07 3500 express. 6.0l non AFM engine. OEM 2007 AC Delco alternator and voltage regulator. battery less than 2 years old. after a very cold spell of - 8 degree weather , van started successfully though start battery voltage reading was only 9.4 v. But as I drove, the battery voltage was steadily dropping with a battery red terrible lite on the d ash. per my Bosch Amwell OBDII reader , b battery volts dropped to only 4 volts, so I returned home. (thus missing work for the day. ) My OBD2 reader has a charging circuit diagnostic so I ran it after battery was charged up overnight to 12.7v w/a three-stage charger. the next morning, KoEo voltage was 12.7, on engine start start the voltage dropped to 9.1. While engine was running at idle, the Bosch OBDII code reader charging circuit showed code P1637 'generator L circuit' . ( there was no red battery lite on the dash. ) the alternator hot wire into the + battery terminal showed good charging amps idle, via a Fluke clamp-on measurement of dc amps 13.2. the o2 sensors and all other available tests were w/in normal limits. neither the Haynes nor the Chilton manuals for GM vans have this P1637 code listed. a search for P1637 code on this chevy forum brought up P1637 as the code for some models of GM - chevy autos. there is no reference to a generator in any of the manuals for express vans. ) the only reference I can find to a 'frequency generating device' is for "the GM 3-wire MAF. " (Paul Danner's "Engine Performance Diagnostics," sec. 12 p. 12-3. ) My MAF is only a year old.

1/ what's the 'generator L circuit ' and what does code P1637 mean ? (note there was NO engine 'service light' or battery warning lite on on the dash. )
2/ does this DTC indicate need for an alternator replacement ? a voltage regulator replacement? or is it a MAF sensor or PCM issue?

my chevy dealer traditionally lets problems like this sit on the lot for a few weeks. And won't listen or look at my stored OBDII Codes, or l reports of symptoms or voltage / amp measurements. ( I'll be taking my van to a good independent mechanic near me. )

thanks in advance for any and all thoughts, as usual.

 

Give this a try.
https://charm.li/Chevrolet/2007/Expr...0Charts/P1637/

 

Wire diagrams.
https://charm.li/Chevrolet/2007/Expr...al%20Diagrams/

Did you test for AC voltage? Possible that it's a diode in the alternator.

 

thought alt is generating amps to supply dc to my engine battery . not ac.

 

wow, this explains a lot. very helpful, thanks 1,000,000.

 

It's supposed to supply DC but starts with 3 phase AC and converts it to DC with Diodes. When they go bad a small AC ripple voltage can turn into a bigger portion of the output. It's always a good thing to check when looking at alternator issues.
https://electrical-engineering-porta...harging-system

 

thanks, that write up was very helpful.

 

Had a similar issue during a harsh winter with my 6.0L Express. The P1637 code refers to the generator "L" circuit—basically the signal wire between the PCM and alternator that controls charging. If it fails, the alternator might still generate voltage, but the PCM can’t regulate or monitor it properly. Even if amps look good at idle, inconsistent voltage and that code suggest either a failing alternator internal regulator or wiring issue—less often the PCM. I ended up replacing the alternator and cleaning up all grounds and connections. That cleared it. Your dealer’s delay sounds frustrating; good call going with a trusted independent. Also, if you’re ever considering a secondary vehicle or camper setup while the van’s down, Truck1.eu lists well-documented vans like this: https://www.truck1.eu/campers/camper...-a3307776.html —helpful when comparing options across different dealers.