97 Z starting issue

I have a 97 Z that is hard to tart after it gets warmed up. It starts fine when cold but, after I get it to operating temps, it cranks forever before starting then the idle runs high for a minute or two. If I let it cool off it starts fine.

I pulled a code from the computer because the SES light is on, p0118. It relates to a high output voltage on the coolant temp sensor.

I replaced the coolant temp sensor but it did not help and the code comes back after deleting it and the SES come back on. I noticed the other day that the voltage gage goes low, below 13v and sometimes dips into the red when I am at a stand still. I found if I put the car in neutral it goes back to
14v. I ran the check on the alternator, it tests out fine.

Any ideas as I don't really have the money to take it to a shop and have them find and fix the problem. Thanks.

I forgot to add that the fan is always on, even if the engine is cold.