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Old 02-23-2009, 05:59 AM
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Alternator selection

I need an alternator for my project and am very electrically challenged.
It will have power windows, air conditioning, stereo, electric fuel pump, and a trunk mounted battery. I've purchased but haven't yet installed a Ron Francis Express wiring system. I don't plan on an electric radiator fan, but it could have to happen. I was going to go with the Powermaster Powergen but can't find the space to mount it, and it's 75 amps may not be enough. It looks like I need a 10si or 12si size and definately a 3 wire but what amperage? My immediate thought would be 140 amp, but I'm concerned about overheating.

Also, can I change the clocking myself?

Thanks in advance.

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Old 02-23-2009, 08:18 AM
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Go with as much amperage as you feel you can afford, minimum 100amp would be a good guideline.

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Old 02-23-2009, 09:17 AM
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Remember this alternators only put out whats needed at the time. If you have a 140 and the load is 90 then it will make 90. When you add load, it will have the extra to make up the difference. With a smaller alt. (75amp) and larger loads you will have dim lights thing not working as they should. And in the end dead battery because the alt could not charge properly. Get the bigger alt.
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Old 02-23-2009, 09:25 AM
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Done deal! Thanks for the help guys.
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Old 02-24-2009, 02:24 PM
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I highly recommend a 12si. They run cooler due to the fins, have better internals and a rebuild kit from summit will cost about 25 to 30 bucks.
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Old 02-24-2009, 04:20 PM
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Make sure you also get a large battery or you will be burning through those alternators quickly.
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Old 03-02-2009, 01:13 AM
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I have had nothing but headaches with big amperage in 10 and 12 cases. They really aren't big enough to make much more than 85 amps without overheating and frying.

The amperage load that the alternator sees is usually only about 25% at most of what your total vehicle load will see. How many times do you plan on running power windows, power locks, stereo, and every single light at once? Even if you have a 30 amp alternator and you end up discharging for 10 seconds because the alternator can't keep up, it will be replaced in 20 seconds.

Here is where it becomes an issue. Those big-amp alternators are designed for people with BIG draws like a competition stereo that can get ahead of the battery. They do fine with big draws for short bass hits into a bank of 12" subs. In a normal car, you only need as much as you need, period. Now, the first time you leave the lights on, or you need a jumpstart, that little 12si case will start chugging out 140 amps and get hot... FAST. It will try to continue to put out 140 amps until the battery is charged, but there is a good chance it won't make it. I have third degree burn scars from my 140-amp alternator. I went through four like that; left the lights on, got a jump start, 20 minutes later it stopped charging and gave me 3rd degree burns. Replaced it under warranty, tried to jump start the neighbor's car, the alternator started smoking. Replaced under warranty again with their new improved design and left the interior lights on. Jump start and the next day it quit charging. I threw it in the trash and went to the parts store for a 73-amp replacement alternator. $36 and 87,000 miles later, it still keeps up.

If you truly NEED over 85 amps (which is ridiculous unless you have an ambulance) then don't consider a 10 or 12 case GM alternator. You need more. Be realistic about your amperage needs. Add up all the loads in your vehicle and multiply by 0.6. That's the amperage you need. Even if you operate every single electronic thing in your car for 50% of the time, it will still be able to keep the battery charged. Since you never operate everything at once, you'll have more than enough buffer to take care of the battery.

My 66 Bonneville is still using an externally regulated 34-amp alternator with power windows, an add-on cigarette lighter heater that I use as a rear defroster, 65/100w quad headlights, and a stereo with two 160w amps. The charge/discharge ammeter rarely dips below 0, but as long as it spends more time above 0 than below it, its fine.
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Old 03-02-2009, 07:01 AM
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When I mentioned a larger alternator, I did not mean a after market unit that's marginal because someone remanufactured it.

What I meant a original factory unit designed to handle 140 amp loads. What I'm using came off of a diesel powered E250 van, they were doing a conversion and installing a 250 amp unit.
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Old 03-02-2009, 10:03 AM
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A 12SI built with good parts will last a long time even under a real heavy load. You run the risk of burining up any alternator if you try to use it to charge a flat battery. They are not designed to do that. Read the lable on any new or reman, it will tell you to charge the battery first.
If the vehicle has high current requirements the first thing is to get a properly sized battery. The battery runs the electrical system and the alternator keeps the battery charged.
A huge alternator with a little battery is not going to work.
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Old 03-02-2009, 12:44 PM
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Quote:
Originally Posted by jaguarxk120
When I mentioned a larger alternator, I did not mean a after market unit that's marginal because someone remanufactured it.

What I meant a original factory unit designed to handle 140 amp loads. What I'm using came off of a diesel powered E250 van, they were doing a conversion and installing a 250 amp unit.
Ah... smart dude. I ended up retrofitting a 140-amp from a 96 Caprice.
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Old 03-02-2009, 12:49 PM
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Quote:
Originally Posted by T-bucket23
You run the risk of burining up any alternator if you try to use it to charge a flat battery. They are not designed to do that. Read the lable on any new or reman, it will tell you to charge the battery first.
Of course they say that, but if everyone did that, we wouldn't need jumper cables.
Quote:
If the vehicle has high current requirements the first thing is to get a properly sized battery. The battery runs the electrical system and the alternator keeps the battery charged.
A huge alternator with a little battery is not going to work.
Big time agree with you T-bucket. Its the battery's job to supply juice, not the alternator. The alternator is the recharger. If you try to run a car by choosing a big alternator it will always disappoint. Buying a large battery with a huge reserve almost never disappoints. I say spend the money on a good battery and skip the hi-amp alt unless its absolutely necessary.

I cringed when I bought my wife's mercedes diesel knowing that its requires a special mercedes-type battery that costs $200... but it has 193,000 miles on a stock battery that is constantly starting a cold diesel with glow plugs. I'm impressed. Good battery, 70-amp alternator, and I guarantee it has more electronic gadgets in it than anything you can build into a hot rod.
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Old 03-02-2009, 01:11 PM
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"Its the battery's job to supply juice, not the alternator. The alternator is the recharger."

The alternator provides power to run everything, the battery is there to start the car and cushion the impact of turning on accesories.
Then what juice has been taken from the battery is replaced by the alternator.
Voltage Regulator, Alternator And Battery Operation.
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Old 03-02-2009, 05:07 PM
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This is correct to a point, my point was a larger capacity battery will take some of the load off the alternator and therefor make the alternator last longer. The battery is the heart of the electrical system and must be sized properly.
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Old 03-02-2009, 11:24 PM
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Quote:
Originally Posted by T-bucket23
This is correct to a point, my point was a larger capacity battery will take some of the load off the alternator and therefor make the alternator last longer. The battery is the heart of the electrical system and must be sized properly.
The battery is there to start the vehicle, once the vehicle is running the alternator tops off the battery to full charge.
The alternator then runs the show, all the accesories.
As I said before when you switch on other items, electric fans for example the battery supplies the extra juice until the alternator can catch up.
My MSD and relays for the two fans I have are connected directly to my alternator, works perfectly.
The alternator is also connected to a terminal block to which is attached the wiring for the interior of the car,remote solenoid and the battery charge wire.
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Old 03-03-2009, 12:51 AM
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10SI & 12SI Alternator Information.
Remote Voltage Sensing

"The voltage regulator will adjust alternator output in effort to maintain that 14.2 volt level, under normal operating conditions. Therefore, most electrical system parts (lights, ignition, and accessories) are designed for best performance when operating at about 14volts."

How the alternator is told what power to supply to used accesories.
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