Porsche 928S4 MY 1989 or 90 had these symptoms:

1.  interior ventilator (blower) does not run at all. Not on any speed.

2. radiator fans (both) do not run.

This happens always together, so non of the fans work.

If you slide the HVAC controller to DEF position the fans start working and keep running even if DEF is switched off. Very strange.

Swapped Fresh air blower relay without success. Measured on the CEB that this relay is not forwarding any voltage to the system (i.e. powered but not engaged).

Turned out that the fog lights also did not work.

Took some time to find it as the problem appeared to be intermittent.

But turned out that a broken or bad X-relay caused the power failure to the HVAC controller  and the Fog lights and the radiator fan system. A chain of seemingly unrelated events happen which are caused by the X-relay...

regards Theo

1992 928gts Midnight Blue

The Netherlands