Quick Alternator Question - Impala Tech
Electrical & Wiring Troubleshooting electrical problems

 
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post #1 of 16 (permalink) Old 07-04-2018, 02:27 PM Thread Starter
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Quick Alternator Question

Hello all,
1968
396/TH400
Would a 12SI 100 amp alternator be enough to run my car and also a drop-in replacement for the current alternator? The current one is 61 amps and I'm pretty sure it's not charging the battery. Pic of the connections on the back attached.
I currently have an HEI conversion (not sure if that counts) and will have an MSD Ignition box and reasonable stereo/amp at some point in the future. Thanks for any replies.
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post #2 of 16 (permalink) Old 07-04-2018, 02:52 PM
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I think the biggest 12SI is only rated at 78 Amps (though you can rewind the stator fields and Armature windings to increase the rated power levels). The 10 was a standard size frame with the 12 being used in marine, cop car and medium duty trucks as it had a higher rated capacity to keep more lights lit. Bigger frame holds more wire but it still bolts in place of a 10.

The problem with an SI series is the transistors are weak compared to the CS that has newer technology and reverse cooling of the transistors and diodes in the voltage regulator to increase power and reliability.

The DN and SI were designed to be nothing more than battery chargers with a bigger battery being used to provide more power for devices with these style alternators. The advent of computerized controllers and digital sensors required GM to power the car entirely off of the alternator (why the lowest rated powered for a CS style alternator is 105 Amps and the run up to 230 Amp; ratings on cop cars). In addition to provided a rock stable voltage they output their rated Amperage (if needed ) just above idle instead of requiring the motor to spin up to 3800 RPM to reach rated out put the way a SI does.

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post #3 of 16 (permalink) Old 07-04-2018, 03:12 PM Thread Starter
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Thanks for the reply Dave. Screenshot of the alternator in question. I trimmed results for case style and amperage.
As far as the better CS alternators, how big of a deal is it to convert? Brackets, wiring? I don't want to gut the electrical system as I'm a little project-weary at this point and just want to drive the car for the rest of the summer before I get really nuts this fall and hang a third pedal. Thanks again.
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post #4 of 16 (permalink) Old 07-04-2018, 05:27 PM
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A CS style is called a one wire as the only wire that has to be connected is the power supply wire to the battery (which has to increase in size because you are doubling the current in the wire, as power goes up the wire size has to increase as well).

The other wires can be installed but because the windings are self exciting. Current will flow at full rated power as soon as you rev the motor off idle (has to get above 2,400 RPM). This is why most like this alternator, it is simple to wire up.

It bolts up using the same brackets as you used with the DN or the SI series alternator (standard GM upgrades go in place of what preceded it).

If you buy one with a serpentine belt do not remove the front shield and reuse the new fan or it will overheat and die. Just remove the serpentine pulley and replace it with a V-belt pulley (if you can replace it with a dual V-belt pulley off of the Cadillac or Buick the belt(s) will last longer).

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post #5 of 16 (permalink) Old 07-04-2018, 05:37 PM Thread Starter
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Dave, thanks so much. The CS I'm looking at comes with a V-belt pulley. Last question. 14.9V or 12V? I'm assuming 12V but I'm terrible at this electrical stuff. Thanks again and have a great 4th.
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post #6 of 16 (permalink) Old 07-04-2018, 08:28 PM
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I am not familiar with a 14.9 Volt alternator. Sounds like a 16 volt system where you had a 16 volt battery (had two positive terminals; one 16V and one 12V) that you ran the ignition off of and everything else was running on 12 volts because even if the battery ran down during your race day you had enough juice to keep the motor running even as your electric fuel pump was only moving half the gas that it did at the beginning of the day.

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post #7 of 16 (permalink) Old 07-04-2018, 09:20 PM
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I bought this one from Jegs, wired her up and haven't had any issues. I did upgrade with a billet bracket for it though because my headers made mounting a PITA with anything less.
It's an easy upgrade, you can do it man!!


Happy 4th of July!

