Z31.9 electric engine speed(s) fan issue

My car is a 1.9 M44, without viscostatic, only electric 2-speed fan (directly assembled that way on bmw at its birth for AC post-montage).

Now, the mistery.
When engine turns hot, the double temperature switch should pull the fan at lower speed reaching 80 degrees, and at higher speed reaching 88 degrees.
But in my car the slower speed NEVER starts. You can only hear the higher speed.

If I jumper the thermostat connector in pins n°1 and 2, it starts regularly. When i re-attach the connector to the thermostat, same issue: slower speed won't start.

Thermostat is NEW, and so hi-speed and lo-speed relays.
I'm now wondering if is a common "issue" of M44 without viscostatic, or there is something strange that can't be easily identified.

Can someone with my same engine configuration make a test?

Into your car (with AC off) the vents starts at the slower speed too, or only at higher as mine does?

solved!
SOLVED!!!!
the guys on my italian club (bmwdrivers.it) solved the mystery!!!

M44 motor had, at that time a DIFFERENT wirings during the years