Hello, im driving a 1996 z/28, all of my mods are listed in my signature. Thanks.

Driving the other night all of a sudden at a stoplight i noticed my alternator gauge sitting right on the middle (12v) mark and slightly moving either direction but when it did move it moved erratically / suddenly and quickly. Just for a bit of info my gauge always used to set between the 18 and 12v marks, i would've guesstimated at about 15 or 16v. I also noticed when i turned on my lights the gauge immediately dropped about 5v's or so (enough to go in the red) but it immediately comes back up and resumes where it once was (all this happens in about one literal second). Another thing i noticed as i had stopped was my speedometer radically moving, it would jump from 5 to 10 or to 20 MPH just suddenly and and then fall. It bounced like this several times. Im not sure if this has anything to do with my current problem but my BCM is not working correctly, the RAP function in it has gone bad (my radio and windows would not work) and to temporarily remedy this i wired in a toggle switch. These gauges only give me these symptoms from a complete stop, when im moving again my alternator gauge takes about 3-5 minutes to slowly climb but will eventually read about where it normally used to and the speedometer immediately functions normally once moving. So in a nutshell, is there any chance that i have an alternator going or gone bad on me?
(btw: car has 150k on it and idk if the alternator has ever been replaced on it yet so keep that in consideration please)

 

You should probably pull the alternator and have it tested at a place like Autozone. They will be able to tell you if it's bad or not. If it is, hopefully it didn't take your battery with it. As a quick check, if you have a volt meter or can borrow one, you can test the voltage output of the altenator while the car is running by testing the post off the back of the alternator. Should have somewhere above 12.5v. Above 13v prefered. Also check the battery voltage while running too. Should be about the same voltage as the alternator is putting out. Maybe slightly lower, but you don't want to see a big voltage drop there. Then check your battery with the car off. Should have 12 volts and above there also. If that is the original alternator, then with 150k miles on the clock, then i'd say your due.

Ken R.

 

If alternator checks out I would really suspect a loose connection somewhere. If volt gauge drops to 5V momentary when lites are turned on it sounds more electrical than alternator. Can you just leave the key on engine NOT running and see the same symptoms. Stick the voltmeter on the battery and see if it still has 12V when the gauge reads 5v. Make sure the 12V distribution post on pass side wheel well is tight, also check ground post right next to it and ground to frame connection. Could also be ignition switch loosing 12V to dash gauges, if lights stay bright when the 5v condition shows up suspect ign switch.