I am not saying that is not possible - I am no expert on such things - but if that IS the problem, I know for a fact that nobody has ever touched that post since I originally purchased the car new and if that is what is wrong, then even if I buy a new starter and install it, I would guess it would just fail on its own again?

 

If a light bulb burns out and you buy another one, it'll just fail again on its own.
The two cars that I have seen with the exact same problem as yours failed on their own. One was a Z06, 2003 with 10,000 miles. The post was burned internally so bad that you could hear it when you tried to tighten it. You said earlier that you knew that nut should be torqued. If the post turns, how can you torque it? In a dc circuit, loose and/or corroded connections produce heat and lots of it. If your car has headers running nearby the starter, it makes it happen even faster due to the heat. Seems like you're willing to try everything else, but not the starter. I'm telling you for a fact, that post should not turn internally. One day, when you turn the key and it doesn't start, wiggle that wire and have someone turn the key. As I said earlier, please post what you find eventually that fixes your voltage fluctuations.

 

The Starter is the only mechanical part with electrical connections that I have not replaced in the chain, so that may very well be the culprit. I just started replacing items that various forum members recommended and most people in this and other posts said to start with the grounds (which I have cleaned and reconnected), then the alternator, then the battery and then the ignition switch (all of which I have replaced with new parts). I also installed new plugs and wires and one person insists that one of those is the weak link? Another thinks it might be the belt tensioner?

One other reason I have put off the starter is that it will be the most expensive single out-of-pocket cost to me of all the parts I have replaced so far (since the alternator was under warranty and the Dealership said the alternator was the source of the problem during their diagnosis). If it has to be done, then I will do it.

When I tightened the Starter nut, I was REALLY putting some torque on it before the post started to move as that was a symptom I was watching closely for due to other forum posts (and for sure the nut was not very tight to begin with). As soon as I saw the post "start" to move I stopped tightening, but the connection was VERY TIGHT before the post started to move. According the the parts guys here, they say that the solenoid has a plate that applies pressure to that stud on the inside and that it is not uncommon to get a small amount of turn as it is tightened. Of course, if you could keep turning the shaft as pressure was applied, then it would be toast, but I don't think that is my case. I guess one way to test at this point would be to try and loosen the nut and if that shaft now turns freely, then I probably trashed it by over-torquing?

I don't have headers - just stock manifolds.

I am not dismissing your suggestion at all - that is just the last major time/expense suggestion that was made and I wanted to exhaust all the free/cheap possible fixes first.

 

Make up a (temporary) jumper wire, connect it to the large terminal on the back of the alt and directly to the postive terminal on the batt. That will bypass any bad connection at the starter.

 

I am experiencing the same problem and am waiting to hear how it is resolved.

 

Good idea (I think). Leaving the suspect Starter connected will not hinder the effect?

Would 12Ga wire be good enough?

 

12ga. would help. If I only had 12ga., I would probably double it.

 

Well, I did as you suggested and doubled the 12Ga to a single eyelet on each end of the jumper and soldered the connectors. Ran one end from the Pos+ Alternator post to the Pos+ Battery terminal. Started the car and drove around a bit - exact same problem. So, I guess that eliminates the "starter connection" as a suspect?

I guess one other thing I could try is hooking my digital multimeter to the Pos+ Battery and a ground and lay it on the outside of the windshield as I drive around the block to see what it says?

Tonight I was at 13.1V while in motion at all speeds and it would drop t0 12.1-12.2 at stop signs and slowly work it's way back up to 13.1 (by slowly I mean 1-5 seconds). Headlights, Highbeams, A/C, nothing seems to have any real effect on this. BTW - the car was running great tonight so I am wondering if the problem is even real or just in the gauges?

 

Today I hooked up my digital multimeter (Red to Battery Pos+ Terminal and Black to G-102) and laid it against the windshield to compare with the dash gauges. For the most part the dash gauges were .5V lower than the multimeter.

Driving...
Multimeter at 13.92
DIC and Dash gauges at 13.4

When coming to a stop sign...
Multimeter dip to 13.25 then back up to 13.9
DIC and Dash gauges dipped to 12.8 then back up to 13.4

Still seems like I used to get 14V or more on my dash gauges - maybe not?

At any rate, still not sure why the voltage is dropping when coming to a stop. It is not the tail lights because even if I let up on the brake the voltage does not increase any faster.

 

The voltage regulator in the alternator controls output based on the needs of the electrical system. The PCM actually controlls the output voltage of the alternator. If the battery if fully charged and there isnt much demand on the electrical system it wont put out as much as it would if it needed to recharge the battery or supply a large load. Try fully loading the system by turing on everything and see if the voltage increases.

Bill

 

Bill,

If I fully load the system it drops about .1V for just a second. If I turn off the load it increases about .1V for a second.

I also went on an open road tonight and noticed that the voltage drops when slowing down (letting off the gas and coasting to a stop) without even applying the brakes - so I guess that clears the brake light circuit?

