type.

Type "si alternator" into Google and read what comes up.


I was looking into smaller ones but I have a march serpentine belt kit on there and they seem to mainly only have one size diameter of pulley. Also I currently have an internally regulated alternator right now. Do you think a 100amp would be sufficient or something bigger incase I ever get a radio or a heater in the truck?
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Old 11-24-2015, 03:07 AM   #4
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Re: alternator suggestions?

I have been wondering the same also. Please have a look here:http://67-72chevytrucks.com/vboard/s...d.php?t=689354

There is a ton of information out there, just which way to go.
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Old 11-24-2015, 05:17 AM   #5
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Re: alternator suggestions?

100 amp will work fine as long as you arn't powering a mega stereo system with 8 sub woofers.
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Old 11-24-2015, 08:35 AM   #6
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Re: alternator suggestions?

You need to figure out how many amps you need at idle to properly answer this question.

Dual fans could be drawing 40 amps-or more-at idle. I believe most of the si alternators had roughly half capacity at idle, so an 100 amp si alternator is about 50 amps at idle. So you throw in ignition and some lights and it won't charge sitting still anymore. The cs144 alternators have better low amperage charging and are very robust. I would not even consider a cs130, they are not even good paperweights IMHO! The cs144 lowest amps is 140 I believe so it leaves plenty of room for future electrical expansion.
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Old 11-24-2015, 12:02 PM   #7
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Re: alternator suggestions?

You didn't mention how the alternator was wired up. Might look at the Mad Electrical web site. He talks about different ways to wire up an alternator and what happens (similar to what you said) when a one wire hook up is used. Read on his "Tech Tips" section or call him if you have any questions. His name is Mark and he's a really good guy. J.M.O.
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Old 11-24-2015, 12:06 PM   #8
GASoline71
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Re: alternator suggestions?

Grounds. Dimming lights at idle could just be bad ground straps. Check to make sure all the straps from the engine to the frame and the frame to the cab are hooked up and not corroded.

Gary
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I love using vacuum gauges as part of the carb tuning process. I hook the gauge to the inside of my garbage can and leave it there.
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Remember Murphys 2nd law of mechanical relationships... "OPPOSING COMPONENTS ATTEMPTING TO OCCUPY THE SAME SPACE, AT THE SAME TIME, GENERALLY END UP OCCUPYING ADJOINING SPACE AT THE BOTTOM OF THE OIL PAN"
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Old 11-24-2015, 03:41 PM   #9
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Re: alternator suggestions?

As stated, fix all the ground straps and ensure the components already in place are at least working correctly before you start replacing components that may be working fine.

Your battery should be between 12.4-12.6V (75-100% charge)
Using a DMM when idling, your battery should read 13.5-15 V, that indicates your alternator is in fact, charging correctly. As suggested again, look at the MAD website to understand the basics of AC generators.

You can check for voltage drop in your charging wiring by using your voltage gauge between the pos alt post, and pos battery post then check the neg battery post, and alternator chassis. (checking at the posts and not the wires includes the resistance of the connection itself, which may be adding resistance). Those readings will indicate if there's any voltage drop between the wiring.

If everything turns out okay, grounds are good, alternator is putting out a charge, but you still feel like it isn't generating enough current, there is another test you can do.

Not everyone can do this test because of the ammeter required, i'm just sourcing it here for the sake of information, and some may find it beneficial. This test will allow you to KNOW how much current your system demands with everything running. You'll need an inductive ammeter that can read high amperage. Do NOT use a 10A DMM for this. A VAT 40/60 will work well also. I'll include the info for other vehicles as well, not just classic trucks, so some may not have all the circuits available, you just use whatever you have that draws continuous current.

The Minimum required alternator output test.

1. After an ammeter is correctly connected in the battery circuit, start the engine and operate to about 2,000 RPM
2. Turn the heat selector to AC if you have it (heater is fine if you don't) and turn the blower motor to HIGH SPEED. (this bypasses the resistance in the blower circuit)
4. Turn high-beam headlight circuit on
5. Turn on rear defogger ( seat heaters if you have them)
6. Turn on windshield wipers
7. Turn on ANY other accessories that may use continuous current ( not power door locks or any of that jazz)
8. Observe the ammeter. The current indicated is the electrical load that the alternator is able to exceed to keep the battery fully charge.

Ideally, you want the minimum acceptable output to be 5 amps OVER the maximum number read.

Something like cooling fans and AC on while sitting in traffic at idle is okay to start drawing a little current from the battery. Obviously sitting for a long time (LA or NY traffic for instance!) can be a danger.

Hope that helps! Best of luck!
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Old 11-24-2015, 03:47 PM   #10
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Re: alternator suggestions?

One other thing. Some people don't care to do tests, or already know the alternator can't supply the amount of current expected through future upgrades, or needed because they know the amp draw of various components. In that instance, they may just want to throw a larger output alternator at it (Certainly nothing wrong with that if you have the $$ and the desire).

In that case, make SURE the wiring is capable of supporting the current supplied, and upgrade as necessary.

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Old 11-28-2015, 12:36 PM   #11
dhamilton62
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Re: alternator suggestions?

Thanks to everyone for all the great advice and help with everything! I made sure that all my straps for properly grounded, and rechecked the tests to make sure it wasn't a bad wire or something wired incorrectly. Again not charging enough at idle so I ended up buying an alternator through summit. First off the pulley on the new alternator was much smaller than the one that came with the March serpentine pulley kit so that made a different in the spinning speed at idle but either way, the battery is charging at idle once again. It's a one wire alternator, but just a quick question, where is the best place to hook up the Volt meter for the cab? Do I use any ign switched 12v source or is there a better spot? Thanks again, you guys are all really the best!
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Old 11-28-2015, 02:09 PM   #12
GASoline71
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Re: alternator suggestions?

Did you test the voltage regulator? It's located on the radiator core support by the drivers side headlight. Looks like a little black box.

Gary
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My 1972 GMC 1500 Super Custom (Creeping Death) "long term" build thread.

The Rebuild of Creeping Death after the wreck

Quote:
Originally Posted by LONGHAIR View Post
I would never rebuild a 305.
Quote:
Originally Posted by prostreetC-10 View Post
I love using vacuum gauges as part of the carb tuning process. I hook the gauge to the inside of my garbage can and leave it there.
Quote:
Originally Posted by Marv D View Post
Remember Murphys 2nd law of mechanical relationships... "OPPOSING COMPONENTS ATTEMPTING TO OCCUPY THE SAME SPACE, AT THE SAME TIME, GENERALLY END UP OCCUPYING ADJOINING SPACE AT THE BOTTOM OF THE OIL PAN"
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