New alternator and truck still dies?

So the battery is good, and we bought a new alternator, but my truck still dies when we take off the positive cable and the battery light is on. What could be the problem? I have a 1993 ford ranger xlt

Take it to AutoZone or your mechanic and have it checked. You need not be working on your truck. Removing the pos cable could result in further damage to electrical system. Thats not the way to check charging systems anymore.

Your alternator (new as it is) may be defective.

Stop testing alternators by disconnecting the battery. That's a good way to kill them. Normally an alternator can produce enough charge to keep itself running. But if you disconnect the battery you COULD cause a problem for the internal regulator. Hence you can lose electrical power.

Here's the RIGHT way to test a battery and charging system: Let the car sit untouched for at least 6 hours. Open the hood and check the battery voltage with a volt meter or a DVM (Digital Volt Meter). The voltage should read 12.0 or more. Likely not more than 12.4 volts.

Next, turn on all the electrics. Everything you can. Check the battery voltage. Should not go below 11.6 volts. If it does the battery is suspect. If it goes into the 10 volt range the battery is struggling. 9.6 volts is considered dead.

Start the car(or truck). Check the battery voltage while the engine is running, all other electrics off. Should be 13.6 volts or better. (not more than 14.4) Turn on all the electrics. The battery voltage should not change more than a couple tenths (13.6 down to 13.4)(for example).

IF the battery voltage is dropping while the engine is running then the alternator is failing or has failed. Shut the engine off and turn off all the electrics. WITH THE ENGINE OFF - NOT RUNNING - I SAY THIS BOLDLY BECAUSE IT'S EXTREMELY IMPORTANT: Never touch any moving parts while the engine is running. WITH THE ENGINE OFF - see if you can spin the alternator. If you can turn the alternator without turning the engine then the belt is either lose or worn. A slipping alternator belt is common. People assume the time to change a belt is when it broke. Not so.

Also check your battery terminals. Make sure there's no corrosion and the terminals are clean and tight. Check the cables for signs of swelling (corrosion inside the insulation). If the cable shows signs of swelling then change both of them. New cables will solve a ton of problems caused by corroding cables.

Above all, stop throwing money at the problem. Fully diagnose it before you spend any cash. Otherwise you'll end up with a new car for your radiator cap.

Sounds like a loose wire or cable somewhere

The problem is you are disconnecting the battery cable. All modern cars stop running when you do that.