Just came across your post
and thought I reinforce -fad's comments in his first post to you. These comments represent very sound advice.
Re temps for fan cutin, check my page (www house below) and click on menu 17. The temps and pressures are described there.
As I remember, the first stage fan is supposed to come on around 100C, second at 107C and third at 115C. In my car, the 1st stage occurs about 102C (without my CH). Maybe the resistor is burned out/broken?
If the A/C comes on, the fans usually start up right away since the pressure build up quickly. If they are not working, then by all means get the car checked before adding the CH!
Regards . . . . JimF
'94 S500 Cpe
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