Hi.
I'm having some issues with my 2005 Ram 2500 (Cummins Diesel). The batteries are good but voltage drops to approx. 10 volts when starting. After start, everything is normal and voltage goes up the 14 volts.
I have a Warn VTC air compressor that takes quite a load. Never had any problems until recently, it now drains the batteries so it seems that the alternator can't keep up with it.
As soon as I start the compressor, the voltage drops. I took the truck to the shop where the batteries were tested. They didn't want to sell me new batteries and stated that the charging seemed ok.
My question to you specialists is: can the alternator fail in the way that it charges in reduced mode, without failing completely? There are no error codes displayed on the Banks IQ and the charging is sufficient for everyday use, except it takes about 2-3 weeks to deplete the batteries if the truck is not started.
I'm puzzled.

 

It's not unusual for voltage to drop under heavy load (such as start-up). How old are the batteries? How much does the compressor draw?

 

The batteries are about 5 years old but were deemed good under load by the shop. The compressor draws around 125 amps.
What puzzles me is the fact that I used to be able to run the compressor without problems. It's only lately that the alternator doesn't seem to be up to the job.