10-SI alternator checks good but discharges when in car.

Aggravation! I’ve got a 10-SI alternator installed on my L98 for my 58 vette and up until recently it worked fine. Now my amp meter is showing a discharge with engine running and a very high discharge with headlights on. I’m showing battery voltage at pin 2 of the connector. Pin 1 is not used on this alternator. Cleaned battery terminals - they didn't need it but I'm groping now. Pulled alternator and had it tested at Auto Zone. Checked good. Reinstalled, checked everything I could think of and still a discharge. Pulled it again and brought it to NAPA – same result. Any suggestions? All are welcome.

Run a separate ground wire right from the battery negative (assuming it's a neg. ground system) to the mount of the alternator. See if it helps.

34-40 prior to removing the alternator the second time I used my heavy duty jumper cables and did exactly that. Shorted the negative terminal to the housing of the alternator directly. No change.

What's the voltage doing?

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Originally Posted by firebird77clone

What's the voltage doing?

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Voltage drops a bit below static when car is running - maybe down to 12.4 or thereabouts
but I don't run it very long after starting. Just long enough to see if anything has changed. How about this - has anyone had problems with an accurate diagnosis on an alternator from a parts store?

I’ve never bothered getting an alternator tested at a parts store. 12.4 volts while running is definitely not good. It sounds like you have an alternator problem.

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Originally Posted by mr-natural

Voltage drops a bit below static when car is running - maybe down to 12.4 or thereabouts
but I don't run it very long after starting. Just long enough to see if anything has changed. How about this - has anyone had problems with an accurate diagnosis on an alternator from a parts store?

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Yes and most of them suck. As stated above you have a bad alternator, probably a shorted diode. They normally show open when bad, but that doesn't mean that it can't be shorted which, IMHO, causes your ammeter to indicate a very heavy discharge.

OK you KNOW the voltage is low, so next question is the regulator internal or external?

If external, then I'd get one and put it in. If that's not it then keep the old one in the glove box.

Another good thing to do would be to swap in new pig tail for the alternator, regulator or both.

If you do, then solder and shrink wrap the connections, no point making a new problem down the road by using a cheap crimp connector.

Here’s a good article on the 10SI and 12SI alternators: Catalog

Almost all electrical problems are due to dirty or bad battery cable connectors. So before you do anything, make sure the connections are bright and well attached. I always like to go directly to the engine block (for the negative) with a #4 AWG if the battery is in the engine bay or a #2 AWG if the battery is in the trunk. I use a hydraulic crimp tool and a tinned copper or solid copper terminal (number 3 below) at the engine. Again, make sure the cast iron is bright and clean. I also use a star washer between the terminal and the block with a stainless 3/8” bolt and a second star washer – it eliminates all connectivity issues and provides peace of mind. I coat with anti-oxidant (NOCO NCP2 is really good – about $12 for a 4 ounce can with a brush) when everything is tight.

At the battery, I use a direct crimp (number 2 or 4). If you want to use a more traditional connector, number 1 is okay in conjunction with number 3. Again, coat with anti-oxidant.

If you have number 5 type connectors you’re courting trouble.

Several quick tests – make sure to use a digital volt meter.

At a reasonable idle (500-750RPM) measure the voltage at your battery with the engine running. A good alternator will maintain battery voltage between 13.9 and 14.8 volts (14.2 is the optimum). With all accessories turned on, there should be at least 13 volts at the battery.

Another test is to measure between the alternator’s output (BAT) terminal and ground. If voltage reads 13.6–14.6, the alternator is fine.

All SI alternators have an internal regulator. To test (with the engine running) put a small screwdriver into the “D” shaped hole on the back and read the voltage. If the voltage goes up, replace the regulator. If the voltage drops, the alternator is bad.

D&B Electrical make a fantastic one-wire, 105 amp direct replacement for $60 – no brainer.
https://www.dbelectrical.com/product...p-10si-se.html

Let us know what you find.

Best and Happy New Year All!
Glenn

Firebird77, Ken, Driver, & Glenn, thank you all for your input. It's reassuring to get such solid advice in such a short time. Put on a new alternator today and all is well. Lesson learned. Now it's time to figure out the BIG issue -- white smoke after the engine comes up to temperature. I know that from that symptom alone the prognosis isn't good.