2012 Kenworth T660 4 batteries, 1 pair not charging

Hey all.

This is on a 2012 Kenworth T660, it sat for 5 or 6 months and I just swapped in a set of good batteries.The truck has four batteries and they are wired into pairs(with jumper wires). It seems that the front pair is for the starter/maybe more and the rear pair is for the ECM/lights etc. The front pair is working perfectly but the rear pair is not charging properly. I took all of the wires off and cleaned the corrosion but it's the came thing. I also traced the wires a good bit and all looks well, the starter has full power, alternator is putting out full power, grounds all look good.

The only thing I can think of is maybe I put a connector on top of instead of under a jumper. I just remembered that the jumpers only can make a connection from the top on one end. I'm about to head back to the truck and freeze my butt off while I try some more. I just wanted to make sure there isn't something I'm not aware of that links those two pairs together or if there might be a fuse or something that's partially blown. I found that one positive wire that runs to the rear pair does have voltage coming from the front BUT it lose about 1 volt(11.5 instead of 12.5) from what I measured on the front pair.

Is it safe to just jump the two pairs together for a full parallel setup? I'm thinking they isolated them to protect the ECM and other sensitive electronics. I tried it briefly with jumper wires and it seemed fine, the positive got a bit hot but it was having to charge up the rear pair.

 

That's not how it is on newer KWs. They have four batteries and the front two are wired together as are the rear two. Then a positive and a negative run to each pair.

Yeah on these newer KWs they are wired in parallel but two pairs rather than all together. The front pair has a good connection to the starter and alternator. IIRC the positive from the rear pair are goes somewhere else(negative goes to the battery box chassis)...or maybe it runs to the starter but being tired and frustrated I forgot. But for sure the rear pair isn't getting good charging current.

They haven't been load tested because they do their job just fine so long as they are being charged. They ran down because the truck wasn't properly charging them. Once I jumped the pairs together, so all four are in parallel, everything worked perfectly.

All of the connections at the batteries are fine so I suppose I need to trace the positive for the rear pair again and make sure it's got a good connection.

 

Ultimately yes, but the connection isn't in the battery box like in older trucks. As Goodysnap explained it's at the starter. So it isn't nearly as clear that they are in full parallel. Yeah it actually cranks just fine off of one pair, but then they are beefy AGMs.

Ah ok, I was tackling a bunch of other issues and in being a bit frustrated I apparently missed that they were all meeting up there. Thanks Goody. I'll have to check the connections there.

Also, what do those two plastic boxes on top of the batteries with three wires each do? Are those breakers or fuses? When I was poking around with the multimeter, iirc, all of those wires were at about 5v.