Very good chance also that if it has been doing this for some time you may have burnt or corroded brush holders under the fan. Mine were completely toasted as it appears that the fan stuck on and the PO never disconnected it. I tjust ran and ran until the brushholders literally melted away adn the brushes turned more or less sideways...

if you follow the main power wires coming off the fan they go down just under teh front air dam whcih is where there is a two pronged connector that the fan plugs in to. You can disconnect it there and then troubleshoot using a voltmeter to tell where it is in the circuit that the sensors have failed causing it to stay on all the time.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique