What would the most likely cause of these codes being thrown (assuming that the alternator is OK).

I am getting these codes about every other day (together with the Charging System Fault message on the DIC) when I start the car for the first time only. They then show up as History codes only. The rest of the day/evening, I never get these codes again no matter how many times I restart the car after clearing the codes.

Since I only get these codes after the car has sat overnight, would that maybe indicate that the battery is getting a slight drain to where it throws those codes. Has anyone ever seen these codes from a low battery condition?

 

How can you assume the alternator is OK ?, its the alternator that's throwing those codes.
DTC P1637
Circuit Description
The powertrain control module (PCM) uses the generator turn on signal circuit to control the generator. A high side driver within the PCM allows the PCM to turn the generator ON and OFF. When Generator operation is desired, the PCM sends a 5 volt signal to the voltage regulator via the generator turn on signal circuit. This causes the voltage regulator to begin controlling the generator field circuit. Once the Generator is enabled by the PCM, the voltage regulator controls generator output independently of the PCM by monitoring the battery positive voltage sense circuit. Under certain operating conditions, the PCM can turn Off the generator by turning Off the 5 volt signal on the generator turn on signal circuit. The PCM has fault detection circuitry which monitors the state of the generator turn on signal circuit. If the fault detection circuit senses a voltage other than what is expected, this DTC will set. The voltage regulator also contains fault detection circuitry. If the regulator detects a problem, the regulator will ground the generator turn on signal circuit, pulling the voltage low. This also causes the PCM to set the DTC.


DTC P1638
Circuit Description
The PCM uses the generator field duty cycle signal circuit to monitor the duty cycle of the generator. The generator field duty cycle signal circuit connects to the high side of the field winding in the generator. A pulse width modulated (PWM) high side driver in the voltage regulator turns the field winding ON and OFF. The PCM uses the PWM signal input to determine the generator load on the engine. This allows the PCM to adjust the idle speed to compensate for high electrical loads.

The PCM monitors the state of the generator field duty cycle signal circuit. When the key is in the RUN position and the engine is OFF, the PCM should detect a duty cycle near 0 percent. However, when the engine is running, the duty cycle should be between 5 percent and 100 percent. The PCM monitors the PWM signal using a key ON test and a RUN test. During the tests, if the PCM detects an out of range PWM signal, DTC P1638 will set. When the DTC sets, the PCM will send a class 2 serial data message to the IPC to illuminate the Charge System Fault message.
Bill aka ET

As I've said many times the PCM only likes the alt/gen found in the original design. If your charging system is still a problem then you have not found the solution in that alternator. its brand new, it should work flawlessly. Those codes are telling you that it is not. whether it happens once a day or once a week .If your seeing a charging system fault, your charging system is not working correctly.

 

In actuality, I cannot "assume" that the alternator is not at the core of my issue. But I have not been able to find one other Corvette C5 owner come forward and state that they are having "ANY" compatibility problems with these alternators.

In speaking directly with Billet-Tech, they say they have sold hundreds of these to C5 owners with no code or fault issues and have offered to give me a full refund if I am not satisfied. I have also spoken with West Coast Corvette (who also sells these for C5s - in fact it appears to be the "only" alternator they offer for C5s). WCC also said they have sold hundreds and never had a single complaint of codes or faults reported?

So, I am deducing that:
(1) I am an isolated case and my problem lies elsewhere (and this new alternator is just revealing a weak link in my system).
(2) All the vendors are lying and they have only sold five of these alternators to C5 owners over the years vs "hundreds" and those five people are living with those codes and not reporting back.
(3) They have indeed sold "hundreds" and many or all of those people are having the same problems and just staying silent?

In essence, I am trying to locate one other person that has installed one of these on an Automatic 2001-2004. Either they are getting these same codes or they are not - either way, it would be enlightening. I need to hear from other end users rather than Vendors.

 

those two codes PCM 1637 & 1638 Codes, are very specific to the communications values the PCM is programed to monitor. the Alternator in a C5 is not an alternator, its an alternator/generator with a specific switch over circuit Its the reason the replacement alternator from GM< requires a PCM re-flash so there are no communications problems. Before the code was written for the new alternator, corvette owners where seeing the same thing that you are, because the PCM did not like the new alternator. I'm not sure that this New alternator of yours even has a switch over from gen to alt. Not sure it even applies voltage to the generator circuit when first spooling up. I said I applauded the vendor for finding a work around, but I wasn't holding out any hope because when I told him how the alt/gen and discrete circuitry works in a C5 he said the manufacturer never heard of it. So my thinking is how could they do a work around to something they weren't aware of? its obvious by all the grief, that the Corvette communications circuit is the heart of the problem, other wise any alternator that fits the physical mounts and put out enough voltage and was regulated properly would work but we all know that no one from any auto parts house has a reliable alternator to work in a C5. So my skepticism is founded in the fact that the manufacturer has no clue about the intricacies of this alt/gen. they have not said anything about how they made this alternator work around a known problem or how they can bolt in a 170 Amp alternator that is monitored by the PCM that is looking for max load at 110 amps or 140. IM almost positive their design does not support an alt/gen design, and with out the gen and the switchover to alternator the PCM is going to have hissy fits.,Believe me, I want this to work but IM not feeling warm and fuzzy over what I've seen them say. I know what you are saying about them claiming that hundreds have been sold with no problems, I wish it were so. I'm certainly not trying to trash anyone here, I'm sure your money is safe with this guy. What I would like to see is a list of the hundreds of C5 owners who bought this alternator and some way to contact a few of them to see what going on. I know if I found a solution to this problem , I would be all over this forum promoting this alternator as a solution to this problem... The problem is I don't see it. Ive also never heard of this company, so how do hundreds of C5 owners even know about them? Most would go to auto Zone, advanced auto, Pep boys or O'Riley's Napa, etc. My suggestion would be to have them send Bill Curlee an alternator, Like my charging system, his works perfectly. He is a well respected member of this community. Give him a couple of weeks with this unit and see what he sees. If it works for him then the potential to sell hundreds here is very possible. After a few weeks, send Bill a call tag and he will return the unit. Now Bill may not want to do this, but rather then sending a unit to me, it would take my skepticism out of the picture. Bill is as respected here as I am or even more because he is a nice guy. I'm not so nice at times, too many pokes in the eye for me over the years here.
Just some thoughts.
Bill aka ET

 

Excellent suggestion. That would be a smart move for Billet-Tech and help their sales here immensely. We desperately need a new high-quality alternator vendor that we can rely on as lot of us use our C5 as a DD and plan on keeping them until the wheels fall off (or alternators cannot be replaced - whichever comes first).