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Old 04-15-2007, 02:03 PM
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How to tell what amp an alternator is.

Is their any way to tell what amp alternator that i have it is on a chevy 1951 truck it is a powermaster. What would be the advantage to go with 100amp if this is not. The alternator is chargeing at 13.8 to14 volts it is a one wire. I do not know if this is going to keep my battery at full charge the battery is an Optima red top. Thanks Turkey

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Old 04-15-2007, 02:26 PM
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Dear Turnkey,

The only way I could figure my old alternator out was to take it off truck it on over to Advanced Auto and have them put it on the tester.

As to the size you need that is more of a function of the total current draw when all electrical systems are running: lights, stereo, a/c and the like.

If you do not have high demands you do not need a high amperage alternator.

Test yours at the battery with your volt/ohm meter and all accessories on. See what it shows at the battery. If you are still showing 13.6 to 14. volts at the battery with all systems running you are good.

Be sure to protect your electrical system with a 50Amp fuse. This is necessary in case the battery goes flat and the alternator is now sending all of it current to every connection (due to demand from the battery) including the fuse block. You interior circuts are not desined to receive more that the fuse in its system i.e. 10-20-30 amps so if they start receivieng the full capability of that high capacity alternator, you'll have major problems.

Good Luck,

MrPhat 40
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Old 04-15-2007, 04:35 PM
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Doc here,

The Way to determine the ampacity of the alternator, Is pull the casting OR PN number off it and go to the powermaster site to confirm it's output..

FLA load testing is another option..but it's hard on the alternator.

The alternator is there to support the system, and TOP off the surface charge on the battery..NOT to constantly charge a battery from dead to full charge..No alternator was designed to do this.

The Common misconception Is : Because it says 100 Amps on the alternator that is going to be a full time output to the battery every time the pulley turns..

This is not true...The alternator is only going to put out the demands of the system until it reaches saturation point (13.95 to 14.4 volts)Then the regulator shuts it off..until a demand needs to be met again. Without it, the battery would fry and explode from overcharge..

If your System is a bone stock system, (no high current audio, Air bags, juice ect..) AND it is putting out 13.95 to 14.4 Volts..AND your not having problems, I'd say the system is functioning fine. You probably have a 63 / 70 Amp alternator in there and it's supporting the system with no problems.

Unless your planning some major electrical upgrades in the future, I would not waste the time or more important, your money..Why pay $299.00 for something that is working fine now..(and probably could be replaced with an over the counter $39.00 part should it ever pay for chrome and a name..)

To size the LOAD at the battery, you need to get a load test..a Voltage test will Show that Current is being drawn..not how much..The higher the current, the lower the voltage..You need that reading to be in series with the load and source, (between the Battery and the draw on one cable) And it must be in AMPS...Since most home meters only go as high as 10'll need the to have a load test performed..(high amp draws)..usually a freebie at auto zombie.

Always use a Fuse link or MAXI fuse on the Alternator output circuit and buss support wires..Unlike a standard fuse, the link or Maxi, require Time/Heat to open, and is more stable than a fuse..If you put a standard fuse in there, it will open every time the fuse sees a spike..(regulator shutting on / off under load) because it doesn't have "The Time Factor" So you usually end up over fusing it..which can lead to a fire..

Fuses are designed to "Protect" the wire they support, not the devices..This is why proper gauge wire is so important when installing harnessing..If one wire melts in a bundle because of a short, it will melt others..causing more short (pun intended) order you will have a the whole bundle melts..under gauge wire and over fusing is Almost always the culprit..AS is bypassing or NOT installing fuse links..

Under gauge/over-fuse causes the wire to heat..(current doing no useful work) and the end user devices suffer in performance. A light bulb will draw as much current as the filament resistance demands..(ohms law) the current available to it is predicated by the source (battery) and wire gauge, and resistance of the circuit (bad connections) If the current is lost in delivering it to the load..the bulb will burn dim..the wire will heat trying to overcome the total resistance of the circuit..So Wire gauge AND proper ground bonding are equally important.

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Old 04-15-2007, 04:55 PM
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The only way to find out the capacity is to have it tested or looked at by a rebuilder. If at any time it was repaired and either the armature or the stator were replaced the numbers on the case are no longer valid. Even if it hasnt been repaired you are depending on the 5 dollar an hour help that is building those mass produced alternators to label it correctly. I worked for a friend part time who is a local rebuilder for a year or so, we load tested every rebuild before they went out the door. The big builders just dont have the time to do this and this lends to their inconsistant quality. We used to see alot of the big name supplier units with very little time on them that failed due to shoddy workmanship. Yes we did see a lot of powermasters. Usually poor soldering or cheap parts caused the failures. Alot of these companies tout that their alternators are all new, well thats not always a good thing if they use inferior parts. You can tell how long they have been in use by checking the brushes
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Old 04-21-2007, 01:40 PM
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I had a stock GM 10si 63 amp alternator on my truck, and made an easy switch to a 12si 94 amp. Powermaster is a good brand, but stock GM 12si alternators are available everywhere. You just need to make sure the one you get has a V-belt pulley and not a serpentine pulley.

I think the rebuilt GM 12si alternator cost about $80 at O'Reilly Auto parts. I spent another $10-15 upgrading to an 8 gauge charge wire with a 12 gauge fusible link. Only NAPA carried the 12 gauge fusible links, since the more common stock GM link is 14 gauge. I ended up getting the 8 gauge wire at Lowe's because the auto parts places didn't have it in stock.

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Old 04-21-2007, 02:52 PM
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Dear 75GMCK25,

Just curious about your wire plan:
Is this how you wired your higher output alternator, 8AWG charge wire runs from the alternator to the battery terminal of the starter, with the fusible link spliced in at the starter post and the fuse box wire?

Thank you,
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Old 04-21-2007, 04:59 PM
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The stock charge wire was connected to the starter with a fusible link, just as you described. For my new wiring I ran a wire directly from the battery positive terminal to the large battery connection on the alternator. I left all the existing wiring in the stock configuration (both the original charge wire and the other two-wire plug on the alternator), so I still have a sense wire and everything switches off normally with the ignition.

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