I tried to follow the big red cable off the alternator but I lose it in the harness. It looks like it goes to the starter. Can someone tell me for sure?

My truck is barely charging. The gauge is always just above (or just in) the red zone. Batteries and alternator are 1.5 years old. Connections are all clean. 11.9v in the batteries, alt puts out 14.9. I made a temporary cable and hooked it from the alt to the drivers battery. Gauge went to 13v+ and batteries read 14v+ while running.

The other strange thing was the turn signals would cause the speedometer to jump 5mph up and down when the batteries were low.

11.9 in the batts? a fully charged batt should read 12.6 volts(2.1 volts per cell). anything below 12 i would consider a dead battery. the large alt wire on my truck attaches to the passenger side battery positive terminal. when idling my volts gauge reads above 14 volts(14.5 or so) and under full load of headlights, running lights, and pushbar lights it still sits above 14 volts. i redid all the battery cables with 2 gauge HD amp wire so that's why mine sits where it does.

Dave's right, look to the batteries first. I'd disconnect all cables, then check voltage on each battery. My guess is one will be OK at 12.6, the other will be below 12. If so you could try charging it by itself to see if it improves, but if you find this it is probably bad.

That's my quick thought on the problem.

you might check the fusable link on the alt wire.

The batteries were 11.9v because the alternator wasn't charging them. After improvising my alternator cable, the batteries are now both at 12.6 volts.

The passenger side battery has 2 wires to it, one from the drivers side battery and the other to the starter. The driver side battery had only one wire to it (until I added the other). All wiring looks OEM and the truck was fleet maintained by local government, so I doubt it was messed with.

I think the red wire from the alt runs to the junction block under the plastic cover on the firewall passenger side then goes to the starter. There should be a fusible link in the red wire some where between the starter and the alt. It should be close to the starter to prevent fire in case of overload. Thats where they were on the older trucks.

when I replaced my alternator with an 160 amp unit, I made new alternator wire from #4 wire, and went through a wafer fuse to the driver's side battery with it. All is well, no charging issues at all......

94K30, what size wafer fuse did you use and how did you make the connection to the battery?