91 940 radiator fan does not start with A/C

[Telespalla Bob]

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.