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Old 09-29-2001, 01:32 AM
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Manual switch for auxiliary fan?

Has anyone seen or installed a manual switch override for the auxiliary fan located in front of the radiator on a 1975 450 SL.

Am wondering if being able to turn that fan on at will, say while stopped at lights on a hot day, to start trying to cool down the engine without waiting for the temperature gauge to hit 200 before the auxiliary fan cuts on...?

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Old 09-29-2001, 02:36 AM
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further information requested

as an addition to the first posting, would like to know what other methods/ideas/tips are out there and working to reduce the heat in the engine compartment for those owner whose vehicles might have or currently experience this issue.
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Old 09-29-2001, 08:28 AM
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You can put an off/on sw. in the cabin and run the two wires to the high pressure sw. on the reciever/drier for the a/c.
Basically, a high pressure sw. jumper.
It is a good idea and have done it to many older models.
Gives you a good jump on the temp rise.
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Old 09-29-2001, 08:55 AM
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you can also use a lighted switch. when wired properly, the light will go on whenever the aux fan is on , whether through the thermoswitch or via override. i had a car wired this way and i would drive with a/c on in summer in stop and go traffic in nyc with never a worry!
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Old 09-29-2001, 02:01 PM
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I tell ya living here in Phx. for me my aux. fan never has a break as the A/c is mostly on for most of the year out here, from the moment you star the car till you finish your mission. and temps still climb with the on
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Old 09-29-2001, 04:20 PM
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thanks for the replies

sounds like putting in the manual override switch can only enhance the cooling of the engine compartment.

now to find out how to install said switch so I can activate the auxiliary fan at will....anyone out there have instructions or can point me to a website with instructions? or is this something that only a qualified mechanic should do?
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Old 09-29-2001, 04:49 PM
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i've never done it on an MB. if you're not comfortable doing electrical work then have it done by someone else. you have to have a fair understanding of electrical switches and the like. the auxiliary fan is turned on by thermosensor which is really just a switch that completes a circuit above a set temperature. in general you wire the override switch in parallel with the thermoswitch so that you can complete the circuit at will...
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Old 09-29-2001, 09:09 PM
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found the thermosensor switch ...

while trying to unhook it, the wire leading into it broke off due to its being so brittle and worn after years of "cooking" in this hot engine compartment of my 450 SL!

so while it was broken I touched the bare end to ground with the ignition turned on and lo and behold the auxiliary fan cut on so I know I had found the right temp sensor.

so I am going to wire it directly to ground (lifting eye bolt nearby) and run the car with the aux fan running anytime the ignition is on to see if it makes a difference for me in terms of running engine temps. will update as I get data.

if it does make a difference then I'll investigate putting it inline with a switch instead of running all the time since it sure makes for more noise when the fan is running.

One question remains: anyone think that running the aux fan all the time will lead to its premature failure or should it be able to take on the load?

thanks again to all who replied.

Last edited by domchang; 09-29-2001 at 09:15 PM.
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Old 09-29-2001, 09:56 PM
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answers to previous posting


yeah, the heat from the catalytic converters are probably the major contributor and yes, I've been told by a MB mechanic that I could replace the catalytic converters with pre-1975 or post 1979 exhaust piping which respectively would either eliminate the catalytic converters or move them downstream under the car where most modern cars have their catalytic converters. Either option is around the $2000-$3000 range and I am not ready for that step quite yet.

I've also been told on this forum that the current running temps of 175-212 are normal for this year 450 SL so am pretty sure my cooling system is working as designed. I was a tad low on coolant which I fixed today (50/50 mix of water and MB antifreeze) and I've also added a bottle of Water Wetter as well.

I'll see how she runs tomorrow when I can take her out for a spin to see how the temps hold up.

ps: I've also filled her up with 93.5 octane gas to help prevent vaporisation of the fuel hopefully.

thanks for your reply and feedback,


Originally posted by blackmercedes
I understand that the 75 450SL's run hot ue to the catalytic converters being mounted in the engine bay. This switch seems like a stop-gap until the hot-running problems are solved.

Is it possible to have the exhaust re-fitted to look like the 79- SL exhaust?

Also, have you checked overall cooling system operation? Thermostat, rad, water pump, lines, etc.

Also, many people are driving around with the improper coolant/water ratio.

As well as fitting your manual override, make sure you check the cooling system...
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Old 09-30-2001, 04:24 PM
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Check to make sure your visco clutch is working, too -- when the engine is warm (above 85 C or so), the fan should roar, and stop within a half turn or so when you shut the engine off. If it spins freely when hot, or doesn't roar, the clutch is bad and you don't have enough air flow through the rad.

I'd also have the rad checked -- with the age on the car, it may be crudded up, with resulting poor cooling performance. You need all the radiator you can get down there in the summer!

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Old 10-29-2011, 11:40 PM
Arizona Real Estate Guy
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How can I wire it to run after the key is off? ( it will shut off when cool) Do I just need to change the fuse input from key to battery? looks like fuse 8.
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Old 10-30-2011, 12:38 PM
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You're reviving a 10 old post with minimal info.

You're trying to have some after-run cooling down using the aux fan, I presume?

You may want to look for a newer temperature controlled relay. Tinkering with the existing system to add on an ignition-off feature risks killing your battery.

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