After 15 years and 150k miles, the original alternator on my bone-stock 2000 C5 finally gave up. I read all the posts on all the threads I could find, and noted some comments about which parts house's remans work and which ones don't. So I went with the Autozone/Duralast, and lo...it didn't switch on like all the threads say. Three different remans from three different suppliers later, same problem: won't switch on until above 2000 RPM, then all is well.

No, I didn't keep my original core. Missed that advice in the posts. But even though I am 99% sure that an incompatible alternator is the culprit, and even though I am ready to dig deep in the pocket for a genuine GM/Delco/Valeo replacement, I wanted to positively eliminate the car as the source of the problem.

I read in several places that the turn-on signal from the PCM should be 10vdc at a low current, but just how low (or high) should it be? Open plug, key on, I get the 10 volts, but connected to the alternator, key on, not running, the voltage drops to .5 volts. Is there any test that can measure if the signal from the PCM is good enough to actually switch on the alternator? Other than borrowing the alternator from my buddy's 04 C5?

Finally, is there a difinitive list of aftermarket alternators that are known to work or not?

 

The "flash" voltage from the PCM is only 5 volts.

Here is some reading for you.

https://www.corvetteforum.com/forums...tem-fault.html


Mr. Sam

 

Mr. Sam,

That thread, and all the threads linked to that one, were all part of my research. I even took a copy of the circuit diagram with me to the parts houses and a couple of shops. The amount of ignorance in the auto repair industry is astounding.

To be certain, I went with a brand-new battery that was on the trickle charger overnight before installation; I've checked/cleaned every connection on every wire in the diagram from battery to starter to alternator and from alternator to PCM; I've verified continuity/resistance/voltage at every point in the circuit. It all comes down to the switch-on "flash" voltage on pin B.

The other threads say 5vdc from the PCM, GM tech ops says 10vdc, local shops (includung the GM dealer) have said both 5 and 10. To me, it's not the voltage that matters but the current. There has to be enough to pull the transistor inside the case to the "switch on" position. Without opening the case and actually getting the values for that transistor, I have no idea what it needs and therefore no idea if the PCM is providing it.

The strange thing is that other than the DIC message that comes on until the alternator starts putting out, I'm not showing a single OBD code.

 

That theory would be valid if the PCM actually switched on the regulator, but it doesn't. The connection to the L terminal produces a small current in the field and that causes a small voltage to be generated when the alternator is rotating once the engine starts. The regulator detects that voltage and switched itself on.

If the case is still a Valeo case there is a place selling regulators. I'd have to look the link up on another computer and get back to you.

 

That is the way even large generators will work, Our emergency diesel generators at the Nuclear plant just use a battery to flash the field and then once generating A/C then some of that output is converted to DC to maintain the rotating magnetic field and the battery circuit is opened.

I say 5 volts since that is what the FSM says.

Mr. Sam

 

I don't mean to be snarky, but if the PCM doesn't turn on the charging circuit, why then is the wire from the PCM to the B terminal on the alternator labeled "Generator Turn On Signal"?

From the FSM:

"The PCM uses the generator turn on signal circuit to control the load of the generator on the engine. A high side driver in the PCM applies a voltage to the voltage regulator. This signals the voltage regulator to turn the field circuit ON and OFF. When the PCM turns ON the high side driver, the voltage regulator turns ON the field circuit. When the PCM turns OFF the high side driver, the voltage regulator turns OFF the field circuit."

The regulator itself is all contained on a microchip, but I found a schematic that shows the entire circuit and there is indeed a transistor that functions as the actual switch. 5vdc is the correct voltage to pull the transistor closed (on). I'm showing 10vdc on that wire when it's disconnected, so something is definitely amiss.

Mr. Sam, I followed your example and used a 9v battery to energize the B pin at idle, and sure enough the field lit up and I had 14.5 volts coming out of the alternator.

So I am reversing my earlier prediction to say that the alternator is fine. Now I have to figure out what's going on with the signal coming from the PCM.

Stay tuned...

 

So which alternator? Autozone,, how did it work out, changed it or kept? Bought new GM?

 

The C5 PCM has no ability to "control the load of the generator on the engine" so that can't really be relied on as a correct operational description.

The factory manual says 10V when running and working and 5V if running with the connector off the alternator.

Is this the schematic you're referring to? The L terminal is connected to the collector of the transistor so it has no ability to turn that transistor on or off.



If you did find an actual schematic of the regulator IC used in the C5 alternator then share the link to the info.

 

 

I never said the PCM doesn't provide the voltage to the rotor to initiate
voltage and current in the stator. It does, but as soon as it does the regulator takes over.

