BMW: Check Coolant /Continues to come on despite new sensor, coolant tank, coolant level

Question
I have a 94 325i convertible...
I took it in for oil service and also because the check coolant light was on..The coolant looked at a normal level...The service people said its prob a sensor---Picked up the car---drove it approx 30 miles then the overflow resovoir CRACKED---total spew of stuff into my drive---they said they DIDNT do anything during the repair that could have caused his to crack ,,,anyway-- after them replacing the resovoir I STILL HAVE THE Check Coolant light coming on---I am soo aggravated that I am NOT going to take it back to the dealer, but I just curious what you might think is the issue

Thanks


Answer
Cynthia,
 Sorry for the trouble you have been going through, I'm sure it's very frustrating.  However you did learn a valuable lesson about the dealerships... they want to make money, and dont really care about your car.  Most of the people in there are just parts-swappers.  This means that they never really find out what the problem is, but they keep replacing parts til it fixes the problem.  This system works, but unfortunately it takes time and money away from you.

 Anyhow, If you the coolant resevoir is filled to the appropriate level, and the coolant light remains on, the sensor HAS to be at fault.  Its a very simple system, there is a sensor plugged into the bottom of the coolant tank, and it has a float on it.  When the coolant level is low, the float will not be suspended, and it will cause the light to come on.  So you know that the tank is full of coolant, so the sensor MUST be bad.  

 Any decent mechanic would have taken 5 minutes to test the sensor and fix the problem right, unfortunately they dont want to worry about small problems like this, because there is not enough money involved.

 So YOU will know what the problem is, and you can have it fixed I will give you instructions.  Open the hood, and located the coolant tank (passenger's side of radiator).  The tank is green/white and has a mark on the outside for the level when the engine is cold.  If you look down toward the bottom of the tank, you will see a black plastic nut holding a sensor in, this has some wires coming out of it.  Take the car to a trusted mechanic, point to the sensor and say "I want THIS replaced, and nothing else".  

 If by chance the light is still on, after the replacement, you have electrical problems in your cluster or something far more rare and serious. (highly doubt this is the case).

 Good Luck,
 Josh