713-7294
Alternator 100A 12SI Nat
$121.99

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
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post #8 of 16 (permalink) Old 07-06-2018, 03:49 PM Thread Starter
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So I went with the same PowerMaster 7294 and did the single wire setup. Do I need to change anything with the old external regulator or is it good to go and I only need to cap and stow the old wiring? Thanks again for all the replies.
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post #9 of 16 (permalink) Old 07-09-2018, 07:52 PM
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You need two out of the four wires. Here is a schematic drawing of the process:


This is for a 12SI but it is the same mod to the wiring. You are by-passing the points part of the regulator as the voltage is now controlled digitally. But you still need those two wires to complete the charging circuit.

Big Dave
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post #10 of 16 (permalink) Old 07-09-2018, 11:19 PM
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I need to look at my wiring to see what I did. It's been a year or 2 and I don't recall how I left that old external regulator wiring.

HOW A NOVICE REBUILDS A 66 IMPALA CONVERTIBLE:
http://www.impalas.net/forums/blog.php?u=1432
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post #11 of 16 (permalink) Old 07-10-2018, 09:41 AM Thread Starter
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Ok so if I'm understanding the mod correctly I need to splice the white/blue leads out of the old alternator into the red/brown wires coming out of the regulator. Attached is a pic of the back of the old alternator.
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post #12 of 16 (permalink) Old 07-10-2018, 10:29 AM
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Yes! American Automotive Wire sells a plug for the alternator end as the SI uses different termination. This way the alternator is "hot" as soon as you turn on the key, you no longer need to rev it to get it to light up.

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post #13 of 16 (permalink) Old 07-10-2018, 12:44 PM Thread Starter
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Ok so I need to get the attached conversion plug and rewire per Dave's schematic for 10DN to 12SI. One last question. I wired up the 12SI per PowerMasters included instructions. Do I still need the alternator-to-block ground wire I added? Don't see a ground in the schematic... Thanks again.
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post #14 of 16 (permalink) Old 07-10-2018, 03:54 PM
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Quote:
Originally Posted by 68WASAGOODYEAR View Post
Ok so I need to get the attached conversion plug and rewire per Dave's schematic for 10DN to 12SI. One last question. I wired up the 12SI per PowerMasters included instructions. Do I still need the alternator-to-block ground wire I added? Don't see a ground in the schematic... Thanks again.
You can not have too many grounds! Every electrical device on my 1958 Corvette had a separate wire for a ground all going back to a bus that was grounded to the battery negative post.

If you installed it leave it be.

I might add that, that pretty Chevy Orange paint is an insulator. It has to be scraped off the block to use the block as a ground. The block is grounded by a cable that attaches to the alternator bracket or the water pump that also has to be devoid of any paint (insulating plastic) to get a good ground. Further iron is a rotten conductor; copper is the best conductor of electricity or heat, then silver, before you get down to gold). The car is grounded back to the block via ground cables (plural). Two flat copper copper cables that attach to the transmission bolts on the bell housing and one that connects the block to the frame.

Here is a picture of new clean grease free ground cable kit for your Chevy:

How many are still on your car? Many are left off by lazy mechanics that remove them to work on the car and leave them off afterwords.

The bigger the copper wire (cable) diameter the more current it can conduct. Up until 1963 Chevy used to use a 00 gage battery cable. After that they dropped the size down to a 02 size cable to save money because the cost of copper ore went up after the world had used up all of the readily available ore lying near the surface. I still use a 00 gage cable on my starter and ground to the block that I bought from Taylor Wire.

I'm not an electrical engineer, (hated Smith Charts) I'm only a mechanical engineer, but I know enough to pay the extra cost to gain electrical performance.

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post #15 of 16 (permalink) Old 07-10-2018, 06:16 PM Thread Starter
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Thanks Dave. Same - I'm a mechanical engineer for a good reason. I also did thermo homework during circuits class and I'm paying for it extra now. Just not very intuitive to me. I grounded the alternator to the block by scraping off the paint from the valve cover and hold-down tab. Will leave it be and take a look at the other grounds, but I think they are all in place. Thanks again.
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post #16 of 16 (permalink) Old 07-10-2018, 06:53 PM
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Don't want to say how old I am but I was playing with cars that had a six volt system (and positive grounds) along with many powered by flat heads having four, or six cylinders.

DC circuits are as easy as PIE compared to a 440 volt Y or Delta circuit where whether the device blows the circuit breaker off the wall is the function of the in-rush current and phase angle. That was a very finicky motor that took us a week to determine why we couldn't get it to work. It was the only time I had to use calculus to solve a problem at work, not to mention some expensive test equipment to measure everything.

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