This may be a dumb question but what is the best way to determine if a problem is the alternator or batteries? Is it a load test of each part? Is it watching volts at start up for batteries and volts while driving for the alternator? 5his is just me inquiring no real issues yet other than corroded pos. cable. Although I do have a strange thing based on what I've read about over charge and under charge. Is it correct thatmif positive side of battery corrodes that it is an over charge issue and if negative side it's under charge issue. With my problem being the positive side on the driver side battery is corroded and the negative side on the passenger side battery just got some started. Is it possible to have both under and over charge at the same time?

 

Typically a battery will fail causing the alternator to put out max amperage for too long and it will kill the diodes in the alternator, so I would remove the batteries and have them load tested separately, if they come back good and the alternator voltage isn't keeping the system voltage at an acceptable level then the alternator is likely on its way out, or perhaps some bad connections on the battery cables, positive or negative. Proper system voltage is around 13.2 on the low side to 14.5 on the high side as measured at the OBDII port after the glow plugs have cycled off.