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#1
05-22-2014, 05:29 PM
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Auxiliary cooling fan always on
The auxiliary cooling fan, which is only supposed to run when the engine temperature gets too high or in response to certain AC conditions, is now on all the time. What should I be looking at to cure this problem?
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Berfinroy in CT Present vehicles: 1973 300 SEL 4.5 1959 Rolls Royce Silver Cloud I 1959 Ford Thunderbird convertible/430 Past vehicles; 1958 Bentley S 1 1976 ex-Max Hoffman 6.9 1970 300SEL 2.8 1958 Jaguar MK IX 1961 Jaguar MK IX 1963 Jaguar E-type factory special roadster 1948 Plymouth woody 1955 Morgan plus 4 1966 Shelby GT350H Mustang 
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#2
05-23-2014, 02:06 PM
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The temperature switch in the receiver/dryer on the firewall may have failed in the on position--there's also a coolant temperature switch that controls the supplementary fan--that may have failed as well.
I've used a manual switch to control the fan on some of my M110 114's. The problem with having the fan on all the time is that the power consumption is close to the entire output of a 55-amp alternator. The fan also interferes with the ram effect above 35 mph or so, negating the value of the main fan clutch which slows or stills the blades above 2000 rpm Tim Kraakevik kraakevik@voyager.net
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#3
05-23-2014, 06:33 PM
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Definitely sounds like a bad thermostat.
In my w114 I also use an auxiliary fan with a manual switch with an adjustable thermostat (you'll need a relay if >10 amp fan). I kind of use this set up for hot days on traffic (flip it on and peace of mind). In addition, if I ever park and forget to turn off, it will cut off by itself when the engine cools (and not drain my battery).
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 Current: 1971 Mercedes Benz 250 (Euro Spec) 1972 Mercedes Benz 250 (US Spec) Past: 1972 Mercedes Benz 280
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#4
05-23-2014, 07:04 PM
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Have you checked the fuse box?
As far as I know there is a fail safe, if a fuse blows the relay will kick in and the fan is on at all time. Also, check the connection on the Thermostat housing, if this connection is lost, the fan will run at all times. Also check, with AC on and blowing cold, is the second (if present) FAN running? The fans are usually only fused with a 16 Amp fuse, so it does not draw 55 Amp of current. I don't know which vehicle is running the fan at all time, so take my post as a general trouble shooting guide. As always, these are my guesses. Sorry, just saw the other cars are past cars, so it has to be the 300SEL.
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Mostly, I don't know notin, I just know where to look. I am looking back, to over 30 years in Electronics Design. Electrons don't care if they move in a car, computer or relay! 95 W124 E320 M104.992 - Because, I love to repair, naaaah! Over 221,000 Miles Cheers, Norbert
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#5
05-23-2014, 08:47 PM
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If this is the 4.5, it is likely one of 2 things:
1) Bad/stuck fan relay, 2) Bad 212° switch. The fan relay IIRC is the canister-shaped one on the firewall. The 212° switch is the single-wired sensor on the front of the engine.  If you unplug that wire and the fan stops it's that switch/sensor. There are other possible causes but those are the two most likely.
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Current: 2021 Charger Scat Pack Widebody "Sinabee" 2018 Durango R/T Previous: 1972 280SE 4.5 2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi" 1999 Chrysler 300M - RIP @ 221k
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#6
05-26-2014, 06:24 PM
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I've been having an interesting off-line discussion on the electrical consumption of the cabin fans and the supplementary fan on the 114/115-chassis models--does anyone know what the heater, A/C and supplementary fans consume in terms of watts or peak amperage at full power?
Tim Kraakevik kraakevik@voyager.net
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