06 9400i Voltmeter

2006 9400i C13 KCB...Voltmeter rarely read correctly while driving. It used to consistently read approx 13.75 V. About three years ago, it started reading low and I thought that the alternator was going bad. So I had it replaced. That helped a little but still wasn't reading what it used to. About 9 mos after that, the truck failed to start one night when I went to leave the house. I found a dead battery. Replaced all 4 batteries. That didn't make any difference.

While driving I have noticed that if I accelerate hard the gauge will go down to 12 - 12.5 V and when I release the throttle it will go up to 13 V. The gauge usually reads around 12.5 - 13 V now. I have a Voltminder digital display and a Scangauge. Both consistently read approx 13.9 V while driving. Why the difference with the throttle?

With the key on (engine off) the gauge reads 12.5 and the Voltminder and Scangauge both read 12.1 V...I know, not a big deal.

With the engine idling the gauge reads about 13 and the Voltminder and Scangauge read 13.9 V.

On a few occassions the gauge has worked properly and showed about 13.75 V while driving.

I guess my question is, could a bad battery from years ago have started this? Also, how can it be fixed? I can keep ignoring it (it has been 3 yrs or so) and just use my voltminder, but it has always annoyed me knowing that it used to be more accurate. I understand that the gauge won't read the exact voltage, but it shouldn't be one volt off should it?

 

Thanks for the replies. I'll look at the wiring. Does the ECM port in the cab read different info from the ECM than the voltmeter gauge would read? I ask because my Scangauge connects to the ecm port in the cab reads a higher voltage than the voltmeter gauge. Thanks again for the help.