My battery is depleting while driving with lights and heater on. The alternator seems to be not recharging battery. Also, for the past few days there is a knocking noise coming from the front end coming up every 1-2 seconds, which leads me to believe it's the alternator bearing or something

I've tested the voltage while car is running and it is 11.7 with an almost dead battery.

I found some other threads that suggested that it could be the 3w bulb in the instrument cluster, but it doesn't seem to be burned out...I tested it with a multimeter

If it was truly the alternator which is not charging, wouldn't it cause the bulb to light up in the cluster?

I also noticed that the bulb is not illuminating during the bulb test, when I turn the key without turning on the engine. Also, the brake-wear light and other lights are not turning on at that point either. Only the glow plug lights are coming on when key is turned

Does anybody have any ideas what it could be before I take it to the mechanic?

I would check your voltage regulator first before anything, as it is simple to check, cheap to replace, and will most likely restore your alternator output. It seems like your dash lights aren't working properly either, which is unrelated. Could be wiring, or bulbs.

Check this page out

Or this page

And yes, you can remove it with the alternator still in the car.

Not all alternator faults cause the bulb to light.
The charge, low fuel, brake wear, and brake warning indicators get a ground through terminal D+ of the alternator. When the alternator starts charging, this terminal goes high extinguishing the bulbs.
With key on engine off, ground the D+ terminal (thin blue wire?) on the alternator and see if the bulbs light. If they do then you have an alternator or grounding problem.

Have the battery charged and load tested. 11.7 volts when measured across battery terminals generally means alternator is not charging.

Mine was voltage regulator that was bad. You cannot trust parts store machine when it condemns the alternator. The measurement only means that proper voltage isn't detected. You have to determine the reason.

Bearing are a separate issue. You may want to have yours rebuilt if it is OR. Parts store alternatives frequently have poor quality control.

The bulb is more than an indicator, it's actually part of the charging circuit. When the car is first started, a bootstrap voltage is supplied to the alternator through the bulb. If the bulb is burned out, the alternator wont generate power.

Your charging system should have a connection point on the right fender. This is a convenient place to test. See photos

http://www.peachparts.com/shopforum/attachments/diesel-discussion/142568d1496454559-alternator-upgrade-wiring-questions-photos-img_3170a.jpg

http://www.peachparts.com/shopforum/attachments/diesel-discussion/142569d1496454576-alternator-upgrade-wiring-questions-photos-img_3169a.jpg

There should be three wires. Two are B+ and if you test them with a multimeter (engine off),both should read the same as the battery. The other is D+, which is the bootstrap voltage that comes back around through the bulb. Turn the key to the run position, but don't start. Then measure the D+ voltage. If it's zero, then there's a problem either with the bulb or the wiring. If you have both D+ and B+ voltage, then the VR should be replaced.

The other idiot lights also feed into the voltage regulator to help flash it in case the alternator light fails.

Since the idiot lights aren't on and the alternator light isn't on, it's a pretty safe assumption that the voltage regulator is toast. Even without being flashed, it should still pick up and generate on its own with some RPM.

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I got the alternator rebuilt and they also put in a new regulator, and it is charging now. It's so strange that the light on the dash wasn't lighting up. The regular brushes were only about half worn, but it seems to have caused the issue

Thank you to everybody that suggested checking the regulator