I have a 1995 GMC 6.5 diesel. It has a BD chip, K&N filter, 4" BD exhaust, and 181000 miles. It started having low voltage, around 9 volts, the battery light came on, and the tach keeps flickering. I was on my way to get a battery to see if one might be bad, and the voltage went back to normal, and the light went out, but the tach is still jumping all over the place. Does anyone have any ideas. Thanks.

Either bad connections at the alternator or a bad alternator.
Tach runs off the alternator.

Probably not a bad time to check all of your grounds. Very strange things happen when there is bad grounds. As previously mentioned the Alternator could be going. Also if you are thinking one batt. is bad, they should be replaced in pairs so the weaker one doesn't drain the stronger one.

Wife had exactly the same problem last month on the Suburban. It also threw a code for the pedal.

It was just a bad alternator. Easy to change your self- just be sure to disconnect the batteries first.

Your Tach get's it's signal from the alternator. Look for corroded battery cables & terminals first. If you can afford to, go with new top-post type terminals. The side posts are a weak area that can loosen up from vibration. Also use the sites search feature for a lot of info on checking all your grounds. This is another known problem area on these trucks. Your alternator may actually be OK. Go with the troubleshooting first before sinking money into a new alternator.