Ok gents, I’ve been having subtle problems with my charging system that came to a head today.

About three weeks ago I got a P0621 code (Alternator warning lamp - circuit malfunction) that went away after clearing and never came back. At that point I started keeping an eye on my battery voltage indicator. I started to notice sometimes it would indicate charging voltage (14+ volts) and other times it would drop to 12 and under while running.

I haven’t driven her in about a week. Today got a one click, no start. Switched it off, back on, and she reluctantly started up. The DIC indicated 11.5 volts and never improved. After a few minutes I got the “Service Charging System” message so I just turned around and took her home.

Based on some digging on the forum I have already checked mostly everything. The voltage at the alternator is ~14v, give or take. All of the engine bay grounds look/feel clean and tight. I’m getting 0.5 ohms from the negative post to various chassis points.

However, between the positive post on the alternator and the positive terminal on the battery I am getting 5.5 ohms. Still low, but not what I would call a good ground. I understand there is a fusible link in the circuit between those two points.

To rule out the wiring/link between the two, I connected a jumper cable lead directly from the battery positive to the alternator post. Sparks, engine loaded down, got 13.1 volts on the DIC, going up slowly.

Im assuming the fusible link is bad/flaky? Visually, the connections at the starter look ok. Any other thoughts or ideas on this?

(Yes, the battery is right now on a charger. I know low 11 volts is dead and bad for the battery. Also, the battery is 6 months old, if anyone was wondering.)

 

Not sure this is relevant but I had charging issue that was resolved by changing the horn fuse in engine bay fuse box. might be worth checking.

 

Oh yeah. Good call. I checked all the fuses, including that one. Thanks!

 

You should really be doing a voltage drop check between the alternator and the battery…ohms is not the correct way…the ONLY thing I use an ohm meter for on any car are communication issues…nothing else !!…you also don’t want to measure grounds with an ohm meter too…with the engine running at 2000 RPM with an electrical load applied measure the voltage with one lead of DVOM on battery positive and other lead on the alternator B+…to measure ground side one lead to battery negative and other lead to a clean spot on the alternator…GM says max of .5 volts but .1 to .3 volts better…you may have a loose connection or corrosion at the starter solenoid where the alternator and battery cables are connected.

 

The high resistance reading caused me to suspect an issue at the starter. Being a worst-case thinker, I assumed fusible link. However, I found that the nut securing the alternator-to-starter and starter-to-battery cables on the starter solenoid was maybe just better than hand tight. Mild corrosion and evidence of arcing on the related hardware. After cleaning all the lugs, post, nut and re-tightening, I now get 0.2 ohms from alternator to battery, indicating a good connection. Back in business!

 

Hey Everyone,

I'm reviving this thread because I'm stumped. I recently bought a 2007 C6 base so my son can learn to drive a stick. (That's my story and I'm sticking with it. I definitely didn't buy it because I just wanted a C6 with a manual...LOL.) But anyway. The car is new to us and mostly fantastic, but has a few issues that we are working through. Every so often the "Service charging system" warning beeps loudly (and startles me because its so loud). It seems to happen only at idling, like when you are sitting at a light, rather than driving on the highway. The battery is relatively new. The voltage is 12.7v with the car off. With the car idling, the voltage at the battery is 14.6v. The voltage drop measured from the alternator to the positive terminal on the battery is only 0.16 volts, so I don't suspect the alternator, the grounding, or a bad connection is the issue. The voltmeter on the dash accurately represents the charging voltage.

The alternator seems to have the "correct" three pins in the connector. If I pull the connector when the car is running, two of the three pins have voltage on them, but the last pin has no voltage on it, as if it's not connected to anything internally. Is it some kind of input back into the alternator from the car? I seem to recall reading something that there are versions of the alternator with only two pins that will cause this warning. I'm wondering if this alternator has the pin, but it's not internally connected and we need a new alternator? I don't really want to replace a perfectly functional alternator, but on the other hand, the beeping is incredibly annoying.

I have to say, I love the C6. What a fun and pretty car! Definitely a future classic.

 

Your alternator connector has 3 wires…what are you seeing on each wire ??…the red/white is the alternator sense wire and receives battery voltage from fuse 2…is your horn working ??..,best to back probe the wires with the connector installed.