backstory: 1996 NAS Discovery. son drives it. it's still in my name.

the tachometer started going crazy and the battery light came on. i tested the battery and it tested fine. so, figured it needed an alternator. ordered a used part from a reputable vendor and installed it. same issue. i will be pulling the alternator tomorrow to get it bench tested. if the alternator tests good what else should i check?

thanks,

 

it's probably the alternator (again) my guess is that the used one wasn't good either...the tach is driven by a one wire feed off of the back of the alternator....just to be sure ....check that the terminals are clean and tight

 

Check the fuse under the hood 100 amp fuse located in the engine compartment fuse box passenger side black rectangular box near the battery and the overflow reservoir, you will need to use a volt meter to test that there is power on both sides of the fuse terminals. the plastic cover pulls up once cover is opened you will see screws that attach the fuse to the terminals.

 

Literally had the same issue the other day. Ended up being a bad MAF sensor. Really strange but I replaced it with a used one and it solved the problem. Had autozone test my alternator and it was fine.

 

put in a new alternator from O'Reilly's on Friday morning. truck is still running fine.

thanks to all that responded.