The fan has two speeds. Low speed is triggered by the AC high pressure switch, the switch with pigtails on the drier. Jump the pigtail ends and the fan should come on. High speed is triggered by the coolant temp switch, the one on the upper radiator hose turret on the cylinder head. Jump the contacts on the 2-pin connector and the fan should come on.

If the fan doesn't come on on low speed, it's either a bad relay or more likely a bad step down resistor behind the left headlight and ABS pump. A bit of a PITA to get to.

The fan doesn't come on just because the compressor is engaged. There's also the high pressure factor.

Sixto
87 300D