My son just gave me his 1987 560SL. I am new to this forum and need help. I am getting no heat, yet the fan is working and the a/c is OK. What should I be looking for. Living in the east, means I need to fiz this before the cold weather moves in. Someone told me to check the monovalve. I took out the inside of the valve, and the rubber seems to be in shape but I don't know what to be looking for, or if that recommendation is correct. Please help. Thanx