I have a daily driver with 129k on the odometer. The voltage indicated by the DIC used to be constant, but recently has started to fluctuate, sometimes through quite a range. It used to start at 14.3, drop ever so slightly to the tune of 0.1 when the brake lights come on, turn signals a little more, as expected, but go right back up and sit at 14.3. But now, when I start drive the car to work in the morning while I ALWAYS see 14.3 initially, as I drive it starts to fluctuate, sometimes going as high as 14.7 and sometimes as low as 12.9. When I drive it home 6 hours later, it seems to not go back up into the 14 range at all. It's typically in the mid 13s, and I've seen it go all the way down to 11.9 for a moment then back up into the 12's then 13s, then sometimes back down into the 12 range. While my Red Top battery is 4.5 years old, I would think if the battery was starting give up the ghost, I wouldn't see voltages above 13 at all, let alone 14, and possibly be experiencing hard starts, and such, which I am not.

For context, I am in sunny inland Northern California, where rust is never heard of, and I've never had a non-start (aside from when I rat chewed up some cluster wires under the computer below the battery several years ago, but that's a whole 'nother story), so I don't think this is a connection issue, Logic tells me that this is an indication of the regulator inside the alternator starting to go out, but I have been know to be wrong.

A brand-new GM factory alternator is over $500, so I would like to narrow this down as much as possible before plunking down the money, so thoughts, and suggestions would be most welcome, as would tips on to how I might test the alternator vs. battery with a multi-meter.

 

Have your battery tested 1st, thats free at most auto parts stores... Then you may want to look thru this thread before you drop that $$ on a new alternator
https://www.corvetteforum.com/forums...ch-repair.html

 

Yep, do this. I bet your alternator is fine and your battery is beginning to go. That is the way to bet.

 

I am not having start or power issues. My voltage reading is fluctuating on the DIC. I'm not so sure an ignition switch contact problem would cause this. Am I missing something?

 

From what I understand voltage readings on the DIC correlate to what the charing system is putting out. If my battery is getting low, why would the readings go up? For reference, what does yours read?

 

Since you guys both mention testing the battery, I will stop by Autozone and ask them to test it.

I wonder if my 2000 Suburban battery would fit in the Vette. Then I would be able to swap batteries for good measure and see what the DIC reads then. What do you guys think?

 

You guys are the bomb! I stopped at Autozone on the way home, and their little handheld device not only checks batteries, it also tests alternators! My alternator reported to be in fine, but my battery was very low. While that saves me the headache of a new alternator, I'm surprised that batteries are so expensive nowadays, too. I had an Optima Red Top that served me very well, and will stick another one in. Thanks again, you guys!

 

The DIC analog Voltmeter and the IPC Digital meter DO NOT tell you battery voltage. The show you what the HOT in RUN & START battery bus is. That buss comes directly out of the IGNITION SWITCH.

If the contacts inside the ignition switch are burnt, corroded, dirty, the voltage will be compromised and something lower than actual battery voltage or alternator output.


The ONLY way to see what actual battery voltage and or charging voltages are is to use a DC Volt meter on the battery terminals when the engine is running. You can also compare the actual battery voltage to the alternator output voltage by reading the BATT TERMINAL on back of the alternator to chassis ground. You can compare those readings to what you see inside the car on the meters.

ALTERNATOR. If you have significant mileage on the car, the alternator rotor SLIP RINGS are probably very dirty. That can and will have a direct effect on the alternator charging voltage. Its VERY SIMPLE to clean those slip rings. There's an EXCELLENT You Tube video on that exact topic.

Just You Tube search "VALEO ALTERNATOR REPAIR" The videos by (WESTRONICSLRL ) are excellent and all you need to self repair your alternator.

EASY AS PIE!

Bill

 

One other possibility is the voltage sensing wire (from the computer) on the starter is not making good contact. Here is a picture that connector. It is badly oxidized.


I was experiencing wild voltage fluctuations that got worse and worse. The bad connection made the computer think the charging voltage was too low, causing the alternator to put out 15 volts and higher. This high voltage cooked the battery - it died shortly after a couple of high voltage events. I replaced the connector with one out of my junk box. Took 30 minutes. The alternator voltage has been perfect ever since.

 

Thanks for that, Bill, as always!

As I mentioned, Autozone diagnosed as a bad battery. They also said my voltage dropped to below 10 volts when I start the car. Is that normal? I put a new battery in, and now I only see it drop into the 11 volt range.

BTW, not to hijack my own thread or anything, but on your ignition switch write-up you included instructions to r&r the IPC. I've never tried to access it, but I thought only the center console and knee booster needed removal to get to the switch. Am I wrong? If I have more questions on that topic, I will post them there, where they belong.

 

When Autozone told me the battery was low, it got me wondering if the alternator somehow knew to apply a higher rate of charge, which might explain why I started seeing voltages starting to fluctuate up towards 15 volts (14.7 to be exact). Do alternators really adjust their outputs depending upon voltage/state of the battery? What does your voltage range read like now?

