Help no tach and alt gone nuts

[dheavychevy38]

Ok I have a 97 chevy 3/4 ton 4x4 6.5 diesel and a 4l80e. The other day while driveing my alt would drop to around 11 volts and go back to 14 volts and back and forth. Once in a while the batt light would appear when that would happen the tach would bounce. So today i install a new alt. Now if im lucky the alt will charge 14 volts ina hour other wise it is at 11. I have no tach while cruiseing or at idle only at 1/2 throttle or more. Any ideas this is my daily driver. Thanks Denny

[Robyn]

Check the battery grounds on both batteries as well as the positive cables too.

Be sure that the cable that connects to the plus stud on the alternator is good all the way to the junction box.

Poor grounds will cause all sorts of grief.

Kepp us posted

[dheavychevy38]

Ok just went outside to check out somethings and start the truck and it is chargeing 13.4 and tach is workings.

[JFerg65]

I'm with Robyn on this one. (Usually a sure thing siding with Robyn's opinion)

Check your battery connections. I had something similar, it was driving me nuts. After I worse case scenario'ed it to death, it turned out to be a damn bad connection on the battery, those stupid side post battery bolts. I also had just enough corrosion to foul things up as well.