I don't think they ever fully engage. They have thick silicone fluid inside which allows them to run at less RPM than the pulley is turning, and at higher RPM's they run substantially less than the pulley speed. The thermal ones have the bi-metallic spring that must tighten up something inside so it turns closer to the pulley speed when it feels the hot air from the radiator. I just put on a heavy duty clutch from flex-a-lite. What I don't know is what temperature they activate.