Q: The heater fan stays on high speed, even when the key is turned to the off position. I have to disconnect the battery to get it to shut off. What is the problem?
A: Your vehicle most likely has a relay type blower motor control system. The blower motor relay is possibly stuck. When the fan speed is set to the HI position, an electrical current energizes a coil inside the relay. The energized coil causes a set of contact points to close, pulling the current through a fusible link from an electrical junction block. The contact points inside the relay may be stuck, keeping the current flowing to blower motor. You will most likely need to change the blower motor relay to solve the problem.