So about 3 days ago, I drove my 1998 C5 to work, on my gauge the battery voltage read 13-13.5 all the way there got back in my car after work, car through a charge system fault message then voltage read between 12.3-12.7. Parked at home, once I got back in my car later that night battery Volts read 11.2-11.4. At this point I know somethings off Woke up to drive to the advance auto, at made it but barely volts dropped below 10 and gauges shut off. So basically my alternator was not charging the battery or providing power to the system. Before doing any real research it seemed obvious to me and the guy at the parts store who was an old mechanic that it was the alternator, so I purchased a new one and put it on, still wouldn’t charge. I then realized I can get my original alternator tested on the machine and it came back good (wish I did this before wasting my money ) . At this point I needed to get my car towed, the tow guy came out and being an old mechanic, did the trick In the below picture, my alternator started charging everything again ! so this tells me something is wrong with the wiring or harness. The next day I started the car up and it immediately read 13-13.5 volts but it would eventually after 15 mins drop to 12.7 then after another 10mins to 11.4 then stabilize at 12.4-12.7. So this worries me that something is still off. why would it drop from 13 after driving, all the way down to around 11.4 just to go back up and stay at 12.7? Battery is an ever start built in 2019.

btw I took off the starter to put on headers a week before this started happening. No issues starting the car,

also can anyone explain if the temporary fix the mechanic did will cause an issue ? He said it would not be an issue and I could leave it like this or search for the issue through all the wiring.

Inserted the black wire from connector (far right red wire) to big wire under boot

 

Could very well be the alternator wire connection at the starter solenoid, also best to check voltage with a DVM across the battery terminals.

 

For future reference, previous posts have stated that it is better to rebuild the alternator rather than buy a new one.

 

I would get the battery tested to see if it is okay as the first priority .

If the battery is on it way out and don't keep the alternator charge current it could be "draining" the alternator voltage like you describe .
My alternator voltage is 14.6 volt and battery voltage is 14.4 volt when I measured it last year (shown below)

 

Put the wiring back to stock and clean the connections at the starter solenoid and see what happens.

 

thank you just did the measurement with the DVM there is 12.3 volts there with key off

 

OK, let’s do a voltage drop check on the positive and negative side of the charging circuit…with car at idle and electrically loaded (lights on, AC on, blower on high) place one lead of DVOM on battery positive and the other lead on the B+ terminal of alternator…what do you see ??…don’t worry if you see a negative voltage…should be .1 to .3 volts !!…now take one lead and place it on battery negative and the other lead on a clean spot on the alternator housing…what do you see ??….0-.1 volts or so is good !!

 

hey I did the battery positive to B+ terminal which I think was the big red wire under the boot, it read 9.38V. I did the check to the housing and it read around .013 but I will try that again. (Btw thanks for help it takes me awhile to reply due to the amount I work during the week )

 

Yes, you check from battery positive to the big red wire under the boot…if you are indeed reading 9.38 you have a HUGE voltage drop….you only have 4 volts getting back to the battery. This is what I see engine running under load !!

 

yea and as I was running it with the load my battery voltage gauge just kept dropping, so yea the voltage from the alternator is not getting back to the battery, and now the temporary wire that the mechanic did is not working at all, so I’m back to square one with no clue on where the issue lies, as suggested by one of the other members I will get a closer look at the started connections I did a brief feel and everything look and seemed tight

 

will try to do this today. Thank you

 

Disconnect the battery and get back on that starter lug. ENSURE the nut is tight! Tighten battery and check voltage again. If still there...
Check for continuity on your main alternator to fuse block wire/cable. If continuity is there...
Check continuity in the harness pins for the alternator...

If all is good, report back.

 

Yes, most likely the solenoid connection. It could also be the wire from the alternator to the solenoid so check the fusible link while you are there. Feel it for rough spots and also make sure you can't stretch it.

Don't just tighten it. Take it apart and clean the lugs before putting it back together. You can put a bit of grease over the lugs to help protect them if you want.

 

I found the bolt on the starter loose, as I was tightening it this is what happen, the connection point came completely off the starter. I’m assuming this is not supposed to happen and this may be my issue.

 

pl
That's called an oops. did it just fall off when u appled little force or u were putting some force to it?

 

well i was applying little force , could only get a small turn with the ratchet as the space was limited , but it kept turning with little force, and then just snapped off,

 

Hate to say I told ya so...but, yeah. I know this suck from first hand experience.

 

That nut is only supposed to be tightened to 89 lb in

 

but is it supposed to still turn with little force ? I wasn’t putting any muscle through the turns

 

hopefully this solves the alternator not charging the battery issue