Sounds like your coolant temp sensor has gone bad. If the engine management system can't get a reliable coolant temp, it goes into kind of a fail safe mode and kicks on the aux fans at high speed to prevent the engine from overheating.

Regardless of whatever other problems you may have, you probably want to replace that sensor. It's very easy to get to. Pull off the smaller plastic cover on top of the engine. The sensor sits smack-dab in the middle of the area under the cover. It looks like a big bolt with two wires coming off of it.

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"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles