Thread: 300 tdi alternator interminant fault????

I posted yesterday indicating my alternator had stopped working.......

Well after I picked up a new one and was driving home and the battery light went out, got home and had 12.8v across the battery with engine running

I did however find an aux battery completely dead....dropped cell, low on acid. I noticed while driving home my traxide isolator started flashing which means it would have separated the aux battery from main battery this is why i check the aux's.........I wonder if this cause the alternator to start working I.e. The dual battery isolator disconnecting, like it was too much work for the alternator trying to charge a dead battery

Is this possible with a dual battery setup???

Should I keep the new alternator just incase or do I return it.......

yes it is possible

if the aux battery is shorted and the main battery is charged then the isolator will permit the 2 batteries to connect that drains the main battery and if the alternator cant keep up the main battery starts to go flat

when the main battery gets to the point where it cant keep the isolator in connected mode the aux battery drops off and then the alternator starts to charge it till it hits the point where it will couple the 2 batteries together.

theres usually nominal minimum voltage that the 2 batteries will connect at and it can be as low as 12.5V so what you might be reading is that your main battery is sitting in the middle of the hysteresis range of the isolator IE above the low voltage cut out but below the cut in voltage then when the unit cuts in because the AUX battery is trying to draw more than the alternator supplies it eventually drops back below and you start charging the now nearly flat main battery again.