Voltage guage reading negative with check engine light on

How was it diagnosed as a bad alternator?

What does "already known problem with computer needing replaced" mean? Was the Engine Computer already replaced or is that just poor English? If it needs a new computer, why?

Measure the voltages on all three wires on the alternator while the engine is running and holler back with those numbers.

This is very confusing. When kids text that something "needs replaced" instead of "needs replacing" or "needs to be replaced", we don't know if you mean something is needed or something was needed and has already been replaced. We don't do texting shorthand here because it always leads to confusion and we can easily give you the wrong information or tell you to do stuff you already did. By the time you come here you have already spent a lot of time trying to solve a problem, and just like when you talk with your doctor, we need accurate information and symptoms.

The Check Engine light and the voltage regulator for the alternator are both associated with the Engine Computer under the hood. That computer has nothing to do with the fuel gauge. You have two different problems that both could be related to the instrument cluster.

It's nice that auto parts stores will read fault codes for you but you have to understand those codes never say to replace parts. They only indicate the circuit or system that needs further diagnosis. This is the perfect example where they should have suggested the alternator was the most likely suspect, but not the only one. That's why I wanted you to measure those three voltages.

As for some wire that was touching something, was that the cause of the original problem or was it the result of replacing the alternator? Is the charging system working now? If it is not, what are all the symptoms? What exactly do you mean by "the voltage gauge reads in the negative"? It can't read a negative voltage. It can only read too low. Has the pointer gone out of sight to the left? Is it just reading low? Measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is, you have an instrument cluster problem, not an alternator problem. If testing by Auto Zone showed the voltage was low, the suspects include a defective voltage regulator, worn brushes in the alternator, which can be replaced for around ten bucks, and a break in the wire between the alternator and voltage regulator. We can narrow it down when I know those three voltages.