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Discussion Starter #1
I have a LS376/480 with a 4l70e that I put in almost 3 years ago. I got the Holley accessory drive which came with a 130a alternator. The first one lasted about a year but then went out 1 day. I got another one and it went out in about 100 miles. I turned that one in for warranty and got another. That one lasted me up until about March of this year and it started acting funny so I swapped to a spare. The spare only lasted a couple months and I just put on another this weekend.

At first I thought I may have just had a bad couple alternators because we all know the quality of parts nowadays. There is no way that I could have gone through this many alternators without some other issue.

I started doing some testing and some research. I am going to add a more grounds just to make sure that isn't this issue. I tested my battery wire to my alternator for correct sizing by measuring voltage drop at full load and I am only dropping .2 - .3 volts so that seems fine. I have a brand new optima battery that I put in about 2 weeks ago. At full load at idle (A/C, cooling fans on high, headlights, and stereo with 2 amps on) and I am getting 14.3 volts at the battery terminal on the alternator which is right where it needs to be. So it seems the alternator is fine with the load. Everything is functioning correctly in the car.

Anyone else have any issues or offer any insights?
 

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When it goes out, what symptoms are you experiencing? What harness are you using? Are you using the Holley alt pigtail?
 

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Discussion Starter #4
I talked to a bunch of different people (GM Tech Line, PCM of NC who did my fan wiring kit, and a really good local alternator and starter repair shop) and got mixed opinions.

The local shop mentioned I may need a resistor on the "excite" or "L" terminal wire because it is sending too much voltage and burning the regulators out. I only talked to them on the phone and I really doubt they fully understand how the wiring on the car all is because they have never seen it, but still I thought it may be possible. They also mentioned to make sure my grounds are all good.

PCM of NC didn't have much advice other than they had never heard of this happening (they specialize in late model GM stuff). I was mainly questioning them on the fan wiring just to clarify how it all functions.

I placed 2 separate calls to the GM tech line and both people seemed very knowledgeable and both confirmed there is no need for a resistor in the wire going to the "L" terminal with this harness. They said grounds could be a possible issue but also mentioned the possibility that I may have just gotten several junk alternators in a row because that does happen. Getting junk alternators was my first thought, but then when you are on alternator #4 it makes you start to wonder.

So here is where I am at now. I added a couple additional grounds because that really cant hurt. I have a ground from each side of the engine to the frame. From the same bolt on the frame I have a ground going to my firewall. I have ohm checked my body, engine, frame in several spots to make sure there is good connection between them all. So I am going to roll with things and see what happens now.
 

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If you don't have resistance on the "L" terminal then I recommend that you add some. The "L" terminal is used to excite the rotor windings when the alternator is not turning but after the alternator starts turning the alternator will provide its own excitation and it also adjusts the amount of excitation to keep the alternator at the proper voltage. Without the resistance the alternator will have a tougher time maintaining proper voltage since the self-excitation current is back feeding all the way back to the battery. In the original circuit the incandescent bulb in the dash for the alternator provided this resistance. Putting a 510 ohm resistor in parallel with this lamp is a good idea in case the lamp should burn out or if you don't have this lamp anymore.

Another thing I would check is the "S" terminal. This is the remote sense terminal of the regulator and if the connection is intermittent it will cause the regulator voltage to swing high and may cause it to shut down. Although I would think you would see this on the voltage gauge if you were watching for it.
 

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I thought the resistance was included in the GMPP harness? At least that has always been my assumption.... I think that is why GMPP tech said no additional resistor is needed.

Don
 

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Discussion Starter #7
I thought the resistance was included in the GMPP harness? At least that has always been my assumption.... I think that is why GMPP tech said no additional resistor is needed.

Don
Yes Don is correct according to GM. The tech said there is no need for a resister because the computer adjusts voltage as needed. Also for reference, I am using a 4 pin alternator with 1 wire going to the "L" terminal. No other wires are used.

My other thinking is there are countless people who have used this exact harness (several on here) and Googling doesn't show me any results for that exact issue. There are enough people on different forums talking about LS swaps that if this was an issue it would have been talked about.

I appreciate the input though. You are 100% spot on for an LS swap where you may be using a wire going to your alternator from a switched 12v source that isn't coming from the computer.
 

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How did you rewire the cars front harness with the old voltage regulator and all that stuff? All the old charge circuit and wires should be bypassed and/or abandoned.

It’s been awhile since I did mine but had the same setup, GMPP LS3 and GM harness. I seem to recall my charge wire going to the starter lug, I used that as a junction for the battery since it was in the trunk. Power center ran from there too. I kept everything as short as possible and nothing ‘extra’.

Your engine harness grounds are very important. The engine to frame ground is needed, one good ground. The factory had the subframe then grounded to the body with a strap. You don’t need anything more than that.

I don’t know where this resistor would go. Charge wire and harness plug was all I remember. Alternators didn’t have all the terminals like the old ones. These are internally regulated alternators.

Have you checked to see if your overloading it? Voltage alone won’t tell you that. If it goes out again I would take the alternator to a rebuilder and have them build it and do some post mortem diagnosis to see if the can figure out what failed.

I don’t think you go four bad alternators.
 

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Discussion Starter #9
How did you rewire the cars front harness with the old voltage regulator and all that stuff? All the old charge circuit and wires should be bypassed and/or abandoned.

It’s been awhile since I did mine but had the same setup, GMPP LS3 and GM harness. I seem to recall my charge wire going to the starter lug, I used that as a junction for the battery since it was in the trunk. Power center ran from there too. I kept everything as short as possible and nothing ‘extra’.

Your engine harness grounds are very important. The engine to frame ground is needed, one good ground. The factory had the subframe then grounded to the body with a strap. You don’t need anything more than that.

I don’t know where this resistor would go. Charge wire and harness plug was all I remember. Alternators didn’t have all the terminals like the old ones. These are internally regulated alternators.

Have you checked to see if your overloading it? Voltage alone won’t tell you that. If it goes out again I would take the alternator to a rebuilder and have them build it and do some post mortem diagnosis to see if the can figure out what failed.

I don’t think you go four bad alternators.
All my old voltage regulator etc are long gone. The wiring going to the 4 pin plug on my alternator comes from the GMPP harness. I have a essentially a battery cable going from the back of my alternator back to the battery. Battery is in the front of the car. By overloading I assume you mean that I am pulling more amps than the alternator can put out. I thought of that as well and I don't have anything out of the ordinary electrical wise. LS3 with a 4L70E, dual electric fans (the superior radiator ones), electric fuel pump, vintage air and a stereo with 2 amps and Dakota Digital gauges. If I have any further issues I am going to take my alternator to a local shop that specializes in that and see what they have to say.
 

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Back in the day when i used to do car audio there was a formula that we used to determine how much power we needed to safely run the equipment

Later i will dig up some of my books and post it
 

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You might get/borrow a starter current meter, the type of meter slips over the cable, and see how much current is being converted.
Is alternator case hot to the touch? The higher the current demand, the hotter the case.
And, if two ground cables between engine and chassis/body, remove one of them - eliminates ground loop, more prevalent in AC circuits than DC circuits.
The battery is a parallel load in the electrical circuitry of car.
There should be a cable from alt BATT stud to distribution block, usually horn relay buss bar, then power lead to fusebox, and lead to battery junction block.
Yes, I read a cable from alt BATT stud to pos clamp, and may not be a bad idea, but, you might try a resistor in the line like OE design.
Can't hurt.
 

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Resistor in what line?
 

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Discussion Starter #13
Resistor in what line?
Alternators need some sort of resistance in the "excite" line or on the style of alternator I have the "L" terminal if they are wired directly to a switched 12v source. This resistance can take a couple different forms. You either need an "idiot" light or a resistor that provides the same amount of resistance as the bulb would.

That all being said, according to the GM tech line claims that the GMPP engine harness (that has a "Gen" plug for the alternator build into it) doesn't need a resistor because the signal is coming from the computer and not just a switched 12v source. From my understanding the reasoning for the resistor is that the voltage from this "L" terminal would be too high and cause the regulator to overcharge and thus burn out the regulator.
 

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I’ve used the GMPP harness at least 6 times or more and all builds still have the alternators I installed running without any problems.
 

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I just went through this with my Holley Mid-mount serpentine belt set up on my LS3. You NEED that resistor...
GMPP harness? It’s incorporated into the GMPP harness. No need to add one. Pretty sure Speartech includes it too....

Don
 

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Alternators need some sort of resistance in the "excite" line or on the style of alternator I have the "L" terminal if they are wired directly to a switched 12v source. This resistance can take a couple different forms. You either need an "idiot" light or a resistor that provides the same amount of resistance as the bulb would.

That all being said, according to the GM tech line claims that the GMPP engine harness (that has a "Gen" plug for the alternator build into it) doesn't need a resistor because the signal is coming from the computer and not just a switched 12v source. From my understanding the reasoning for the resistor is that the voltage from this "L" terminal would be too high and cause the regulator to overcharge and thus burn out the regulator.
I see. I just didn’t remember any other terminals l guess. Remembered the alternator had the harness plug and charge lug.
 

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Discussion Starter #18
I just went through this with my Holley Mid-mount serpentine belt set up on my LS3. You NEED that resistor...
I know the pigtail that comes with the Holley kit has a resistor in it. GM says their harness doesn't need it. The other thing that reaffirms things for me is that I know there have been literally thousands of crate LS3's sold and yet there is nobody talking about needing a resistor for the alternator on any forums. It surely would have came up in multiple places if it was a known issue.

UPDATE: I drove to a car show this weekend with my new overkill alternator 253a. The one thing I noticed is that previously when running the air and everything fully heated up I would be running around 13.5 volts at cruising speed instead of the 14.5 I would get without the air and fans running. Now at cruising speed I get 14.5 volts continuous and the only time it dips slightly is at idle. Maybe the alternators were struggling to keep up and thus burning themselves out? I guess time will tell.
 

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no... my harness is a Holley Terminator & Holley Mid-mount accessory drive. The resistor was included with the alternator. I didnt use their alternator pigtail at first... and it wouldnt put out more than 12v. Added their pigtail with resistor... 14.5v!
 

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Discussion Starter #20
no... my harness is a Holley Terminator & Holley Mid-mount accessory drive. The resistor was included with the alternator. I didnt use their alternator pigtail at first... and it wouldnt put out more than 12v. Added their pigtail with resistor... 14.5v!

I agree with you. The Holley harness needs the resistor pigtail.
 
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