91 940 radiator fan does not start with A/C

Hi, in my 1991 940 with B204E engine I have a problem with ECC A/C
system that randomly do not start the front radiator fan.
I checked the radiator fan relay by short-circuiting coolant temperature
switch in the radiator, and fan start.
I checked, in engine compartment, the connector between ECC unit and
radiator fan relay, and it seems ok.
All other function of ECC control unit works, but very rarely, and
specially in winter, the A/C OFF button light flash, however is
sufficient to switch off instrument cluster and re-turn it on and
problem disappear.
How I can check?

Thanks.