According to my gauges, with A/C on, alternator seems to be fine, but at idle, (in gear at light A/T) voltage drops below 12v, (11.9 or 11.8v).
Is this normal or does it need rebuilding? Just bought new battery, and still the same. Thanks in advance for any help....
89 coupe w/90,000mi.

 

....... Take the car to Autozone or Advance Auto Parts and have them load test it for you ... no charge ( no pun intended ) ....................

 

Thats somewhat normal....although these are supposed to be a high output 105 amp alt instead of the usual 90 (?) and they DO keep up when new, as the system ages they tend to idle at about what the alt can produce with all the drain.
If it jumps up to 12.5 or 13 just above idle it should'nt be an issue. I've been thru many alt in the last 23 yrs on C4s and MANY do not do this good....alot depends on how the voltage regulator is set, and messing with it seems to shorten its life so...

The real test is immediagtely after start up the gauge should show 14V + to build the battery after the starter sucks a bunch of amperage out...you should see that for a few minutes then have it settle to 12+ or 13...

You;d be amazed at how much the HEI sucks out ! The ignition system uses as much as the lites and a/c on high put together.

 

GM has issued countless Bulletins - one in particular chastizing Dealers for replacing good alternators - about seeing Battery Voltage, dimming headlights, etc with high loads at idle. Your alternator is putting out about 25, maybe 30 amps at idle. Worn Blower Motors can suck up 25 of those amps, the Coolant Fan another 10, add everything else along with 21 year old wiring and it's on Battery. And all of this is a good reason not to run the Aux Fan on these years (which shouldn't be running anyway if the system is right). The real solution of course is to yank out the Harness and replace all the components, but doing that might indicate a need for some quality couch time with a shrink. I would get yours Load tested. Check the Blower Motor connector for discoloration and if it's there, then it's heating up and dropping volts because the crimps on the wire connectors are toast. Take apart the Weatherpak and solder it together. You might also check the Coolant Fan Connector or if you have Clampon Meter, see how many amps it's yanking out of the system and if it's higher than 10 amps, replace the motor. You can also try some of the solutions - like a higher low speed output alternator - offered here: www.alternatorparts.com - Note: one of our Members did try an "Iceberg" version years ago and toasted it in 2 weeks. Finally, what you're seeing on the Display usually reflects a voltage drop across the ignition switch that runs anywhere from .2 to .5 volts. You can clear it up (well, for awhile at least) with some Electrical Contact cleaner. That'll get the Display back into the 12's which you should see if you put your Meter across the battery.

 

might check idle speed at those low readings, if its low also, increase it a bit. i actually think a rebuilt alternator would be the way to go, you may have a diode goin bad

 

The only way to increase idle speed is to re-program. You can step down in Pulley diameter - there's an OEM that's either .5 or 5 mm smaller (I can't remember). Check the Link or go to a rebuilder.

 

This is exactly what is happening. If I increase the idle just 100rpm, the voltage will rise above 12v. Voltage DOES jump to 14v after startup, then drops to around 13.5v afterwards. I really don't want to increase my idle because I would have to hold the brake pedal much firmer to prevent the car from going forward. It is at proper idle speed now, and I don't want to mess with that. If it is common for the voltage to drop below 12v at idle with a/c on, that is really what I needed to know.
I WILL get it "load tested" just to make sure. TY everyone.

 

The internal voltage regulator should accommodate AC and give you around 13.5 volts. If you are dropping below 12 volts while idling smoothly you need to get the alternator checked. I just got mine rebuilt for under $60 with tax.

 

Is this true even at idle? My only problem is when sitting at the light, in gear (a/t), with A/C on, voltage drops below 12v. My idle is smooth, but sounds like it's laboring. Once I'm driving, everything seems normal. I haven't had it load tested yet, but that's next.

 

All I can tell you is that mine was doing what yours was doing and I got a rebuild. It would drop to 11.8 volts. It is not doing that anymore. The difference in our cars is yours is auto, mine is 6 speed, but I think target idle in neutral is about the same. Your Idle in gear may be a little lower than mine. Not sure.