Another test to see if it is time to change antifreeze is using a digital volt meter. Connect the negative lead to the battery - terminal. Insert the positive lead into the coolant water. If you see a reading of .3volts or more, change the coolant. This not only helps protect the cooling system, but if you have a high voltage, it can alter the readings from coolent temp sensors etc and make the car run a little out of spec.
I know I will get some negative responses from this post, which is fine, but also I know it is correct.

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84 500 SEL (307,xxx miles)