BMW E28 heater blows only cold or hot – water valve
The E28 heater blows only hot or only cold – what is the fault?
The typical cause is the electric water valve, which seizes with age. The same symptom can also be caused by the heater control unit or the interior temperature sensor. If the engine is warm but the air is cold, air trapped in the cooling system can prevent hot coolant from circulating.
Where is the E28 electric water valve and how is it tested?
The valve is on the bulkhead near the brake booster and master cylinder, where the heater hoses go into the cabin. It has an electrical wire and two water hoses. The control unit supplies voltage to the valve only when heat is requested. If voltage is present regardless of the control unit setting, the fault is in the control unit or the valve.
How do you fix a seized water valve?
The valve's cover can be opened, and cleaning and lubricating the internals often gets it working. If the rubber seal or diaphragm is broken, a repair kit (part number 64118390132) is available for the valve. An alternative is a mechanical water valve.
Why do the E28's centre vents blow only cold air?
The centre vents and the large grille on top of the dashboard bring in only fresh outside air. Warm air goes to the windscreen, the footwell and the round side vents of the dashboard. The middle control lever determines which vents the cold air comes from, so this is not a fault.