GM Alternator Questions

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Feb 7, 2002
Pendleton Oregon
My stock alternator wasn't doing its job. I couldn't run the wipers, heater, and lights at the same time without everything slowing down or getting dim. So the amp needle would be on the negative side a lot, especially when I step on the brakes.

So I replaced my alt with a GM 135amp beast! I followed directions from the web to a T. I got everything hooked up correctly (far as I know). The cruiser runs great only problem is that the amp needle fluctuates from positive to negative now depending on the RPM.

So here is my problem:

1. At idle no accessories on: amp needle is below the mid mark on the negative side. (Volts @ battery <mailto:Volts@battery> 12.7)

2. RPM 2000 no accessories on: amp needle midway on the + positive side. (volts @ battery 13.99)

3. At idle all accessories on: amp needle pegged on the - negative side. (volts @ battery 12.16)

4. RPM 2000 with all accessories on: amp needle midway on the + side. (volts @ battery 13.96)

Why does my needle fluctuate between + and - so much? The battery volts never drop below 12?

When the car is running, voltage at battery or at Alt. should not be lower than 13, ideally 13.8 I believe. It looks like it is not charging at idle, is this a internal or external reg alternator? Perhaps you need a smaller pulley to spin the alternator a little faster at idle. Somethings up with the way the alternator excites itself to start charging. If this was a stock alternator it would probably be the voltage reg.
BTW, I had the same problem with the stock alt, I replaced the brushes and the reg and it now charges more than enough.
I think you are correct on the not charging at idle. It already has a small pulley, but I did come across this.

I need a different regulator! See below

Stolen from:

When used in the one wire configuration, the sensing wire is connected directly to the BAT terminal. The field terminal is not connected. If the engine is run at a high enough rpm the regulator will "leak" enough voltage to the field to energize it and the alternator starts. The trick is to get the regulator to "leak" at a lower rpm, and thereby "self energize". Replacing the regulator with one that "leaks" is covered in the next section.

So it looks like I will be buying one of these one wire conversion kits

I really hate anything to do with electricity! (troubleshooting that is)

I thought when you replaced the paltry 30amp Nippondenso alt. with a 130 amp GM you have to change to a voltmeter or at least take the stock ammeter out of the circuit since its not built to handle more than 30 amps? I would like to know cause I might need to go to a H.O. alt sometime in the future.
Very good question. Looking at the tech links and on the web I haven't heard anyone replacing/disconnecting the amp meter. Though it does make sense! It seems to be handling it fine at high RPMs.

Sounds good! let us know how the new reg works out for ya, I'm sure there are others contemplating the upgrade as well.
I will...I'm not looking forward to taking the alternator apart though  :dunno:
It's not bad at all. At least the stock one wasn't. An hour for dissassembly and install of brushes, 15 min for reassembly and install. I bet alternators have changed in 30 years tho.
Just a shot in the dark if you can't figure out the problem... I remember considering upgrading my Mustang's alternator a few years ago with a HO replacement. Every place I looked for one (I remember JC Whitney specifically) had a small disclaimer indicating that at idle speed, the alternator actually had LESS output than stock ones. This seemed to be universal of all HO alternators at the time. They actually seemed to give a little at idle to gain a little beginning at around 2k. This is why I decided against getting one, figuring a lot of my driving in that vehicle is spent in traffic and/or going slow.

I'm sure there is a remedy for this, i.e. a smaller pulley but just wanted to make sure you were aware of that.
Why did you switch to a 1-wire setup?

It seems simpler to just connect the field wire to a switched 12V source with an idiot light inline. No need to take the alternator apart unless it has faults.
It seems that you need to replace the stock regulator with one that will enrgize at a slower RPM. This way you will get more volts/amps at idle.

Are we talking about your alternator or mine?:dunno:

I use the stock Delco 3-wire setup and have no problems (yet!:D). I'm just wondering if maybe now that you know why yours doesn't charge at idle, you might think it easier to add a wire than to replace the regulator. Either way will work, you just have to decide which you like better.

Also, Pete has a good point. You generally have to sacrifice some low rpm output when you go to high amperage alternators. That means you will see your lights go dim at road's end on a cold, stormy night (headlights on bright, heater on high, wipers beating, stereo pumping), when all you wanted was a place for you and your girl to watch the submarine races!

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