It comes on for two reasons. One is if the pressure of the refrigerant is at a high level. The fans will come on low.
IF the engine is running hot, the fans will come on high.
IF your fans are not coming on the low speed with the A/C on in the summer, you have a problem. (We see the 15a fuse under the fuse box cover blow, it should be updated to a 2Oa.)
1992 500E (very soon I hope
1981 280GE SWB
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