I have noticed recently especially with the hot weather more of a drain on the electrical system, where my alternator is reading only marginal Volts at idle (13 and below), with the AC on and sometimes the lights, which is probably normal under the circumstances.
But what I am concerned about is that the AUX Fan would be running when I pull into the garage (with the AC on) and only a very slight rise in my temp. I have a 180 stat in the system. In fact if I shut the engine down and turn the key onto the on position, the AUX Fan just runs. This only started in the last week or so. Now that fan is not supposed to come on until 238 degrees and shut off at 201 degrees. So I imagine that the switch in the block is starting to go. Disconnected said fan, battery voltage semed better, but not quite where it should be, at least for me.
Picked up my grandson from "LEGO " Camp earlier today (wish I had that when I was his age) the Vette was fine, got to his house and the idle dipped, Volts dropped, and the CEL & Battery light came on. Engine almost stalled, kept it running and it smoothed out. went home, checked for codes, nothing, that's when I undid the AUX Fan plug.
Went for a test ride, Volts are better, but still will dip when I add more accessories at idle.
Alternator is a rebuilt from Autozone, (Dura-Last) and supposed to be an 85amp unit. I did not add a bigger gauge wire to the starter with the increase of amps from the 70 amp unit to the supposingly 85 amp unit, which I probably should do, and was also thinking about getting a different brand, and maybe a 100amp unit that has the low idle capability to keep the Volts up at idle and through the rpm range.
Thoughts or suggestions welcomed...........thanks....Tom

 

the switch in the block may be okay .I would check the temp readings .

 

The volts sound normal for an older car with ac and lights on at idle. Maybe a hotter alt will help some, but may be limited by your wiring. The efan running would concern me though. Are you sure your not really that hot?

 

But it's a volt gauge, it tells you everything that's going on.

 

I should do that, but I am really sure that the temp gauge is working fine. With the AUX fan disconnected today, seemed to charge better, but the longer I drive, especially local, it still drops slightly.


I know, I guess I want that reading of 13.8 Volts all of the time, no matter what. If I go with a bigger amp Alternator, I will add an 8 gauge wire to the system.

ok "noonie" let's get your real thoughts.....
I know your just anxious to voice your opinion, and I am anxious to hear it......

 

I surely thought mrvette would chime in, he loves those voltmeters.

If you have a 72 amp 10si alt charging at 1600rpm shaft speed is 0 amps
A 78 12si alt charges 23 amps at 1600rpm shaft speed
10si has an open blade fan and 12 si has a closed blade fan.
Aftermarket conversions can be anything.
First I would check the amp output at idle and then at rpm. You should be getting at least 40 amps at 2000rpm engine speed.

Without testing it, it sounds like your alt can't keep up at idle (normal) and also isn't keeping up at normal driving speed. In other words, if you went on a 300 mile non stop trip with your accessories running, you would drain your battery completely.
BTW, an amp gauge would be showing you a discharge.
If your alternator can keep up at all rpms, then you should get roughly a 14vdc reading minimum. Your alternator should be able to run all your accessories plus keep the battery charged.

I would clean all the connections, battery, grounds on frame starter solenoid and bat+ on alternator first, then test again. Also clean the 1 & 2 terminals and connectors on the alternator.
Best way is to mix up a baking soda solution, dip the cable ends in until they stop fizzing, then dry and reinstall. Same with the battery posts, using a small brush. I like to use copper anti seize on the battery and frame connections to prevent any future oxidation.

Assuming your alternator fails the tests, you can go to a 94 amp Delco 12si alternator off the shelf for pretty cheap. Late 80s Buick with all the elec options. Direct bolt on.
There are CS alternators that put out 100 amps at idle, but require some conversion.

I would bump the charge wire up to 6ga if you go that way, especially if running a lot of accessories. It can never be too big.

The only reason I would bump up the amperage is to be able to run the Aux elec fan anytime the a/c is on, not for engine cooling, but a/c efficiency.

Let us know what the tests show.

 

"noonie" thanks for your reply.... I appreciate the time you took to explain. Can't do what you suggested until the weekend. Have looked at the grounds and connections before, and will do again, this time taking them apart, making sure that they are clean and secured correctly. Thanks again....Tom

 

I have electric dual fans on my 82 and my volt gauge will drop below 12 at idle. Usually if the lights are on or the wipers but our alt. are rated at 80amps.

I was either going to buy a mr amp alternator or try cutting down my pulley on a lathe. It looks like Mr amp is doing that too.


PS I have no problems at all in temps from 80* and lower.
Never drained the battery to the point of not starting.
Heavy traffic with AC and lights on will cause over heating as the volts get lower and lower but I only have electric fans.

 

Well although I haven't done the tests that "noonie" has suggested yet, I have noticed that the other day, I did not need the AC on during the drive home, as a cold front moved in briefly, and that resulted in positive reading on the Volt gauge in stopped traffic. Was reading above 13 or the 13.85 Volts as it's supposed to be. Of course as soon as I put on an accessory, you can see the drain on the system. I know it is normal, I guess that I am looking for that eternal 13.85 Volts all of the time, no matter what demands I make on the electrical system
I was also under the impression that the Alternators were 70 amp units from the factory. The rebuilt one I have is from AutoZone, while stamped 70A on the unit, is supposed to be an 85 amp unit according to their catalog. There are some companies out there, TuffSuff is one of them that offers the Alternators with different amps for the 10si & 12si units and have "an internal voltage regulator with a low idle cut-in to charge at very low RPM’s". I am looking at a 100 amp unit, and will sell the 85 amp unit on there now. So we shall see....
My temp does rise slightly under heavy load conditons along with the outside heat, but never goes above 190 or 195 degrees no matter what (I have a 180 stat), I know the PO had a new radiator put in and his friend owned the shop, so I imagine that the radiator is heavy duty, as it cools great.....