XC70: General Climate

[Tatsfield]

You shouldn't have difficulty in buying a replacement control unit but if you are of the make do and mend mindset, the actual component that fails is a power transistor. These components run very hot and that is why the unit usually has cooling fins which project into the cold air intake of the ventilation unit.

I have broken open these units, identified the power transistor which is firmly stuck to the unit's heat sink and used its number to order a replacement transistor for a couple of pounds. The replacement procedure is merely unsoldering the transistor from the circuit board, decoupling it from the heat sink and replacing it with the new component, soldering it into the board and sticking it back onto the heat sink with heat conductive adhesive such as J-B Weld which is an epoxy with steel powder incorporated for conduction.

This repair becomes cost effective if the price of new control units is exorbitant or they are no longer available. For owners who intend to keep the car long term, it might be worth the bother to repair the control unit cheaply and put it into storage for the day when spares become difficult to obtain. I recall the last time I repaired a control unit of this sort it was listed at £40 but was a Chinese clone of doubtful reliability while the replacement transistor was of reputable manufacture and only £3 delivered.

[apersson850]

Funny, I've actually done the same to the fan driver in a 745 SE I once had. That one short circuited in such a way the fan was running always, engine on or off.
Found one dealer in Sweden who had the proper MOS-FET transistor available.

Did a similar thing with the interior light in the ceiling dome in the same car. But that went constantly off. A small transistor that went bad.