1989 90 idiot light?

[MattJoslin at aol.com]

Hello,
I am happy to have happened upon this list.

I have been the proud owner of a 1989 90 for a little over 2 months. 

The problem that I have is the warning light for low coolant, high temp is 
coming on. This has happened before and I believe due to a small leak. Now 
the water level appears OK.
I need to replace the water temp sensor. I have an oil temp gauge and this 
remains steady so I think it helps to determine any overheating problem, 
although this probably is giving me a false sense of security. 
In other vehicles without temp gauges when the idiot light comes on it is 
often too late and the car is overheated. I drove for quite a while and it 
didn't overheat.

What am I missing and what should I look for?

Thanks,
Matt