My problem is related to a sticky thread on Battery/Alternator issues and it's got my shop challenged as well. Started with Charging Fault message, no check engine light, and got P1638 code. Shop advised that PCM was bad. Got Genuine GM Reman, paid to get the GM temporary download to change the VIN and Operating System and then uploaded my HPTuners Calibration. Charging Fault again, only this time it is P1637 code. Discovered that there are two different designs for the Alternator in the 02-03 period of time and it was told the correct one was needed so to match so that the Alternator's internal voltage regulator would shake hands properly with the PCM. Put on correct Alternator for 02 (pn 103161182). Still had problem with code. Shop said I needed new battery as it was not holding charge. That done, battery fully charged, but the message and the code reappeared. Following GM diagnostic instructions to check voltages and for shorts etc shop noted that the PCM pin 15 to red wire (terminal B) at Alternator was showing 10 volts on voltmeter. Ran a bypass wire to confirm. Test say 5 V is what is needed to turn on the voltage regulator. But shop is getting steady 10 V. Why would I be getting a low voltage condition (P1637)? What do I do now?

 

UPDATE: Fixed both codes. After reading all the remarks from a few years ago, we worked backwards to find the solution. Basically it required a new (Reman) PCM, new battery and three different alternators before we hit the right combination. The alternators are the biggest hassle because if they don't work, you may have a problem returning them. Got an extra two brand new alternators because of that! One is Powermaster and the second is a AC Delco. Sellers are reluctant to take back an electrical part as being wrong unless the manufacturer agrees. A lot more work is ahead of me to get my money back.

Our conclusion is that the PCM will not talk to the Valero IR (internal regulator) if the voltage deviates from its expected range. So if the voltage rise time is slow, or the level doesn't reach the expected value, the PCM will detect missing voltage for a brief window and throws the fault P1637. And if the voltage is too high, or you are charging a weak battery, it indicates an overcharge condition P1638.

Note: the Powermaster alternator is a 14V, 220 amp unit; and the ACDelco pn-182 is a basic 12V, 100amp unit. The correct alternator for me is an ACDelco pn -440. It is rated as 14V, 145 amp. You can see the mismatch. The Powermaster may have been the right voltage but it's putting out too much current. And the ACDelco was a too low in Voltage.

Here is an important discovery: Most sellers/parts stores don't distinguish and sell all three as for ALL C5's. In fact are that the pn-182 is probably ok for manuals, but it requires the pn-440 for Auto. One of the setters showed this distinction but only for years 03-04. Mine is 02.
Hope this message helps someone else in future.

 

Glad you fixed it. I know some of the smarter electronics guys on here have long suggested that you keep your original alternator and have it rebuilt to avoid some of the issues that show up with different replacement alternators......

 

Dealing with something similar on mine. My voltage was dropping erratically. My 03 Z06 calls for 145A replacement everywhere I looked. So I bought a Valeo 145A... the car ran fine with it but threw 1638... then over time more and more 'issues" came up. They included TPS, TAC and even lean codes. The car began to go into Reduce Engine Power mode with any of these.... could be cleared early on, but in time became more difficult to clear. Now I am back working with my original alternator which I have since found to be a 110A Valeo. I put on a new voltage regulator and been running it. 1638 is gone. But alternator voltage is still not consistent... but other issues appear to have cleared up unless voltage drops to low 11s... then car goes REP and throws random codes. So my next step is getting a new Valeo 110A alternator and see if consistent voltage is a fix.
This journey has been a PIA as my car is a LS3 swap. Every shop and tuner I spoke with told me my issue was either my xlink or Lingenfelter box. It appears not many understand the scope of issues the C5 platform can have with voltage issues.

 

Probably would have saved me a lot of money. BUT there is only one alternator rebuild shop on Maui, with not the best reviews on Yelp, so it would have been a crap-shoot at best. Maybe this will help someone else though.

 

Many of the sources I researched, including a GM Tech only service, suggest avoiding any off-brand Alternators, including the Valero, as they don't have to meet the tight voltage requirement of the IR included in The Genuine GM or AC Delco alternators. That surprised me since Valero makes the Voltage regulator incorporated into the alternator. All I am sure of is that the 110 volt unit (pn xx-182) not work with an automatic trans and neither did the Powermaster 145 Volt unit (pn xx-523). And we tried two different PCM's before buying the Genuine GM 145 volt unit (pn xx-440) which worked and has continued to work correctly.

 

You did mean to say 110 amp and 145amp right?

 

Yes, you are right. Thank you for the observation. At least I got the pn right.