As far as 5 versus 10 volts for the turn on, all I can say is the FSM in the diagnostics says to disconnect the connector and turn the ignition to ON, and that should measure 5 volts.

Outside of that the PCM only monitors the duty cycle of the alternator and if it is outside of preprogramed parameters it will throw the Charge System Message.







Charging System



Hope that helps. If, not we can discuss on the phone.


Mr. Sam

 

This is way more educational than Ms. Smith's English class.. I'm in..

 

Mr. Sam, I was definitely not referring to you. Your posts here and elsewhere on the forum have been the most helpful. I took issue with lionelhutz telling me that the service manual doesn't say what it says and the regulator circuit doesn't do what it does. Ironic that he chose a screen name of a lawyer that doesn't know anything about the law.

Anyway, there is obviously way more to the description of the turn-on signal than the small section I quoted. When I pulled the PCM wire from the B terminal on the plug and used the 9v battery to flash the field, it turned on. When I took the 9v off the B terminal, it didn't turn off, and I didn't expect it to. Understanding the intricacies of the regulator circuitry was just to satisfy my own curiosity; it really has nothing to do with solving my original problem.

The question I posed in my initial post was what was supposed to be present on that wire from the PCM. I think we've beat that question to death and the answer is that what I'm getting is not correct. The next question is why? And I'm hoping the answer to that isn't a bad PCM.

The only new information I have now is that all my wire-chasing must have upset something. Initially, I only got the "Charging System Fault" DIC message on startup. Once I got enough RPM to light up the alternator it went away and stayed away. Now, after fiddling with the harness, the message is coming on and going off at random after startup even though I'm getting normal charging voltage.

And still not a single fault code.

 

The regulator watches the voltage at the "AC sensor" arrow in the schematic and that's what turns-on the regulator. Voltage only appears there once the alternator is rotating. The L wiring is necessary to get a small field current which initiates the whole sequence but it doesn't turn-on the regulator. If the L terminal turned-on the regulator then the F terminal would be at 100% duty cycle before the engine started, but it's not. Also, your test just proved this fact. If the voltage turned-on the regulator then the regulator would turn-off again when you removed it.

As for the crap about no faults. The P1637 & P1638 codes are set to no error reporting and no SES. The DIC is used to indicate charging system faults. I believe the manual also says not codes are stored.

I did measurements to help but apparently you're insulting and think you're smarter so good luck fixing your car.

 

Okay boys and girls, after weeks of research and a fortuitous meeting, I'm going to put a nail in this electrical coffin.

I was introduced to a gentleman from a local car club. He's a hard-core tuner, a computer genius and best of all, a retired GM engineer. And based upon the contents of his shop, he's been to a few NASA garage sales. Here is what I gleaned from him:

The alternator on the C5 is GM's "4-wire" CS130D case. Some believe that the "D" stands for digital, but he wouldn't confirm that. What he did say was that GM made a subtle change in the charging circuit during the 2000-2001 production run, but it's VIN-specific, not date specific, and that's where the aftermarket rebuilds get jammed up. Here are the basics (and I know much of this is repeated in other threads):

These alternators are identified by the four wires on the plug and are known as PLIS or PLFS. The P (pin A on the diagram) is Phased AC right off one leg of the stator. Mostly unused, its common function is to drive a tach on diesel applications. L (pin B) is still known as the Lamp circuit, but in modern applications that is a misnomer...more on that later. I (pin C) was switched Ignition, once used as a "turn-on" signal for the regulator on non-PCM/BCM vehicles. It was replaced by F (still pin C) for Feedback, the duty cycle measurement to the PCM. S (pin D) for Sense, the "in" voltage to the regulator from the battery telling the regulator the charge state of the battery. So far, so good.

GM's concern as electrical systems demanded more and more current was with "field ramp-up," a condition where a regulator would sense a partially depleted battery on startup and command full charging current right away, resulting in sudden loads on the engine and belt slippage. This became a big issue with the intruduction of serpentine belts driving everything, and the initial solution was a centrifugal clutch on the alternator pulley. That worked well enough, but GM decided to go even farther, and that's where the subtle change to the L terminal crept in.

In days of old, the field got its initial excitement through the idiot light. One side of the light was wired to +12v, the other to the L terminal. With the alternator not producing power (not turning or failed), the L terminal would go to ground and light the light. Once rotating, the L terminal provided the initial field voltage. With the alternator producing power, charging voltage on the L terminal would cancel out the voltage coming from the lamp, and it would go out. This system carried over to vehicles with something more sophisticated than just an idiot light. The PCM/BCM would provide +5v, +10v or +12v (depending on application) to excite the field, but they were still having issues with field ramp-up.

