You'd need to check the wiring schematics and see what the recirc switch
connects too - my understanding is that in the 1985+ (maybe 84+) 900's
there's a small solenoid device that's associated with the recirc function
and it's located in the space above the right hand side wheel well near
where the vacuum tank sits. I think it's got an electrical connection and
three vacuum line connections. My 1985 900i has one of them but I haven't
looked at that car's engine bay for a while.

Maybe that's shorting when activated, or has a fault that overloads the
circuit.

Craig.