14 volts at the battery and alternator and 12 volts on the gage

on my truck the dash volt gauge was showing just above 13 but with lights, fan on it was below 13.trided several different readings, from nothing on to everything on, wrote down all the readings. then I took out the dash gauge, used my 12 volt tester to see which prong was hot, then put my meter on the correct prongs & went thru the process again. it showed the exact same as gauge, so I know know gauge is good.

then I started truck, put every accessory that I could think of on & put red lead on hot at back of alt/black lead at ground back of alt. got 13.9 volts. put leads on batteries got 13.8 looked at dash volts it read 12.5. I knew my alt/ batt & charging sys was good. I disconnected my battery cables, so no current would be in the wires. I then hooked my continuity tester to each prob on the dash gauge & after about 15 min of tracing ground & hot from dash to where they went, found the hot wire from alt going to dash gauge, was corroded real bad & more than 1/2 of the wire strands were broke. cut out bad wire, replaced with quality wire connector, sprayed corrosion eliminator on it. siliconed it & shrink wrapped it. all is showing good & accurate now. with my intl coe it is more difficult to fix these kind of problems, need ladder, long arms e.t.c. I do not know if I went thru the right process to fix this, or if I could of done it a different way, but it did work, at least for now. I will not be going on the road til mon, to do a real test.

 

Well, I thought I would let the group know what I found on My 07 387 Peterbuilt. I'm 100% sure this is issue is complete now.
I thought it was a connection problem from last time because the positive connection was bad on the battery with low voltage and I fixed it by simply cleaning up the connection but this last week the problem returned.

Low Voltage 12 volts or so on the dash and 14 volts at the alternator.

I bought this truck used and it has always kind of cranked slow to start but never and issue starting. I didn't think twice about it.

This last week the truck starting doing the same thing it did in the original post so I get my DMM out and start checking.

Just like last time I found that the one battery (one on the far left) connection was bad and when I went to clean it up I had a really hard time getting the bolt off the stud.
Well bells went off in my little head this time because I knew that wasn't right so I suspected that there was a wiring issue.
There was.

This truck has 3 batteries. The battery on the left was the one that was showing low voltage. It was completely separated from the other batteries in the box and didn't have any parallel straps on it.
The parallel straps were positioned on the back 2 batteries.

I disconnected the leads and moved the parallel straps to the first two batteries. (left to right)

I also noticed that there is a fused bus located in the battery box that fed 3 smaller positive wires that ran out into the truck so I put that bus on the 3rd battery (far right) by itself and viol la, motor cranked over like it was supercharged and the ISX sounded much better idling than before. All batteries reading 14 plus volts.

This wiring scheme is not normally what you would see in an old truck. They just simply parallel all the batteries in the box in older trucks. This is not that way. I'm guessing it has something to do with protection while cranking maybe. Maybe the third battery is switched in and out during cranking or something. I don't have a schematic but apparently Peterbuilt does not simply parallel all the batteries in the box or at least they didn't on this truck. What I wound up with is the front 2 batteries were in parallel in the box and the last battery had two wires of its own that were not paralleled in the box. I would love to see a schematic on this but this is correct now.

So FYI to the group if you purchase a used truck with this problem.

 

Yah, good to hear. Who knows what happened in the past. Lots of wiring gets monkeyed with. In order to save $10 per truck, some manufacturers have batteries connected by pairs with each pair having it's own positive cable to the starter. So with a typical 4 battery truck, only 2 pairs of batteries are connected in parallel in the battery box and the two pairs makes their final connection together at the starter from the two main battery cables. What happens is one of positive cables, or ground starts to go bad so that pair isn't really charging normally or providing properly during crank. Eventually they get so weak that weird stuff starts to happen. The cab and or engine will draw off of one pair or the other from the battery box. Murphy's law says the pair the engine draws from is also the pair with the bad cable and not charging properly. So to prevent this, a $10 battery to battery jumper is all you need to ensure each battery is connected to each other to eliminate issues like this.

 

Having the same exact problem on my 05 Volvo. could one bad battery cause this.