BMW E34 heater blows only hot or cold air (water valve)
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E34 heater blows only hot or only cold air - what is wrong?
The most common cause is a stuck heater water valve. Other causes are no power to the valve, a broken circulation pump, air in the cooling system and the flap actuator servo coming loose from its mounts.
How do I test the E34 heater water valve?
The valve is open when unpowered, meaning the heat is at full. The centre pin gets a constant 12 V, and the control unit closes the valve by grounding the control wire. The resistance between the outer pins is about 12 ohms. With 12 V applied directly, the valve should turn.
How do I get a stuck water valve working?
Loosen the valve screws and tap it lightly while moving the temperature controls from hot to cold and back. You can also take the valve apart and clean it. Coolant will run out, so keep extra coolant at hand. Be careful with the diaphragms and resistance wires.
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