Auxiliary Battery Not Charging

I have an 2009 3500 sprinter with an auxiliary battery under the hood. It is not being charged, The starting battery which is under the drivers feet is charging fine. The auxiliary battery runs the roof AC, the automatic side door and the interior lights.
I have replaced the auxiliary battery but it continues to run down. I can charge it up manually but within an hour of use it is spent.
Couple of thoughts: does the alternator have two charging circuits? If so, could it be partially bad? It has been making some strange noises so I think this is a possibility or it is about to go out, however the main battery charges fine. Also, does the auxiliary battery only charge when the Auxiliary Battery Cutoff Relay is engaged? Not sure how to test the relay but when I jump the relay the ac, doors and lights work fine. This makes me think the relay could be fine. I tried to trace where the relay gets its ignition power but could not locate as it goes through a long wiring harness.

I don't know if this will help, but it is worth a try.

View attachment AuxBattery.pdf

Thank You 3mbusa, I have this one The wires that energize the cut off relay come from somewhere else other than Terminal 1 D+ . Not sure where and can't find a blown fuse anywhere

nymt said:

Thank You 3mbusa, I have this one The wires that energize the cut off relay come from somewhere else other than Terminal 1 D+ . Not sure where and can't find a blown fuse anywhere

Click to expand...

I have very little electrical skills, but I had this problem, only it started very slowly. At first I wasn't getting the proper voltage during the charge. Then I would check it now and again and it would be down around 11-12 volts. I found that my "Y" cable was the culprit, the cable that connects both batteries to the alternator. I order a new "Y" cable and replaced it myself (PITA!). After that, no more problem. So, check the entire length of the "Y" cable, the connections, etc, for any signs of any kind of issue. For whatever reason, these break down and the proper current doesn't pass through to complete the needed charge voltage.

Thanks Synergy, I replaced the Y cable last year when it went bad on a different sprinter I have. The cable rotted out right under the transmission so I replaced it on this one as well. Good thing too as it was on its way to bad.
The Y cable splits at the starter and one end goes to the starter battery, but I believe the other end goes to the engine compartment where there is a red plastic cover which slides back, exposing a positive pin (terminal). I don't see how the Y cable goes to the Auxiliary battery.