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Old 07-04-2002, 11:22 PM
Registered User
Join Date: Aug 2001
Location: Missouri Ozarks near MountainView MO
Posts: 107
Auxcillary Fan "Switch"

My auxcillary fan does not operate on my 79 450SLC. The fan does run if I hot wire it to the battery, so it is not the fan itself. It does not run in conjunction with the AC or by any other method that I can discover.
HOWEVER, on my previous engine and my current engine in this car there is an "Auxcillary Fan Switch" located on the water pump but nothing is connected to it. It looks like an oversize stereo plug.
I have tested it with an ohmmeter and it registers no resistance when cold and high resistance when hot. This would indicate to me that it functions with some sort of biased relay that turns on the fan when the bias is reduced and the fan runs until the temp reaches some value preset by the relay/switch combination.
Does anyone know anything about this "phantom" circuit. There are no loose wires or plugs around that would appear to connect to this "switch".
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Old 07-05-2002, 12:46 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The fan works off the 212 switch at the front of the motor and the temp switch on the reciever-drier. Both switches ground the relay. You must be on the wrong switch or it is working improperly. Try the switch at the drier, by placing both ends together.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 07-07-2002, 07:18 PM
Registered User
Join Date: Aug 2001
Location: Missouri Ozarks near MountainView MO
Posts: 107
Steve: Thanks very much for your reply. I did check the switch/wires at the drier. Put them together and could hear the relay clicking on the passenger side wheel well directly behind the radiator reservior. It still did not turn on the fan however. I could not find where any other wires went because this would entail removing the reservior. I started to go somewhere the following day and the battery was dead. I removed the relay, jump started the car and everything worked fine. This of course indicates that the relay is bad.
where on the front of the engine is the other switch/ground for that fan relay?
Thanks again for your help.
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