this sound like a normal range for voltage

Shouldn't drop that much. Since all those accessories are powered from the cab, it may be just main power supply to the cab that is bad. Could be batteries or alternator too though. Check simple things first, check all your cables and connections including your cab power and ground connections.

 

To check the batteries you need to load test one at a time. If you don't have a load tester,you can unhook and let them sit a couple hours preferably over night. Then check each one. They should be within a couple tenths 12.7-12.8 if you have one read low 12s high 11s that would be a suspect.

 

I had same thing myself, except mine would go down to 12 volts with just lights & fan on med. I took off wire ends on alt hot & ground & a couple at the starter & put new ends on & crimped with good crimp tool, now it's back where is was. not sure which wire had a bad connection, or corrosion, loose ground, e.t.c. but it was in the wire contact somewhere. now driving at night with everything on it is about 13.8 idle & nothing on 14.3

 

a loose alt belt can cause your prob too. 1 way to tell is if the alt has the old type pulley, if the belts are slipping the pulley will be shinny from the belt slipping & polishing the pulley.

 

I'm assuming that since you used a multimeter and not a test light that you are fairly good with electrical. If you are suspecting a bad connection somewhere in the alternator/charging circuit I would run the engine with some electrical accessories on (leave them on and the truck running when you do the following tests). Check your alternator output voltage. If that's good, put one meter lead on the alternator output terminal and the other on your positive battery terminals (one at a time) What you are doing is checking for a voltage drop. If you read 0 to 0.25 volts you likely don't have a connection issue on the positive side. Next put one lead on the alternator ground terminal (or on the alternator housing itself if it doesn't have a dedicated ground terminal) and the other lead on your battery ground terminals one at a time. Again if you see a voltage reading under 0.25 volts I wouldn't suspect a bad connection. If your meter reads above 0.25 volts during the voltage drop checks, you have a poor connection somewhere in the circuit between where your leads are hooked up and you should check for loose or corroded wires/terminals.

 

i just put a 270 amp alternator on my 2007 freightliner Columbia because it as 8 batteries and has an epu unit. before some one had a 135amp unit, and when you started the truck in the morning after running the EPU, the volt metter would show about 12.5 after driving all day it would show about 13.5. once i found out the alternator was to small i went and got the 270 amp unit. Once i installed the new unit and started the truck the volt meter on the dash showed 144.1-14.3, and after driving for about an hour it started showing 14.3-14.7. i just wanted at ask is this normal, or is there something wrong? i have four regular batteries, that are the main batteries and start the truck, and then the are 4 AGM 800CCA batteries for the EPU.
thanks!

 

double post