| If I isolate the heater valve electrical supply on the valves themselves and feed 12v direct from another source then they switch off OK and the heater core goes cold. Wiring to the valves has been checked using the BMW diagnostic system and is OK. |
Well if you supply power to the heater valves and the system goes cold then obviously the system/wiring was NOT checked correctly using the BMW diagnostic system.
The heater valves are powered closed meaning that they should have a constant power and ground present unless the heater is turned on, then the power or gound is taken away (I think ground) and the valve opens because it is built to be sprung open anytime there is no signal present (safety feature)
Sounds like you may want to take it back to whoever you paid to diagnose it and have then recheck it (should probably be free) since you did pay them to diagnose the problem and it sounds like they overlooked this.
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