The Low Coolant light keeps coming on, and the tanks full. Any idea why this would be happening?

Even if your recovery tank is full, it doesn't necessarily mean the radiator is. Check your radiator level, and if it'* fine, either disconnect or replace the level sensor in the radiator. It may simply need to be removed and cleaned (both the face of the sensor in contact with the coolant and the electrical contacts).

Your recovery line from the radiator to the recovery tank may be kinked, disconnected, or leaking if the radiator isn't full, but the tank is.

I'm going to vote for a flaky sensor here; mine goes off routinely for no apparent reason. I'd love to know how to clean the contacts, but there doesn't seem to be any moving parts to this sensor and the little metal pins that stick into the coolant don't seem to be dirty or corroded, so...? Maybe it'* just been cooked too much over the years and its internal resistance has gone wrong, leading to a bogus Low Coolant signal. I haven't been able to find anything in the service manual about what its resistance is supposed to be in the first place; I imagine GM would just say to replace the thing if it gives false alarms.

You can simply unplug the wiring harness at the sensor to get the alarm to shut up, but then you'll probably forget to ever get around to fixing the sensor after that, and I can tell you from experience with our Trans Sport that it is a Good Thing to have that sensor working for you.

P.*. Have a rag handy to stuff in the hole if you remove the sensor for testing or replacement, since obviously your coolant is going to all come running out when you remove it...

Yeah thanks for your help

I just removed and cleaned the low coolant sensor on my '92 SSEi today. (I had the same problem. Low coolant warning with coolant full to the top of the radiator.)

I made a clip from a nail to insert under the plastic tabs on the sensor to lift them clear of their catches. I just shoved the clip in place under the tabs and left it there while I wiggled the sensor out of the hole in the radiator tank. I used a 00 rubber stopper to plug the hole to minimize the loss of coolant.

I was surprised at how small the metal tip was in the center of the plastic sensor. I just sort of gently scraped the metal with a tiny screwdriver. I didn't see two separate metal tips, just one sort of rectanglular in shape and, there was a deposit on the end of it. So far, the light has not come back on, but I have only driven it for about 10 minutes. Sometimes, it would take 30 min or so to start giving me false readings. Anyway, this little clip made a one-man job a little easier and saved the part from breaking.