pff and? works

Sounds like it's working fine to me. I've never heard of them going solid, but even if it has that's a hell of a lot better than the fluid degrading and the fan not doing the job of cooling the motor. Cars used to have solid fans, all that did is waste a bit of energy sometimes, and make more noise, and chew the radiator if you drove through deep water. In fact, a radiator specialist advised me to make my dud one solid, rather than buy a new one. But as i say, i've never heard of them going solid, but a radiator specialist would know heaps more than me about this.

To restate what i said earlier, viscous coupling's become more solid the hotter they get. They get hotter when the air passing over them is hotter, when the shaft they are on is hotter, and i reckon they mostly get hotter when the difference between the shaft speed and the fan speed is greatest.

Imagine a coupling that worked the other way. Imagine a coupling that offered less and less airflow the longer the motor sat there idling, for example. That would be no good.

We want more airflow the hotter things get, so we want more resistance within the coupling's fluid the hotter things get.