I don't mind replacing both if it ends up that way as I'm updating many parts on this 2001 DRV 3500 5.9 w/88,000 mi.. But given the following scenario, even before taking any readings, which might "you in the know" suspect- Alternator or V/R (all wiring has been checked);
4 times in the last 2 months my dashboard chimes and indicates "check guages". The battery charging level then shows at or just above 0 instead of around 14v where it usually is with engine running. A restart or 2 and everything is back to normal for a couple weeks. One time I did happen to see while driving the needle slowly creep down from 14 to zero and the dash doesn't chime until almost to zero. Again a restart of the engine and things are OK.
I find it unlikely since it will run so long between these events that it's actually a faulty alternator and suspect the V/R which is a circuit built into the PCM. Anyway it would be appreciated if you could let me know which part you suspect and just a guess would be fine.
Thanks

 

The next time to voltage drops off you should ignore the dash gauge and without turning off the engine, use a DVM take a voltage reading at both the battery and alternator output terminal. Knowing these two reading when the charging system is acting up could help narrow down the issue.

 

Thanks. Will do this although it could take weeks for this to happen again.
Is there any possibility this could just be a malfunction of the gauge circuitry? I'm sure that involves the PCM also but I don't think I would replace the PCM just for an infrequently faulty gauge.

 

Yes it could be that or a few other possibilities. It's all guesswork without having the readings I mentioned as a starting point for the diagnostics.