Voltmeter reading low, alternator and batts good

Really inspect all of the cables and the jackets, not just the grounds. Battery box all the way to starter to alternator and out. I was disgusted at what I found under mine after giving it a very hard and long look.

 

Just remember, a bad connection anywhere in the circuit can cause issues. Everyone always shouts "bad ground" when playing with electrical but a poor connection on the power side will behave the same. Gotta think of electrical as a complete loop from positive battery post back to negative battery post. A poor connection at any one spot in that loop will cause voltage problems.

 

Well thats just it. If you would, buy one of those cig socket things you can shove in there and it will monitor your voltage for you. Buy one and go down the road. If it jumps from a green light to yellow you really have an issue. If it stays green just put tape over the volt meter on the dash and keep going.

Some issues are better ignored than traced out. Especially if another method can show that it's really not an issue at all and that trying to fix it could cost you way more than its worth.

If you have an alternator light on that truck and its NOT illuminating, I'd say that helps cross check it as the gauge.

 

The best place to tap for a voltmeter is right off the battery cables. This tells you what is actually going on at the all important battery bank and if those batteries are being charged correctly.

If you have any bad connections between your battery bank, or the point where the alternator ties in, and the rest of the systems, you will get "in system" voltage drops. Doesn't take a lot with 12v DC systems. Only 1/2 ohm at a bad connection at a fuse box or other area will produce a 5 volt drop with only 10 amps flowing! Plus a lot of destructive and hazardous heat, or even a fire. Truck systems will usually have more amperage than this during the day, add at least another 40 for night with modern headlights, LED markers, sides, and trailer lights.

Check for both bad connections and shorts.

 

A lot of good information on this post so I will share my two cents. In the picture attached is the truck or one of its sisters parked by my car I was working in the shop 2nd shift. Driver reported the alternator was bad. Voltmeter reads low and the head lights are dim. The truck did not have 100,000 miles on it. The alternator and starter had already been replaced. Seeing that I pulled the file to see what had been done to it. It had already had something like 5 alternators and 3 starters along other electrical work.
With the engine running and every electrical devise on I started doing voltage drop tests. Found the meter read a good bit of voltage between the frame and cab. The cab ground strap was near the steering column. The cab had been under coated before the strap was installed and then undercoated after it was installed.
You cannot see many electrical problems. The best way is by testing not trial and error. Voltage will very some along the way and testing it without the engine running is an interesting idea. Some volt meters were connected directly to the alternator and will read different than voltages most other areas when running. Test both sides + and -. Hope that helps along with the other posts and you get that worked out.

 

Multimeter is an invaluable tool for testing. I had an old bush beater pickup with a charging problem I chased on and off for about 6 months. Voltmeter would randomly peg out at 18 volts. Could not find the problem anywhere. Drove the truck with the multimeter attached to the battery and even when it acted up I still only had 13.8 volts at the battery. Eventually I managed to catch it screwing up while idling in the driveway. Negative battery cable looked to be the culprit. Cut off the insulation and it was rotten inside. New cable from battery to engine block and life was good.

 

I had an issue like that on a Pete, it was a broken fifth wheel ground strap, truck was less than a year old.

 

Just my 2 cents...... I had a similar issue and it was the relay connections. The cables where a little loose and dirty. I cleaned them up and tighten them up and the voltage meter never went under 14 again. The relay I’m referring to is tuck in the frame in front of the starter.