I haven't looked, but is that connector accessible from the top or bottom? Is it an easy reach, either way? And, what color is the wire to that connector?

 

NO, You do not need to remove the IPC to gain access to that ignition switch.

As Pjensen pointed out, the alternator VOLTAGE FEED BACK signal comes from a wire (fused) that is attached to the starter solenoid. The alternator BATT Terminal outputs to the battery. It ALSO goes to the main terminal on the starter solenoid. All that being said, the wires on the starter solenoid MUST be clean and tight to have the proper connection and allow the alternator to work properly. With out that feed back wire making proper contact, the alternator output can and will be all over the place.

IF your battery doesn't have the proper reserve capacity and cold cranking amperage,YES, I have seen them drop very low when the starter draws lots of amps to turn the engine. Battery voltage is INVERSLY proportional to current draw. I have seen a bad battery drop as low as 8 volts under starting current draw and still be able to crank the engine. The C5 electrical modules DO NOT LIKE LOW VOLTAGE.

Its normal to see 11 VDC on initial starter engagement. As the engine spins the engine and draws less current, that voltage reading should increase until the engine runs and the alternator starts charging.

NOTE. The alternator will not always output it's full charge voltage. It can and will adjust the output to meet the demands of the electrical system and needs of the battery. 13.5 to 14.7 is the normal range that you should see (at the battery terminals)
REMEMBER,,,,, The volt meters in the cabin DO NOT represent actual battery charge voltage!

 

I hope i didn't buy a new battery for nothing. The Autozone guy checked it with the engine off, when starting, and when running. He said he couldn't even get a good enough reading with the car off, but I had no trouble starting it. In all scenarios he said it was low. When i asked him about the alternator, he said it was fine, and showed me so on the device. I don't see why he would lie, though, because he doesn't work on commission, and if he did, i would think he would say the alternator was bad, since they cost more, you know what I mean? I think that after 4.5 years as a daily driver, it's possible that the battery was getting low, and for the first time in my life I was fortunate to have checked and replaced it BEFORE I had a problem. I'm just not used to working on a car before I have a problem

Based on what you said about how the alternator works under start and run, it sounds like my DIC is reading correctly, but I can compare to direct readings on the battery and Alternator, to verify.

Also, maybe i should check the starter wires just to be sure they are sound. Is that what you're saying? How hard is it to get to them?

 

You will have to jack up the car. DISCONNECT the Neg Battery cable!!! The solenoid and starter are not out in the open BUT they are accessible.

 

The alternator's output voltage is totally controlled by the computer through the field wire. The bad connection is like a resistor. So the computer was seeing 11 volts (for example) even though the alternator was actually putting out 14 volts. Because the computer thought it was low, it kicked up the alternator to 16 volts (for example). Then the voltage on the that crappy connector looked like 13 volts.

It kept getting worse, sometimes jumping up to 16 volts. I turned on the head lights to load down the voltage to 14.5. Sometimes the display would say HIGH VOLTAGE and shut down the entire dash. Scared the crap out me. I was worried that it would destroy all the electronics and computers if it didn't get fixed immediately.

Got home and started reading all of Bill C. posts on the electrical system. Final came across one thread that explained the alternator/starter wiring. Ten minutes later, I was under the car and found the oxidized connector on the starter.

Ever since, the alternator voltage shows to be 14.1 to 14.3. When I have the a/c and lights on, it might get down to 13.9 or so.

By the way, thanks Bill C for the incredibly helpful posts.

 

Listen to Bill.

 

I would have checked the connections at the starter solenoid before replacing the battery. They get weak and start causing voltage fluctuations. The connectors all need to be taken off the solenoid post and cleaned before being re-installed to do it right. Just tightening the nut is mostly pointless.

Lots of poor connections get "fixed" by constantly replacing the battery since it's become such a forum mantra to keep a new battery in the car at all times.


The PCM has NO control over the output voltage of the alternator. The F or field wire is only so the PCM can monitor the PWM level of the regulator. Stop spreading this false information.

The alternator on a C5 does NOT vary it's output voltage based on system loads or driving conditions or anything like that. The alternators on most of the newer GM vehicles do, but the C5 PCM does not have any control over the output voltage of the C5 alternator.

There is a wire from terminal D of the alternator connector that runs to the starter solenoid. THIS wire is the output voltage feedback used by the regulator inside the alternator. A bad connection on this wire at the solenoid could provide a lower voltage to the regulator which would cause it to in turn increase the alternators output voltage level trying to compensate.

 

Yes you are correct. I was thinking of the typical alternator field control setup.

Weirdly I knew about the voltage regulator on the back of the C5 alternator (and that voltage sensing wire that goes down to the starter). Late night posting mess up. Sorry.

 

Alternators do not 'put out' volts; they put out amps. The voltage reading is the 'push' required to get the electricity into the battery. As the state of a low battery charge increases, the voltage increases, as it's harder for to push the electricity into it. Think of of it as a water faucet. ...the water coming out of the faucet is the amperage and the pressure is the voltage.