 

With respect only to 12 Volt battery voltages, normal is considered as anywhere between 12.8 to 14.2 volts during charging, depending on load conditions (lights, A/C, large stereo amp at full blast, etc). The GM service guys at my local Chevy dealership state that the C5 charging system can run as high as 15 volts and be normal (the RED zone is at 16 volts). To me, that is too high.

Open circuit (good battery, key off, negative terminal disconnected) should read between 12.2 and 13.2 volts. Most likely, the high end will be about 12.8 volts with a good, used battery. Each cell, with a new an fully charged battery, should put out about 2.1 to 2.2 volts.

The gauges in the C5, according to Chevy, are guides only. The best way of checking voltage is with your DVM. Here's another thought and possibly something else to check:

You need to ensure that there is a good, solid ground connection between the battery to car frame and engine to car frame. If the ground potential is different at the engine and the frame, the gauges will most likely read low. Using your DVM, measure for a small voltage between the frame and the engine. If you get anything other than a 0 voltage, you have a ground problem with the frame.

Hope this helps!

 

Just ran that check - had to drop the scale on the DVM. Only measured .7mV between G-102 and the block.

 

That's what I was mentioning earlier. Any time you measure a voltage between 2 grounds, it means that one has a better path back to absolute ground which is the alternator's chassis, not the battery - post. In a perfect world, there would no potential difference between any grounds.

 

A perplexing problem tackled by sharp minds from every corner of the USA ,all who offer tips ,solutions, advice are what make the forum great.THE FORUM IS MY FRIEND

 

Well.....with a reading that low, you should be OK. Got another thought, though. If you haven't already, perform the check with the engine running (right after you've started it), with the A/C, stereo, and anything else you can think of (4-way flashers, etc.) turned on. If you've got a more substantial voltage across the two grounds, then you have a high resistance connection between them and need to replace the ground wire/strap or repair/solder the crimped connectors at either end of the strap.

 

Took this reading - shows 57mV.

 

Is there a reason nobody has suspected the PCM? Does that not receive/send info to the regulator AND the gage cluster?

 

Hi, The PCM only tells the voltage reg to get into action, the reg then controls the output. My 03 vert, auto, shows 13.8 at idle and 14.4 at driving speeds. It was like that when the batt was brand new, and still is a year later. My battery is always a close to full charged as possible as I use a maintainer on it whenever it is not in use.
It is possible that when they rebuild alts they do not replace the regs, they usually only replace the diodes which are what usually fail. Never seen it happen, but maybe the fusible link is going bad, see below in italics.
Good Luck

SEE Below
++++++++++++++++++++++++++++++++++
Circuit Description
The generator provides voltage to operate the vehicle's electrical system and to charge its battery. A magnetic field is created when current flows through the rotor. This field rotates as the rotor is driven by the engine, creating an AC voltage in the stator windings. The AC voltage is converted to DC by the rectifier bridge and is supplied to the electrical system at the battery terminal.

When the engine is running, the generator turn-on signal is sent to the generator from the PCM, turning on the regulator. The generator's voltage regulator controls current to the rotor, thereby controlling the output voltage. The rotor current is proportional to the electrical pulse width supplied by the regulator. When the engine is started, the regulator senses generator rotation by detecting AC voltage at the stator through an internal wire. Once the engine is running, the regulator varies the field current by controlling the pulse width. This regulates the generator output voltage for proper battery charging and electrical system operation. The generator F terminal is connected internally to the voltage regulator and externally to the PCM. When the voltage regulator detects a charging system problem, it grounds this circuit to signal the PCM that a problem exists. The PCM monitors the generator field duty cycle signal circuit. The system voltage sense circuit receives B+ voltage that is Hot At All Times through a fuse link that is connedted to the starter.. This voltage is used by the regulator as the reference for system voltage control.

Charging System Indicator(s)
CHARGE SYSTEM FAULT
The IPC illuminates the CHARGE SYSTEM FAULT indicator in the message center when the following occurs:

The PCM detects that there is a problem with the electrical charging system. The IPC receives a class 2 message from the PCM.
If the tachometer is less than 500 RPM and the power mode state is in the crank mode then this message will not be displayed. Instead the CHECK GAGES indicator will illuminate.
HIGH VOLTAGE
The IPC illuminates the HIGH VOLTAGE indicator in the message center when the IPC detects that the ignition is in RUN and battery voltage is greater than 15.75 V for more than 5 seconds.

LOW VOLTAGE
The IPC illuminates the LOW VOLTAGE indicator in the message center when the IPC detects that the ignition is in RUN and battery voltage is less than 10 V for more than 5 seconds.

 

Got it.

I have had this behavior now with two different batteries, two different alternators and two different ignition switches in the past month.

The alternator the dealership replaced mine with under warranty a couple weeks ago was not a "rebuilt" unit as far as I can tell, but a new alternator (made in France).

Is there a way to test the Starter on the car to see if the problem is through the post connection?