Ran into an issue with my '98 T100 - basically same drivetrain as a 4Runner so figured might be able to find some opinions here.

The alternator died, but the truck didnt give any indications until it was too late - no battery light, and the battery gauge reading where it should be. All of a sudden lights got dim, track started sputtering, THEN the battery light came on for about 20 seconds as it was really dying and bucking like a bronco.

Anyone know from experience about what voltage the battery light gets triggered at? For the record, in the photo below, alternator isnt functional and the battery is at 11.8v. The battery gauge is useless as it is frustratingly where it would be if the alternator was pumping out 13.x!!

 

Are you sure the alternator is your issue? (You can get them tested for free at many auto parts stores.) You need very little electrical power to keep the truck running once started.

Your description also matches a good alternator, but with the engine dying for some other reason. As the rpms drop, the headlights will dim as the voltage drops from ~13.6v from the alternator to ~12v from the battery alone. The "Alt" light will come on once the alternator is spinning too slowly.

 

Good question; alternator output is 0. If the truck is running, disconnect the battery, dies right away. i put in a fresh battery at my house, drove 2 miles to my shop and the voltage reads about 11.8v. Im fairly certain the alternator is dead; more curious as to why the gauge doesnt read properly.

 

First, with KeyOn-EngineOff, do you get the ALT light? If not, you could have a broken wire or something as simple as a burned-out bulb.

How did you measure the alternator output? We start at the battery post, but if <~13.9-15.1v there, you need to check it at the B terminal of the Alternator (the fat wire). If the fat wire is open the alternator can be fine. If you're getting 0v (or something low), you can check the L wire (yellow, oddly enough). When that is at ground, the ALT lamp lights.

If the alternator is making output, the easy way to go is pull the alternator and have it tested. If doing that is difficult (you're on a desert island), you'll first want to check for battery voltage on IG (red) and S (white).