I'm certain that your problem is a faulty fan sending unit which is not
working on slow speed, but before you buy anything perform the very
simple check which Ken Wilford described on his website. In the short
term you can cause the low speed fan to be on by just leaving the jumper
in place while driving or if you have air-conditioning you can turn the
air conditioner on which causes the low speed fan to operate no matter
what the engine water temperature, turn your A/C thermostat off so that
only the fan is running. The link to Kens troubleshooting page is below:

http://www.vanagain.com/disc2_frm.htm

Hope that helps,

Tony 85GL "Westfailure"