W140 Cooling fan Won't Shut off?

[EmDee]

93 500SEL. Runs great, temp is fine, oil is fine, yadda, yadda yadda. Sometimes when we park the car in the garage and turn the engine off, you hear the cooling fans stay on for a while. We go inside and think nothing of it.

But lately, as the summer has come, we go inside and come out like 4-5 hours later and they are still on. And a couple of times they've stayed on all night and the battery doesn't have enough juice to turn the car over in the morning. But it does seem ambient temp sensitive.

We live in the Seattle, WA area, so high temps are usually not an issue. But the last few weeks we did have a few 95°+ days and on a few occasions, this type of problem arose. I've even taken the car out in the evening, after its been sitting with the fans on for several hours, for a quick spin around the block to make sure everything internally was cooler. When I park it and turn off the motor, the fans are still going and as stated earlier, stay on the entire night, causing a very low battery.

Any ideas as to what would be causing this?