Good morning, My brother in law has a 2017 T660 and is experiencing starting issues sporadically after sitting for an hour or so but on other occasions the truck will start for days with no issues. He was told to replace all 4 batteries but because of the issue's behavior I find it difficult to believe that all 4 batteries are bad but also think there could be a dirty connection. He's on the road and I'm trying to help guide him through some diagnosing of the system. I have several years in the automotive electrical field and feel confident in helping him although my knowledge on heavy truck electrical is (0). The other day I had him get a jump box to have in the truck, of course the truck has started as it should for the last few days until yesterday. It wouldn't start he connected the jump box to the jump post and it started. So we started to start diagnosing through text messages. He sent a picture of the batteries and from what I can tell they are wired in parallel, I asked him if there's an isolator or battery disconnect solenoid. He said he didn't know and what that would do. I explained that either would make a main set of batteries from 1 pair and an aux set from the other 2 batteries. I asked him to trace the battery cables to see if they went to any device, he told me it appears that they go directly to the starter but also said it was hard to see the path. A while back the truck had set for a few days and wouldn't start when he went back to it, I gave him a set of cables so he could jump it. The cables I gave him have a digital volt gauge on them and I had told him to keep them in the truck. Yesterday I had him connect just 1 end of the cables to each battery and give the voltages in the following conditions: 1) key off, 2) key on and 3) crank attempt Battery 1 - 1) 12.8 2) 10.6 3) 10.3 Battery 2 - 1)12.8 2) 10.8 3) 10.7 Battery 3 - 1) 12.9 2) 10.7 3) 10.6 Battery 4 - 1) 12.9 2) 11.0 3) 10.6 These all seemed good so I had him disconnect all the battery (-) cables then reconnect the ground to one battery at a time and perform the test in the following conditions: 1) key off 2 ) key on Battery 1 - 1) 6.1 2) 3.5 Battery 2 2) 12.9 2)12.9 Battery 3 - 1) 12.4 2) 10.6 Battery 4 - 1) 12.9 2) 12.5 going through several test steps at one point battery also had a quite low reading. I'll be getting back to him today to go through a couple more steps, his notes are throwing me off this morning. My questions are: 1) are all 4 batteries really connected in parallel with no separation (house and aux)? 2) Does the truck start on 12 volts or is there a relay energized in the crank mode that series the batteries for a 24 volt start? 3) is the jump post connected to all or only one battery? Does anyone have any suggestions on steps to try? Thanks in advance for any assistance you might be able to offer. 0
Looks like the first battery is a problem per your test of 6.1. One battery can ruin all the batteries. Have him replace that battery and should fix the problem.
From your post it seems battery 1 is the problem. I have had one battery in my set of batteries cause the symptoms you describe. While some drivers will insist the only solution to one bad battery is to replace all batteries, that has always seemed overkill. I am certain big trucks use only 12 volts and LOTS of amps, for starting.
I bad battery will drain the others. has he checked that the fridge hasn't been left on etc. Kinda' like tires, replace them in pairs, (replace all 4 batts)
Always replace batteries in a complete set. One weak battery will draw every one in the group down to its level. Trucks at one time had 12 volt electrical systems with a series-parallel switch for 24 volt starting. That was phased out over 30 years ago and to be blunt, they were junk. All 4 batteries are for the truck, there are no starting and house batteries. Only place you'll see "house" batteries is if there's an electric APU.
Thank you to all who replied, I really do appreciate the help. My original thought was battery #1 also, I had him take another reading after they sat for an hour disconnected and #1 started rising to 11 volts which I know is possible but not have any real amperage so with a load it'd most likely drop voltage quickly. I had him make sure that all his accessories were turned off as well as 2 inverters he has plugged into a lighter adapter, when I was a dealer tech we had vehicles all the time coming in with dead batteries and the only parasitic load I would find was their phone chargers plugged into the lighter port. On battery #4 there's a black plastic box with a battery cable going into it and one exiting along with what he thinks are grounds. Any idea what that is? I can get him to send me a picture of it if needed. So all 4 batteries are ran in parallel that was very helpful, I'll get with him today. He couldn't do much yesterday, it was raining where was at and waiting for today to drop the load. Thanks again for your assistance.
If you have pictures its helpful to send them along to us as well, but as others have said, that single bad battery is probably messing with the rest, frequently the primary battery (or 2) will be "starting" batteries, it doesnt mean the others arent in the batch, but it means that the starting load on that particular battery (or 2) will be higher than the rest. Either Have him move battery 1 to battery 3 position till he can get it replaced or just eliminate it from the circuit entirely
You can bring the batteries to AutoZone to check ... I was replacing twice just one battery( not same battery ) before i had any issues ( just preventative measure ) to avoid issues . Also clean all connections really good with sand paper , i had starting issues before i cleaned them perfect .
Here's a picture of the box I had mentioned, it may just be a junction block but I haven't looked at it directly myself. I asked him to cut the wire tie and take a picture of the inside.