Enter the digital L terminal. Working in conjunction with the battery voltage (charge state) and the duty cycle input, the PCM sends a digital signal down the L circuit. The regulator recognizes this signal as either "active" or "inactive." There is measurable voltage on that circuit with the alternator not producing power, but there is no current (amperage). Zip, zero, nada. And the L terminal no longer goes to ground when the alternator is not charging. A test lamp between the plug and socket on the L terminal will not light. Further, running a test light from the battery to the L terminal is a good way to fry these regulators. The kicker is that the digital L terminal is proprietary GM, and the vast majority of aftermarket rebuilds don't have it.

The regulator schematic in the wiring diagrams shown on this thread and elsewhere are useless as they are not complete and don't accurately reflect the circuits on the computer chip in the regulator. I was shown a schematic for both the conventional and digital regulators, and only about half of it made sense to me.

So why do the "conventional" aftermarket rebuilds start producing power when revved up beyond 2000-2500 RPM? Well, the nature of electronic regulators is that a tiny amount of current "leaks" through the regulator to the field if there is measurable voltage on the S terminal. Current and rotation are needed for the field to induce current in the stator; if you have a lot of one, you don't need much of the other. Spin the rotor fast enough, and even the tiny current that leaks through the regulator is enough to light it up.

As to why I wasn't seeing the P1637/P1638 codes, well, that's on me. I was using a Bluetooth dongle in the OBDII port and an app on my phone to do my troubleshooting. I got the free app, not the $5 one, and it doesn't recognize manufacturer-specific codes. They were all there on the DIC when I went looking.

Final score: Auto Zone - nope, O'Reilly - nope, Pep Boys - nope. Delco - works like a charm.

Post script: When the aftermarkets were producing power, either through revving them up or manually flashing the L terminal, they all went straight to 14.5 volts regardless of battery state or electrical load. With the correct Delco now doing its job, I'm back to seeing the usual 13 -14 volts that was normal before all this started.

 

Thanks NEWTOO for the detailed information and follow up.

I have a question for you. Over the life of the car, do you know if the battery was replaced upon failure or a specific time period? The reason I ask is that many people wait until battery failure which places undue stress on the alternator. As the battery degrades, the alternator must make up for the battery's inefficiencies which reduces the life span of the alternator. I'm just curious.

 

Just a watch on battery failure as it relates to the alternator. IF your battery goes so dead you have to jump the car, after start TURN OFF ALL ELECTRICAL you can. A/C radio etc. Failure to do so puts a very heavy load on the alternator causing it to heat up and fail components. This is how I failed my last alternator by not turning off the load after jumping. IT was raining, foggy windshield/ a/c and had wipers on so I just burned out the alternator after jumping off.

 

I know that a bad alternator can kill a good battery just as quickly as a bad battery can kill a good alternator. And nothing damages today's maintenance-free batteries faster than letting them go dead. Jump-starting and charging with the alternator is extremely stressful to both. Even charging a dead battery with a trickle charger doesn't seem to spare them from life-shortening damage.

My OEM alternator lasted 15 years and 150,000 miles. The OEM battery lasted a respectable 8 years, and 5 of those were spent living in the Palm Springs area. I'm on my third replacement battery right now - all Sears Die Hards. I'm convinced they are designed to fail just as the warranty expires. The first replacement went in the first time I noticed the engine was slow to crank.

The second went in when the battery started discharging after the car had been sitting for a day or two. I learned later that the OEM alternator had one or more bad diodes, allowing the battery to discharge to ground. I figured the Sears Auto Center "technicians" would test for a bad alternator before sending me on my way. Well, their "test" is to disconnect the battery with the engine running (I didn't know they did that until afterwards), and doing that finished off the regulator as it ceased charging altogether after that.

The third went in a week later after the first aftermarket alternator was installed. As I mentioned earlier, all three aftermarket units would output 14.5v all the time once the field started working, and I knew that would eventually cook the battery.

Since I went with the genuine Delco (p/n 10316182 for my VIN), all is as it should be. But my next battery is NOT going to be another Die Hard!

 

Disconnecting causes the system to lose ground and go to max voltage out put which can damage other components. Think BCM or PCM. so don't remove the battery connectors if possible engine running.

 

It's not really that it loses ground. The alternator provides a 3-phase full-wave rectified output, as can be seen in the following graph (green trace):



The battery acts as a capacitor to "smooth" it out. When the battery is disconnected, the filtering effect is lost and the "wavy" output plus any surge or spike that may occur is passed to all circuits, which could result in damages.